Chapter 2
Aligned 2016 CACREP StandardsStandard 2.F.3.e. Biological, neurological, and physiological factors that affect human development, functioning, and behavior
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Neuroscience is fundamental to our understanding of child and adolescent development. In this chapter, we review fundamental information about the structure and function of the nervous system to prepare you for subsequent information about childhood development and problems or disorders that occur during the developmental process. This chapter has strong connections to the next chapter, on genetics, epigenetics, and neuroplasticity. Several concepts mentioned in this chapter are further explored in Chapter 3, such as the role of the stress response system in a child’s overall functioning. This chapter was informed by several sources, including Neuroscience: Exploring the Brain (4th ed.) by Bear, Connors, and Paradiso (2015); From Neuron to Brain (5th ed.) by Nicholls and colleagues (2011); Neuroscience (5th ed.) by Purves and colleagues (2011); and Principles of Neural Science (5th ed.) by Kandel, Schwartz, Jessell, Siegelbaum, and Hudspeth (2012). We encourage you to explore these textbooks if you would like more grounding in the basic anatomy and physiology of the nervous system.
The terminology used in this chapter might be challenging to you. In writing the chapter, we attempted to fully describe each scientific term that we used, knowing that the experience can feel like learning a new language! To help you digest this information, we recommend consulting the glossary at the end of this text to clarify unfamiliar terms.
Before we begin, we want to emphasize an important principle in understanding the function of the brain and body. The systems of the brain and body are intricately connected and often work synchronously with each other. Although we often study isolated brain regions, the reality is that all sections of the brain and body work in an interconnected rather than an isolated manner (Field, 2019; Jasanoff, 2018). This is crucial to emphasize, because we tend to divide and separate the functioning of the brain and body, which limits our exploration of the linkages between mental and physical health (Field, 2019; Jasanoff, 2018). Throughout this text, we emphasize the interconnected nature of the brain and body.
We now explore the systems of the brain and body, giving special attention to the central nervous system (CNS) and peripheral nervous system (PNS). We also briefly examine the endocrine system. The divisions and branches of the CNS and PNS are depicted in Figure 2.1.
FIGURE 2.1 Nervous System
The CNS comprises the brain and the spinal cord (Bear et al., 2015). The brain processes incoming sensory information and sends signals down the spinal cord to communicate with the rest of the body. The spinal cord also transmits information to the brain from the body. This system is called the central nervous system because of the brain and spinal cord’s ability to influence and coordinate the rest of the body (Bear et al., 2015). The CNS is composed of several cells, most notably nerve cells called neurons and glial cells. The purpose of neurons is to receive and transmit electrical messages (Nicholls et al., 2011). The neurons in the brain are largely pyramidal neurons, whereas the neurons in the PNS are largely sensory and motor neurons (more about this later).
The purpose of glial cells is to support the functioning of a neuron by transporting nutrients to neurons, holding neurons in place, and cleaning up debris and waste from areas of injury and cell death—in particular during sleep (Nicholls et al., 2011). There are multiple types of glia, such as astrocytes, oligodendrocytes, and microglia. Microglia have important roles in inflammatory responses. Figure 2.2 provides a basic diagram of neurons and glia.
Neurons have several different structures that are essential to their functioning. Each of these structures is displayed in Figure 2.3. A neuron’s functioning is directed by the cell body (also called soma), which contains the cell’s nucleus. As we explore in Chapter 3, the nucleus produces and stores DNA and enzymes that are crucial to the cell’s function. Each neuron also contains structures for receiving information (dendrite receptors) and sending information (axon terminal buttons) via chemical signaling. Each part of the cell has an important role in the communication of information from one neuron to another, known as neurotransmission.
FIGURE 2.2 Glial Cells of the Central Nervous System
Note. From Openstax, 2016. In Wikimedia Commons, used under Creative Commons Attribution 4.0 International License.
The neurotransmission process begins when two neurons are in close proximity. An electrical signaling occurs when a neuron comes into close proximity to another neuron. The first neuron moves neurotransmitters from repositories known as vesicles to the axon terminal buttons, where they are released into the synaptic cleft between the two neurons (Bear et al., 2015). Many of these neurotransmitters are received by the next neuron through dendrite receptors. Some of the neurotransmitters are reabsorbed back into the original axon through a process known as reuptake. The rest of the released neurotransmitters remain in the synaptic cleft, where they are broken down by enzymes. When neurotransmitters bind to the receptors of the next neuron, it opens the neuron’s ion channels. If enough ions enter a cell, it creates a polarized electrical charge known as an action potential. If enough of a charge is created, the neuron will fire an electrical impulse from the dendrites to the neuron’s axon terminal buttons via the axon. The axon plays a critical role in conducting the electrical current and carrying it to the axon terminal buttons. When conduction occurs, this neuron then signals to another nearby neuron and releases neurotransmitters from its own axon terminal buttons into the next synaptic cleft. This process continues, sending messages from neuron to neuron throughout the nervous system. The collective series of synaptic connections are known as neural pathways. A visual diagram of this process is provided in Figure 2.4. Note that even at the cellular level (i.e., that of neurons), the structure and function of the CNS only makes sense when we examined interconnected relationships (i.e., neurons to neurons). In other words, the function of a neuron can only be fully understood by examining its relationship to other neurons (Nicholls et al., 2011).
FIGURE 2.3 The Nerve Cell
Note. From National Institute on Drug Abuse, 2005. In Wikimedia Commons.
The CNS is composed of gray matter and white matter (Kandel et al., 2012). Both gray matter and white matter consist of neurons and glia. The major difference between gray matter and white matter is that gray matter consists largely of neurons that have unmyelinated axons, whereas white matter consists largely of neurons with myelinated axons (Kandel et al., 2012). Myelination is a process whereby a plasma membrane extension, known as a myelin sheath, is wrapped around the axon to form a tight membrane and insulate the cell (Nicholls et al., 2011). This fatty sheath of myelin allows for improved electrical conduction and transportation throughout the cell. For example, an unmyelinated axon needs a diameter approximately 500 times the size of a myelinated axon to conduct electricity at the same speed and requires 5,000 times more energy (Snaidero & Simons, 2014). The color differs because the fatty myelin sheath is white, whereas the unmyelinated cell bodies of neurons are light gray with pinkish tints from blood vessel capillaries. Disruption or deterioration of myelin leaves the axon exposed to damage. Thus, disruption in myelination can result in a host of maladies, such as a disturbance of gene transcription in the cell nucleus (see Chapter 3 for more information about gene transcription). Problems with myelination are also associated with multiple sclerosis (Duncan & Radcliff, 2016).
FIGURE 2.4 Neurotransmission
Note. Image modified from “The synapse,” by OpenStax College, Anatomy and Physiology. Used under Creative Commons Attribution 3.0 Unported License.
Multiple types of neurotransmitters are released from a neuron’s axon terminal buttons. Some commonly known neurotransmitters are the monoamines, also known as catecholamines (e.g., dopamine, norepinephrine, serotonin). Amino acids, such as glutamate and gamma-aminobutyric acid (GABA), are another form of neurotransmitter. Neurotransmitters can have excitatory and inhibitory effects. An excitatory effect increases the likelihood that postsynaptic neurons will experience an action potential, whereas an inhibitory effect reduces the likelihood (Nicholls et al., 2011). The major excitatory neurotransmitter in the CNS is glutamate. Glutamate helps to balance out the major inhibitory neurotransmitter, GABA (Shabel, Proulx, Piriz, & Malinow, 2014).
Each neuron has thousands of receptors per cell and many receptor types (Nicholls et al., 2011). Because neurotransmitters can bind to multiple types of receptors, they can have different effects based on the receptor type. Ligand-activated ionotropic receptors change in shape when a neurotransmitter binds, resulting in the opening of calcium ion channels. When ion channels open, a positive or negative charge is created in the cell, depending on the type of ion (positive or negative) that enters the cell. Metabotropic receptors do not directly open ion channels. Instead, these receptors trigger another signaling pathway that may indirectly open or close an ion channel. As you can see, the process of neurotransmission is quite complex and not always predictable. A list of neurotransmitters and their effect on receptors is included in Table 2.1.
Each neurotransmitter has multiple functions. Dopamine is associated with motivation and reward prediction alongside voluntary movement of muscles (Kandel et al., 2012). Norepinephrine is related to dopamine and is associated with attention and arousal (Kandel et al., 2012). It has important roles in activating the PNS following the detection of threat, resulting in increased heart rate, among other physiological responses. Norepinephrine is also related to other important neurotransmitters and hormones, such as epinephrine and adrenaline. Serotonin has important roles in the sleep cycle, appetite and temperature regulation, as well as mood (Kandel et al., 2012). Melatonin is related to serotonin and has important roles in preparing the body for sleep. Melatonin secretion is influenced by exposure of the eyes to light and has an established role in circadian rhythm (Bear et al., 2015). Humans are diurnal creatures, and darkness prompts our bodies to prepare for sleep. Glutamate is the most common neurotransmitter and is associated with synaptogenesis (the growth of new synapses; Kandel et al., 2012). GABA is a metabolite of glutamate and has a role in sleep and in moderating anxiety (Kandel et al., 2012).
Now that we have learned how messages are communicated throughout the CNS, we explore the different and interconnected regions and structures of the brain. We describe these from the framework of which brain regions developed first evolutionarily. Although we describe functions of these regions and structures, it is important to emphasize that each works in a connected and synchronous fashion with other regions and structures.
TABLE 2.1 Neurotransmitter Functioning
| Neurotransmitter | Type | Activates Ionotropic Receptors | Activates Metabotropic Receptors |
| Dopamine (DA) | Monoamine/Catecholamine | Yes (excitatory) | Yes |
| Gamma-aminobutyric acid (GABA) | No | Yes | |
| Amino acid | Yes (inhibitory) | Yes | |
| Glutamate (Glu) | Amino acid | Yes (excitatory) | Yes |
| Norepinephrine (NE) | Monoamine/Catecholamine | No | Yes |
| Serotonin (5-HT) | Monoamine/Catecholamine | Yes (inhibitory) | Yes |
The brain is a large, sponge-like mass that has three central parts: the brain stem, the cerebellum, and the cerebrum (Bear et al., 2015). The cerebrum fills most of the skull, though it is largely believed to be the last of the three brain areas to have developed fully (Hofman, 2014). Figure 2.5 depicts the different brain structures that we explore in each of these three areas.
The brain stem is located at the bottom of the brain and connects the brain to the spinal cord (Bear et al., 2015). It also connects parts of the cortex to the PNS. It is composed of structures that include the medulla oblongata, midbrain, and pons. The brain stem is associated with basic survival functions. The medulla oblongata is involved in cardiac functioning (e.g., blood pressure, heart rate) and respiratory functioning (e.g., breathing). The midbrain regulates body temperature alongside sleep and wake cycles and motor movement. It also has an important role in hearing and vision. The pons relays sensory signals to the thalamus and has a role in balance, bladder control, body posture, eye movement, and facial expressions and sensations. Damage to areas of the brain stem can result in difficulties with motor coordination, dizziness, disturbances in hearing and vision, and problems with speech production (Bear et al., 2015).
FIGURE 2.5 Brain Structures
Note. From Patrick J. Lynch, medical illustrator and C. Carl Jaffe, MD, cardiologist. Used under Creative Commons Attribution 2.5 License, 2006.
Two sets of nerves connect the brain and body. Cranial nerves begin at the brain, whereas spinal nerves emerge from segments of the spinal cord (Bear et al., 2015). There are 12 cranial nerves (CN I to CN XII), such as the olfactory nerve (CN I), which is associated with smell; the optic nerve (CN II), which is associated with vision; and the vagus nerve (CN X), which is associated with parasympathetic activation (more about this later).
The spinal cord is made of nerve fibers that run down the center of the spine. The bones of the spine (i.e., vertebrae) protect the spinal cord. Spinal nerves have an important role in sending messages throughout the body. Cerebrospinal fluid provides nutrients to the brain and spinal cord and removes waste and debris. Upper motor neurons reside within the spinal cord to send messages up and down the cord, whereas lower motor neurons are found in spinal nerves that branch out from the spinal cord. Damage to the spinal cord can result in loss of communication between the brain and body, resulting in serious sensory and mobility issues such as loss of sensation, numbness, and paralysis (Bear et al., 2015).
The cerebellum, Latin for “little cerebrum,” is a fascinating structure that is located close to the brain stem and below the cerebrum. The cerebellum contains more than half of the total neurons in the brain, despite its relatively small size (only 10% of brain volume; Klein, Ulmer, Quinet, Mathews, & Mark, 2016). The cerebellum plays important roles in motor movement and coordination by receiving motor commands and making movements more accurate and adaptive to the situation. For example, the cerebellum has an important role in motor learning through trial and error, such as learning to balance on a bike or calibrating a baseball swing to hit the ball. It also has roles in cognition and emotion, such as attention, executive functioning (or planned behavior), working memory, and pain (Strick, Dum, & Fiez, 2009).
The cerebrum consists of several important regions, such as the subcortex, limbic region, and cerebral cortex (Bear et al., 2015). The various regions and structures of the cerebrum work in concert to send messages both within the cerebrum and to different areas of the brain and nervous system.
The subcortex is located dorsal to (i.e., above) the cerebellum and brain stem though still ventral to (i.e., below) the surface of the cerebral cortex (Bear et al., 2015). The limbic region of the subcortex contains several important structures for processing sensory information. The thalamus is known as the central relay center of the cerebrum and sends messages to various parts of the subcortex and cortex. The hippocampus has an important role in memory formation and consolidation and is involved in the processing of new memories, the storage of long-term memories, and memory retrieval. Memory is essential to social functioning in terms of being able to remember relational histories with people, remembering their preferences, remembering whether information has already been shared with certain people, and even making predictions based on prior knowledge (e.g., What activity might my friend want to play with me? Rubin, Watson, Duff, & Cohen, 2014). Recent studies have also found that the hippocampus has an important role in mediating sensory processing throughout the brain (Chan et al., 2017).
The amygdala is a complex structure composed of several parts (or nuclei) and is known for its role in processing emotions (Bear et al., 2015). The amygdala has an important function in threat detection and fear processing alongside the hippocampus, hypothalamus, and prefrontal cortex. When threats are detected in the environment, the amygdala communicates with the hypothalamus to send messages to the pituitary gland, the master hormone gland of the brain (Bear et al., 2015). The pituitary gland then sends messages to the adrenal glands, comprising the adrenal cortex and medulla, located above the kidneys (Bear et al., 2015). The adrenal glands release stress hormones such as adrenaline and cortisol. The hypothalamic-pituitary-adrenal (HPA) axis therefore includes structures of the subcortex and parts of the endocrine system. The endocrine system is composed of glands that release hormones (e.g., the pituitary gland).
The subcortex contains other structures that are located between the cortex and the limbic region, such as the basal ganglia, cingulate cortex, and insula. The basal ganglia is located near the limbic region and contains several structures, such as the caudate nucleus, putamen, globus pallidus, and striatum. These structures are associated with smooth motor movement and planning. The striatum and its structures (e.g., the nucleus accumbens) is associated with motivation, aversion, and conditioned learning and has an important role in addiction processes (Bear et al., 2015).
The insula is also located close to the limbic region and has a variety of functions, including emotional and sensory processing and higher order cognition, such as empathy (Uddin, Nomi, Hébert-Seropian, Ghaziri, & Boucher, 2017). It is activated when children become aware of internal bodily sensations (known as interoception) and the emotional experiences of self and others. For these reasons, it has been dubbed the visceral part of the brain (Stephani, Fernandez-Baca Vaca, Maciunas, Koubeissi, & Lüders, 2011). It has connections to the anterior cingulate cortex and ventromedial prefrontal cortex.
The anterior (frontal) and posterior (rear) cingulate cortexes work together and have somewhat different functions. The anterior cingulate cortex is involved in emotional processing and regulation, with linkages to both the limbic regions and prefrontal cortex. The anterior cingulate cortex is implicated in conditioned learning, especially related to emotions, such as when a child experiences pain on touching a sharp or hot object (Stevens, Hurley, & Taber, 2011). Over- or underregulation of affect is problematic. For example, overregulation of affect can result in less conditioned learning in response to emotional experiences (Stevens et al., 2011). The posterior cingulate cortex (PCC) is believed to be crucial to information processing, with linkages to the ventromedial area of the prefrontal cortex (Leech & Sharp, 2014). It has a key role in the default mode network (DMN), which we review later in this chapter. The PCC becomes activated when retrieving autobiographical memories. It is also associated with regulating attention. The PCC is therefore active when a child is reflecting and processing their experiences. Abnormal functioning of the PCC has been connected to cognitive impairment.
The cortex is the outer layer of tissue that covers most of the brain. It covers the subcortex and is dorsal to the brain stem and cerebellum. There are thought to be four lobes of the cerebral cortex: the frontal lobe, the temporal lobe, the occipital lobe, and the parietal lobe (Bear et al., 2015). Before we describe the lobes of the cerebrum, it can be helpful to review basic terminology pertaining to location. Dorsal means “above,” whereas ventral means “below.” Anterior means “toward the front,” whereas posterior means “toward the back.” These terms are displayed around the periphery of the diagram in Figure 2.6.
FIGURE 2.6 Lobes of the Cerebrum
Note. From Patrick J. Lynch, medical illustrator and C. Carl Jaffe, MD, cardiologist. Used under Creative Commons Attribution 2.5 License, 2006.
The frontal lobe has several regions, such as the primary motor cortex, the premotor cortex, the prefrontal cortex, and the orbitofrontal cortex. The primary motor cortex and premotor cortex send messages to the brain stem and spinal cord to direct voluntary movements. They receive information from the somatosensory area and thalamus regarding movement positioning, timing, and coordination.
The functions of the prefrontal cortex are often described by their location to the skull (Bear et al., 2015). The medial prefrontal cortex is located toward the midline of the forehead. The dorsal area of the medial prefrontal cortex is associated with self-referential thought (Raichle, 2015). We often understand information in our environment through our own frame of reference. For example, when asked to identify tasks that their parents or friends enjoy, young children often respond by identifying activities that they themselves personally enjoy (which may not be the preference of their parent or friend). As children mature, they are better able to understand the perspectives of others, known as mentalization or theory of mind. The ventral area of the medial prefrontal cortex (vmPFC) is located just above the orbitofrontal cortex and is associated with emotional regulation. It has an especially strong connection to the amygdala and has an important role in the regulation of anxiety and fear responses (Motzkin, Philippi, Wolf, Baskaya, & Koenigs, 2015). For example, if a child is frightened by a loud sound in the environment, the vmPFC will be activated during their cognitive processing of the potential threat (e.g., “It was just a door slamming shut, no need to worry”). The vmPFC is also a key structure in the DMN (more on this later) and is believed to be one of the main regions damaged in the famous case of Phineas Gage (Raichle, 2015).
The lateral prefrontal cortex comprises the largest portion of the frontal lobe and is located above the orbitofrontal cortex. The dorsal area of the lateral prefrontal cortex appraises sensory information (e.g., from the eyes and ears) to make decisions and is thought to be responsible for rational decision making and planned behavior, known as executive functioning. It helps children make decisions in unscripted, unpredictable, and open-ended situations (Collins & Koechlin, 2012). It receives and sends information throughout the brain and has especially strong circuits with the thalamus and caudate nucleus of the basal ganglia.
The orbitofrontal cortex has connections to the limbic and olfactory systems and alongside the vmPFC plays an important role in emotional processing and regulation. It also functions to discriminate between smells. The orbitofrontal cortex helps children to inhibit their impulses to adhere to social norms and conventions. For example, the orbitofrontal cortex is activated when a child controls their impulses to inappropriately snatch a desired object from another person.
Broca’s area, which is associated with both speech production (i.e., spoken words) and language comprehension (Rodd, Davis, & Johnsrude, 2005), is also located in the frontal lobe. When children communicate verbally to their parents or counselor, Broca’s area is likely active. Underactivity in Broca’s area can result in problems such as stuttering and aphasia (i.e., knowing what to say but being unable to form the words).
The other three lobes of the brain are the occipital, parietal, and temporal lobes. The occipital lobe is located posterior to the skull (i.e., at the back of the head) and contains the primary visual cortex. It processes visual stimuli and sensory information. It can be implicated in seizures that are triggered by flicker stimulations of flashing colors or lights. The parietal lobe is located behind the frontal lobe, in front of the occipital lobe, and above the temporal lobe. It contains the somatosensory cortex and is implicated in awareness of oneself in space and navigation (proprioception). It is also the main area of the brain that receives input from the skin, such as touch sensors. The temporal lobe is located beneath the parietal lobe. It houses structures (e.g., the hippocampus) that play an important role in the consolidation, storage, and retrieval of long-term memories, known as declarative memory. Wernicke’s area, which alongside Broca’s area is associated with language comprehension, is located in the temporal lobe. The temporal lobe is also associated with visual object recognition.
Earlier we briefly mentioned the DMN. After learning about the cortex and subcortex, it is worth revisiting the different structures involved in a child’s ability to engage in defocused and wandering thought. The DMN is active when a person is in a resting state and not actively engaged in attention-demanding tasks (Raichle, 2015). The brain structures involved in the DMN are the ventromedial prefrontal cortex, the dorsal medial prefrontal cortex, and the PCC. You may remember that the vmPFC is involved in emotional processing, the dorsal area of the medial prefrontal cortex is involved in self-referential thought, and the PCC is involved in autobiographical memories and regulating attention. When connected during DMN activity, these structures help people to process their experiences and emotions, make sense of them, and relate them to their sense of self. Such activity helps children to learn from their experiences. Reduced DMN activity has been associated with depression, juvenile conduct problems and impulsivity, and schizophrenia (Raichle, 2015).
Like the CNS, the PNS is composed of neurons. These neurons are often bunched together into fibers, and multiple fibers form a nerve. Each nerve also contains tissue and blood vessels. The nerves of the PNS connect the CNS (brain and spinal cord) with the body’s various glands, limbs, muscles, organs, and skin (Nicholls et al., 2011).
PNS nerves are classified as afferent and efferent (Nicholls et al., 2011). Afferent nerves carry messages from muscles, glands, and the senses, whereas efferent nerves carry messages to muscles, glands, and the senses (e.g., touch sensors, eyes). These nerves often transmit information that is not known to us consciously but is crucial to our body’s overall functioning. For example, afferent nerves transfer information about the current inner state of the body’s organs, such as the organ’s current energy intake, need for blood supply and oxygen, and so on.
The neurons involved in these functions differ from the neurons of the CNS. Motor neurons and sensory neurons have a different structure than CNS neurons. As their name suggests, motor neurons are associated with muscles and muscle movement, whereas sensory neurons are associated with the body’s sensory systems (e.g., touch, sight, smell).
For example, if the hand touches a painfully hot object, sensory neurons in PNS send rapid messages to the brain in the CNS. The brain then sends messages from CNS neurons to PNS motor neurons. The transfer of information from CNS neurons to motor neurons occurs in the spinal cord. The motor neurons are instructed to move the hand away from the hot object. The motor neurons then transmit information across the neuromuscular junction, where PNS nerves connect to muscles, and the hand moves. This example demonstrates how the CNS and PNS often function in concert to carry out important functions.
The PNS is organized into three primary divisions: the sensory, autonomic, and enteric systems. The sensory nervous system contains afferent nerves composed of sensory neurons that transfer information from our five senses to the CNS. It also contains efferent nerves made of motor neurons that carry information from the CNS to the muscles to initiate voluntary movement required for activities such as reaching and grabbing an object.
The autonomic nervous system (ANS) is more complex and has important implications for child development and mental health. In the following section, we describe the function of the three branches of the ANS.
The ANS controls involuntary functions related to heart functioning, respiration, and other reflexes such as vomiting (Bear et al., 2015). It is self-regulatory, which means that the ANS can increase or reduce the amount of energy, blood flow, and so forth to the organs of the body without a person being consciously aware of these regulatory actions. The ANS is divided into three separate branches: the sympathetic, parasympathetic, and enteric branches (Rao & Gershon, 2016). The first two branches (sympathetic and parasympathetic) have opposing actions and function to help a person maintain a balance of activation and recovery, known as homeostasis. The third branch (enteric) has a close relationship to the other two branches, though it is considered an independent system (Rao & Gershon, 2016).
During sympathetic activation, the sympathetic-adrenal-medullary axis and the HPA axis become activated, and stress hormones such as adrenaline and cortisol are released (Bear et al., 2015). Adrenaline functions to prepare a person for action by increasing heart rate, dilating the pupils, and slowing digestion. Cortisol assists with controlling blood sugar and blood pressure levels, regulates metabolism, and reduces inflammation. Hormone release involves structures from the CNS, PNS, and endocrine system (otherwise known as the body’s hormonal system, which is described later in this chapter). The process of sympathetic activation was described by Smith and Vale (2006) and is summarized here. In response to an event in a person’s environment, structures in the limbic region, in particular the amygdala, hippocampus, and thalamus, evaluate sensory information. These subcortical structures evaluate the sensory stimulus relative to prior knowledge and memories. If a threat is detected, a message is sent to the hypothalamus, which in turn triggers the release of corticotropin-releasing hormone. Corticotropin-releasing hormone sends a message to the pituitary gland, which releases adrenocorticotropic hormone (ACTH). ACTH is carried down to the adrenal cortex and medulla, which are located above the kidneys. ACTH stimulates the release of glucocorticoid hormones, such as cortisol, at the adrenal cortex. It can also activate the release of adrenaline (also called epinephrine) and noradrenaline (also called norepinephrine) by the adrenal medulla. Eventually, glucocorticoids such as cortisol provide feedback to the pituitary gland and hypothalamus to stop the release of cortisol. This is the stress response system’s negative feedback loop.
The release of these hormones prompts physiological activation in response to the detection of threat (Smith & Vale, 2006). Adrenaline and cortisol prepare a person for action, such as by increasing heart rate and saving energy by reducing the energy consumption of other organs such as the stomach and digestive system. The function of this activation is to prepare the person to take action to address the potential threat in the environment. Because the person needs to respond fairly quickly to the threat, the process of sympathetic activation can happen very quickly and does not necessarily involve the frontal lobe. Instead, subcortical systems, such as the limbic structures, are mostly active. This means that the process of initial sympathetic activation is more reflexive and less associated with higher order, rational, and conscious thinking (Field, Beeson, & Jones, 2015). If a person were to pause and process their options rather than immediately respond, the person may place their survival at risk (Field et al., 2015). Thus, the reflexivity of sympathetic activation is believed to have evolutionary origins. A person’s ability to prevent sympathetic activation and immediately engage parasympathetic recovery is limited, because the body responds automatically to perceived threats. Therefore, asking children to think before acting is often a tall order because their bodies are priming them for a quick, reflexive response to a threat in their environment (Field et al., 2015). See Reflection Question 2.1.
Children’s behavioral response to sympathetic activation can vary. They might demonstrate an approach response to the perceived threat and become agitated and aggressive. They may demonstrate an avoidance response to the perceived threat by withdrawing from interpersonal contact or leaving the situation. They may also demonstrate a motionless response and feel unable to verbalize thoughts and feelings. Böhnke, Bertsch, Kruk, and Naumann (2010) found that nonacute activation of the HPA axis and cortisol secretion provokes avoidance responses, whereas acute activation prompts aggressive responses. It is believed that acute HPA axis activation might activate the negative feedback loop and reduce cortisol secretion, enabling approach rather than avoidance responses (Montoya, Terburg, Bos, & van Honk, 2012).
Once a person’s sympathetic branch of the ANS has become activated, the person can eventually initiate recovery through parasympathetic activation, which results in reduced heart rate, increased digestion, and so forth. Actions associated with parasympathetic activation include engaging in diaphragmatic breathing and relaxing muscle tension. Research by Ma et al. (2017) found that diaphragmatic breathing activates parasympathetic recovery and reduces cortisol levels compared to a control condition. There is therefore research that supports the importance of children using deep, diaphragmatic breathing to activate the parasympathetic branch of the ANS. Note that parasympathetic activation can also occur when a person is in a relaxed state, not just when the person is recovering from sympathetic activation.
Because of these activating and recovery functions, neurons in the sympathetic branch of the ANS often transmit excitatory neurotransmitters (e.g., noradrenaline, which eventually triggers the release of adrenaline), whereas neurons in the parasympathetic branch of the ANS largely transmit inhibitory neurotransmitters (e.g., acetylcholine, GABA). Note that the neurotransmitter acetylcholine has an excitatory effect in the sensory nervous system yet an inhibitory effect on the ANS (i.e., it stimulates parasympathetic activation).
The ANS is always activated in either a sympathetic or a parasympathetic state. Situational variables and environments can activate either branch of the ANS, and thus the ANS is crucial to the body’s dynamic adaptation to the environment. For example, an elementaryage child might have minor sympathetic activation during school, especially when facing a mildly anxiety-provoking test like an exam, which helps with attention and concentration. The child might have parasympathetic activation when relaxing in the evening and at bedtime as they prepare for sleep. As the Yerkes-Dodson theory goes, a moderate and balanced amount of activation (e.g., cortisol secretion) is important for daily functioning (Yerkes & Dodson, 1908). It is when hormones such as cortisol are excreted at chronically high levels that it becomes problematic and causes dysfunction in systems of the brain and body. See Reflection Question 2.2.
The ANS has a third branch, the ENS. The ENS is a division of the ANS that functions to send and receive messages related to the gastrointestinal system. For example, the ENS regulates the secretion of gastric acid, the release of hormones in the gut, and changes in blood flow. The ENS also contains neurons related to afferent (sensory) and efferent (motor) functions and operates autonomously from the CNS. It does, however, have strong connections to the CNS and PNS, with nerve strands that communicate to and from the rest of the body, including the brain and spinal cord. One of these nerves is the vagus nerve (CN X), which connects the brain stem to the gastrointestinal tract. In addition to performing other functions, such as modulating heart rate, the vagus nerve regulates the contraction and stretching of the gut, known as gut motility. The ENS also has connections to the ANS, perhaps most obvious when one considers that the digestive system slows down to preserve energy during sympathetic activation. Knowledge of ENS function is perhaps lesser known compared to knowledge of the sensory nervous system and ANS.
The gastrointestinal tract contains trillions of microorganisms that are collectively called gut microbiota (also gut flora). The collective unit of gut microbiota is known as the gut microbiome. Most microbiota are located in the colon, with some microbiota located in the stomach. (Intense acids and enzymes of the stomach make it less hospitable to microbiota than the colon; Dieterich, Schink, & Zopf, 2018.) These gut microbiota function to metabolize foods and nutrients and maintain the immune system (Tremaroli & Bäckhed, 2012). Gut bacteria also care for cells within the gastrointestinal system, such as neurons and even glia in the intestines (Kabouridis et al., 2015). The relationship between microbiota and the brain is believed to be bidirectional. The brain sends messages to the microbiota regarding metabolism and the functioning of the immune system (e.g., absorption, secretion, blood flow). In addition, gastrointestinal functioning can impact brain functioning. For example, celiac disease can have impacts on cognition, such as impaired concentration and “brain fog” (Yelland, 2017, p. 90). The strong relationship between the CNS and ENS has been called the microbiota gut-brain axis (Mittal et al., 2017). Microbiota have an important role in producing metabolites during metabolism, such as fatty acids, which affect the release and modulation of neurotransmitters (Mittal et al., 2017). Neurotransmitters have a role in influencing the absorption of nutrients, activation of the immune system, and gut motility (Mittal et al., 2017). Neurotransmitters (e.g., acetylcholine, norepinephrine) are also intricately involved in the stress response system, which itself has a strong relationship to gut functioning.
Although most microbiota are important to our overall health, some microbiota are pathogenic and cause health problems. You are likely familiar with microbiota that cause illness, such as salmonella. In part because of the linkage between stress and gut function, there is emerging evidence of the role of gut microbiota in the development of mental health conditions (Kim, Yun, Oh, & Choi, 2018). Alzheimer’s disease, autism, depression, and Parkinson’s disease all have correlates with alterations in gut microbiota composition (Kim et al., 2018).
When a body fights an infection (i.e., a foreign microbial invasion of a bacterial pathogen such as a virus) or initiates repair of an injury to cells or tissue, white blood cells (sometimes called leukocytes or immune cells) are activated. There are several types of white blood cells. These cells take on several different functions to protect the organism against infection. The gastrointestinal system contains a large concentration of white blood cells and is a crucial part of immune response.
There are two forms of immune system response: innate and adaptive (or acquired) immunity. Innate immunity refers to the immune system’s identification of known pathogens such as bacteria and viruses (National Institute of Allergy and Infectious Diseases, 2014). Once a pathogen has been identified, white blood cells work to neutralize and eradicate it. Types of white blood cells associated with innate immunity include basophils, eosinophils, monocytes, and neutrophils. Basophils are active during allergic responses, when they release histamines that dilate blood vessels. This dilation enhances blood flow to the site of infection or injury, sometimes resulting in swelling. Monocytes become macrophages, which also engulf (i.e., eat) invading bacteria by phagocytosis. Like lymphocytes, monocytes also create cytokines (more about this later). Neutrophils are first responders to microbial infection. They also engulf invading bacteria, though they are less effective at phagocytosis than macrophages, because they are smaller cells and thus engulf fewer microorganisms. In addition to these cells, natural killer cells have a role in identifying and destroying virus-infected and tumorous cells through apoptosis. They form holes in target cells that result in cell death (National Institute of Allergy and Infectious Diseases, 2014).
Adaptive immunity is the body’s ability to respond to new and unknown pathogens and problems (National Institute of Allergy and Infectious Diseases, 2014). Adaptive immunity involves a type of white blood cell called a lymphocyte. B-cells and T-cells are types of lymphocytes that divide and multiply rapidly to address the identified infection. Once dispersed, B-cells make antibodies that neutralize pathogenic bacteria. T-cells create cytokines that help to coordinate immune system response. T-cells also recognize prior infectious bacteria and initiate an antibody response, thus serving as the memory of the immune system.
Vaccination trains the immune system to respond to a strain of pathogen. During vaccination, dead cells are entered into the body. This creates T-cell memory for the pathogen so that the body is more prepared for later infections by the same bacteria or virus.
The microbiota in the gut often have a role in the inflammation process when the immune system becomes activated (Dieterich et al., 2018). Inflammation is crucial to overall immune system functioning, as this process enhances blood flow to the site of infection or injury so that white blood cells can address the pathogen. Cytokines are proteins that have a crucial role in inflammatory responses, as they draw and recruit white blood cells to the site of infection or injury. Those white blood cells then engulf the pathogen, destroy degraded cells, and dilate blood vessels to increase the amount of white blood cells flowing into the area. Proinflammatory cytokines promote inflammation, whereas antiinflammatory cytokines inhibit inflammation. Gut microbiota can provoke pro- and antiinflammatory cytokines (Dieterich et al., 2018).
Acute inflammation is useful for controlling infection and repairing cells and tissue. Yet if inflammation persists for a prolonged period, it becomes harmful and will damage cells and tissue. Cytokines are able to cross the blood-brain barrier (a blood vessel lining that protects the brain from infection) and activate the HPA axis. In the brain, proinflammatory cytokines can alter neurotransmitter metabolism and reduce brain-derived neurotrophic factor and glutamate, resulting in fatigue and mood changes, including depression (Farooq, Asghar, Kanwal, & Zulqernain, 2017; Felger & Lotrich, 2013). High numbers of cytokines have been especially associated with manic states in bipolar disorder (Bai et al., 2014; Brietzke et al., 2009). It also appears that higher amounts of circulating cytokines have been associated with nonresponse to antidepressants (Fitzgerald et al., 2006). High numbers of circulating cytokines are also associated with physical health conditions such as cardiovascular disease and diabetes (Farooq et al., 2017).
The stress response system has an important bidirectional relationship with the immune system (Farooq et al., 2017). Cytokines can activate the HPA axis, resulting in the release of cortisol. In addition, high levels of cortisol can cause immunosuppression (i.e., reduce the responsiveness of the immune system) to reduce overactivation of the immune system and inflammatory damage to cells and tissue. Thus, the CNS, PNS, ENS, and endocrine systems are all intricately connected in their function and communication, especially during stress response.
To summarize, the ENS has an essential role in not only digestion and metabolism but also the immune system and stress response system. In subsequent chapters, we further explore the complex interrelationship between the CNS, PNS, ENS, and endocrine system, especially in relationship to the chronic stress response pathway and psychoneuroimmunology.
Earlier in the chapter, we mentioned the endocrine system when describing the stress response system. Although not a formal nervous system, the endocrine system has many linkages to the CNS and PNS, evidenced by descriptions of sympathetic activation. For example, structures of the subcortex (i.e., the CNS) send messages to the adrenal cortex to release hormones (i.e., the endocrine system) via the ANS division of the PNS. Because of the endocrine system’s importance to the stress response system and to sexual development and maturation (which we describe in subsequent chapters), it is worth briefly summarizing the function of the endocrine system here.
Figure 2.7 depicts components of the endocrine system. The endocrine system comprises the glands of the brain and body that release hormones to regulate the activity of cells and organs. Hormones have a role in energy metabolism and in the body’s growth and development. When multiple glands signal one another in sequence, they are referred to as an axis (e.g., the HPA axis).
There are several important glands in the endocrine system. The pituitary gland is located in the brain and is considered the master hormone gland of the brain and body, as it sends messages to several other glands to secrete hormones (Nicholls et al., 2011). For example, you may remember from earlier that the hypothalamus sends corticotropin-releasing hormone to the pituitary gland, which then secretes ACTH. When ACTH reaches the adrenal medulla, located above the kidneys, it releases adrenaline and cortisol. Adrenaline and cortisol are hormones that assist with the regulation of metabolism and blood pressure. Because adrenaline and cortisol are also stress hormones, they tend to raise blood pressure, elevate heart rate, and constrict metabolism when they are released when the body is prepared to respond to perceived threat.
FIGURE 2.7 The Endocrine System
Note. From OpenStax College, 2013. In Wikimedia Commons. Used under Creative Commons Share Alike Attribution 3.0 Unported License.
The thyroid gland is located in front of the neck. The pituitary gland releases thyroid-stimulating hormone to the thyroid gland, which in turn releases hormones such as thyroxine and triiodothyronine. These hormones control metabolism (i.e., how the body uses energy). The ovaries and testes are also considered glands, as they contain sex hormones (e.g., estrogen, progesterone, testosterone). These hormones are responsible for sexual development and are essential to reproduction. The ovaries excrete estrogen and progesterone, whereas the testes excrete testosterone. These hormones assist with the development of sex characteristics (e.g., breast development, pubic hair, and genital development) during childhood and adolescence. In addition, progesterone has a key role during pregnancy. The pituitary gland plays a key role in the secretion of these hormones. The hypothalamus sends gonadotropin-releasing hormone to the pituitary gland, which then excretes luteinizing hormone. This hormone stimulates ovulation in females and the production of estrogen and testosterone.
Like the pituitary gland, the pineal gland is located in the brain. The pineal gland produces melatonin, which is an essential hormone involved in the sleep cycle. Melatonin production is stimulated by darkness and inhibited by light. Melatonin is related to serotonin, which itself has a role in sleep function.
Finally, the pancreas is another crucial gland in the body that excretes glucagon and insulin hormones, which regulate blood sugar levels.
The brain and body’s nervous systems (i.e., the CNS, PNS, ENS) and endocrine (i.e., hormonal) systems are intricately connected in their function and communication. The brain and spinal cord of the CNS often direct the activities of the rest of the nervous system and receive messages about the body’s functioning from the PNS and ENS. The endocrine system has an important role in the stress response system and in maturation and development.
When working with children and adolescents, it can be helpful to understand the structure and function of their brain and body. Children’s behavior (both verbal and nonverbal) is often strongly impacted by their neurophysiological development. The information in this chapter provides a foundation for subsequent chapters about how and why children develop and which counseling approaches are helpful at various stages of development. You may find it helpful to return to this chapter as you explore further chapters to better understand the interconnected functioning between systems.