Taking a detailed look at the CNS, the brain, and the PNS
Getting down to a cellular level — neurons and neuroglial cells
Relaying impulses through a cell and across a synapse
Receiving impulses: your five senses
Noting a few nervous system disorders
An organism’s awareness of itself and of its environment depends on communication between one part of its body and another. In biology, such internal messaging is accomplished by several different mechanisms. Humans, like all mammals, use mechanisms involving chemistry and mechanisms involving electricity. We discuss the chemical messaging system (hormones) in Chapter 8. This chapter is devoted to the body’s electrical messaging system.
The nervous system is the body’s electrical communications network. It generates and transmits information throughout the body in the form of electrical impulses. An electrical charge creates electrical energy, which has two important characteristics: It moves in distinct “packets,” called impulses, and it moves very quickly.
The nervous system’s structures reach into every organ and participate one way or another in nearly every physiological reaction. Perceiving the beauty of a flying bird and digesting your breakfast can happen simultaneously, and each action is dependent on the nervous system.
The human nervous system is, in fact, the single most distinctive feature of the human species. In particular, the human brain, an organ of the nervous system, functions differently from the brain of any other species. Although just how this system contributes to human nature (consciousness, for example) is being actively researched, the goal here is understanding the mechanics.
Integrating the Input with the Output
The nervous system has just three jobs to do, and these jobs overlap.
Sensory input: Specialized neurons called sensory receptors collect information from the entire body, create an impulse, and transmit the impulse to either the spinal cord or brain stem, and then to the brain. (See the “Making Sense of Your Senses” section later in this chapter.)
Integration: The central nervous system (CNS) makes sense of the input it receives from all around the body. (See the “Central nervous system” section later in the chapter.)
Motor output: In response to the integration of the sensory input, impulses are sent out through the peripheral nervous system (PNS) to muscles, glands, and other organs capable of the appropriate response. (See the “Peripheral nervous system” section later in the chapter.)
The PNS is the route of communication between the brain and spinal cord — the central nervous system (CNS) — and the rest of the body. The sensory information coming in does so via the PNS just as the outgoing messages do. When discussing the PNS, important distinctions are most relevant to the motor output, but don’t forget that the classification does include sensory input.
Nervous tissues
Nervous tissue is made up primarily of two categories of cells — neurons and neuroglial cells. The structure and distribution of the cells differ throughout the different specialized tissues of the nervous system.
Neurons
A neuron is an individual cell and is the basic unit of the nervous system. Neurons are highly specialized for the initiation and transmission of electrical signals (impulses). The neuron is able to, in an instant, receive stimuli from many other cells, process this incoming information, and “decide” whether to generate its own signal to be passed on to other neurons, muscles, or gland cells.
Here are the three types of neurons:
Sensory neurons: Also called afferent neurons (afferent means “moving toward”), these neurons respond to sensory stimuli (touch, sound, light, and so on), passing the impulses ultimately to the spinal cord and brain.
Motor neurons: Also called efferent neurons (efferent means “moving away”), these transmit impulses from the brain and spinal cord to effector organs (muscles and glands), triggering responses from these organs (muscle contraction or release of the gland’s product).
Interneurons: Also called association neurons, these connect neurons to other neurons within the same region of the brain or spinal cord.
Neurons in different parts of the nervous system perform diverse functions and therefore vary in shape, size, and electrochemical properties. However, neurons have a special cellular anatomy adapted to the quick transmission of an electrical charge. See Figure 7-1. All neurons have the same three parts, all enclosed within their cell membrane:
Cell body: The body of a neuron is similar to a generic cell. It contains the nucleus, mitochondria, and other organelles.
Dendrites: The dendrites are extensions that branch from one end of the cell body. They receive information from other neurons and transmit stimulus toward the cell body.
Axon: Each neuron has a single cable-like projection called an axon. Extending many times (tens, hundreds, or even tens of thousands of times) the cell body’s diameter in length, the axon carries the impulses away from the cell body and toward the next neuron in the chain. (Think of electrical transmission wires.)
Illustration by Kathryn Born, MA
FIGURE 7-1: Motor neuron (a) and sensory neuron (b), structure and path of impulses.
Axon means “away.” You can remember this because they both start with a.
Fully differentiated neurons don’t typically divide and may live for years, or even the whole lifetime of an organism. The creation of new neurons is an active area of research.
The longest neuron in your body runs from the tip of your big toe to the base of your spinal cord. That’s a single cell around 3 feet long!
Neuroglial cells
The numerous cells of nervous tissue that aren’t neurons are collectively called glial cells (or neuroglia). These cells can communicate with each other, but they lack axons and dendrites and do not generate impulses. Though the ratio varies throughout the brain, glial cells outnumber neurons by at least 3:1.
Until recently, most neuroscientists thought glial cells were merely supporting structures, effectively gluing the neurons together (glia is Greek for “glue”). However, with new imaging technology and in vitro (outside of the body) lab techniques, we’ve learned that these cells do far more than that. Table 7-1 outlines these functions.
A nerve is a bundle of peripheral axons. An individual axon plus its myelin sheath is called a nerve fiber. Nerves provide a common pathway for the electrochemical nerve impulses that are transmitted along each of the axons. Nerves are found only in the peripheral nervous system. Nerve fibers can be of two types: motor, which send impulses away from the CNS, or sensory, which send impulses toward the CNS.
Ganglia and plexuses
A ganglion (plural, ganglia) is an aggregation of neuron cell bodies. Ganglia provide relay points among the body’s neurological structures, especially at the spinal cord, serving as the junction between the CNS and the PNS.
Ganglia may be connected to form a chain. For example, the sympathetic nervous system contains a chain of ganglia referred to as the paravertebral ganglia or the sympathetic chain of ganglia that spans the length of the spinal cord.
Plexus is a general term for a network of anatomical structures, such as lymphatic vessels, nerves, or veins. (The term comes from the Latin “plectere” meaning “to braid.”) A neural plexus is a network of intersecting nerves. The solar plexus serves the internal organs. The cervical plexus serves the head, neck, and shoulders. The brachial plexus serves the chest, shoulders, arms, and hands. The lumbar, sacral, and coccygeal plexuses serve the lower body.
Integrated Networks
The nervous system comprises two physically separate but functionally integrated networks of nervous tissue. Working together, these networks perceive and respond to internal and external stimuli to maintain homeostasis and simultaneously move the genetic development program forward. The following sections take a closer look at the central and peripheral nervous systems. Check out the “Nervous System” color plate in the middle of the book for a detailed look at the anatomy.
Central nervous system
The central nervous system (CNS), which consists of the brain and spinal cord, is the largest part of the nervous system. It integrates the information it receives from the sensory receptors and coordinates the activity of all parts of the body.
Both the brain and spinal cord are masses of neural tissue protected within bony structures (the skull and the vertebral column, respectively) and layers of membranes and specialized fluids, reflecting their prime importance to the continuation of the organism’s life.
The brain and spinal cord are made up mainly of two types of tissue, called gray matter and white matter. Gray matter consists of unmyelinated neurons, neuron cell bodies, and neuroglial cells. White matter is made up of neuroglial cells and the myelinated axons extending from the neuron cell bodies in the gray matter. (See the “Nervous Tissues” section earlier in the chapter for more on neurons and neuroglial cells.) The myelin has a high lipid content, which results in white matter’s white color.
In the brain, the gray matter forms a thin layer on the outside (the cortex). The white matter is beneath and makes up the brain’s big data lines, carrying information around the brain. In the spinal cord, the tissue is arranged in a long cylinder; the gray matter forms the inner layer, and the white matter forms the outer layer.
The spinal cord extends from the bottom of the brain stem down the vertebral column within a cylindrical tubular opening created by the vertebrae and three tough membranes with cushioning fluid between them (see Figure 7-2).
The three membranes that surround the spinal cord, collectively called the meninges, continue up to encase the brain. The outermost layer is the dura mater, which is followed by the arachnoid mater. Between these two layers flows a fluid much like interstitial fluid. The innermost membrane, the pia mater, contacts the nervous tissue. Between the arachnoid and pia flows a fluid unique to the CNS called cerebrospinal fluid (CSF).
Peripheral nervous system
The peripheral nervous system (PNS) consists of the nerves and ganglia outside of the brain and spinal cord. Unlike the CNS, the PNS isn’t protected by bone or by the blood-brain barrier, leaving it exposed to toxins and mechanical injuries. Structures of the PNS include:
Cranial nerves: Twelve pairs of nerves that emerge directly from the brain and brain stem. Each pair is dedicated to particular functions — some bring information from the sense organs to the brain, others control muscles, but most have both sensory and motor functions. Some are connected to glands or internal organs such as the heart and lungs. For example, the longest of the cranial nerves, called the vagus nerves, pass through the neck and chest into the abdomen. They relay sensory impulses from part of the ear, tongue, larynx, and pharynx; relay motor impulses to the vocal cords; and relay motor and secretory impulses to some abdominal and thoracic organs.
Spinal nerves: Thirty-one pairs of nerves that emerge from the spinal cord. Each contains thousands of afferent (sensory) and efferent (motor) fibers.
Sensory nerve fibers: Nerve fibers all over the body that send impulses to the CNS via the cranial nerves and spinal nerves.
Motor nerve fibers: Nerve fibers that connect to muscles and glands and send impulses from the CNS via the cranial and spinal nerves.
The PNS is further divided into the somatic system and the autonomic system.
Somatic system
The somatic nervous system regulates activities that are under conscious control. Its sensory fibers receive impulses from receptors. Its motor fibers transmit impulses from the CNS to the (voluntary) skeletal muscles to coordinate body movements.
Autonomic system
The motor fibers of the autonomic system transmit impulses from the CNS to the glands, heart, and smooth (involuntary) organ muscles. The autonomic system controls internal organ functions that are involuntary and that happen subconsciously, such as breathing, heartbeat, and digestion.
The autonomic system is made up of the following:
Sympathetic nervous system: Nerves originate in the thoracic and lumbar regions of the spinal cord. The sympathetic nervous system responds to stress and is responsible for the increase of your heartbeat and blood pressure, among other physiological changes, along with the sense of excitement you may feel due to the increase of adrenaline in your system. This system is responsible for your “fight or flight” response.
Parasympathetic nervous system: Nerves originate in the brain stem and sacral portion of the spinal cord. The parasympathetic nervous system is evident when you rest or feel relaxed and is responsible for such things as the constriction of the pupil, the slowing of the heart, the dilation of the blood vessels, and the stimulation of the digestive and urinary systems. The parasympathetic nervous system is known as the “housekeeping” system because it maintains your normal functioning when you’re not under stress.
Enteric nervous system: This system manages every aspect of digestion, from the esophagus to the stomach to the small intestine to the colon.
Thinking about Your Brain
The brain is one of the largest organs in the human body, third to the skin and liver. It makes up 3 percent of our body weight but uses about 20 percent of our energy, illustrating its importance. As the task master, the brain manages its workload by compartmentalizing its functions. The different parts of the brain are in constant contact with each other, influencing one another, but they’re responsible for different functions.
The brain’s major parts are the cerebrum, cerebellum, brain stem, and diencephalon. The brain’s four connecting, fluid-filled cavities are called ventricles. In this section, you find out some details about the parts of your brain and its ventricles. Take a look at Figure 7-3, and refer to it as necessary.
If you’re conscious, you’re using your cerebrum. The cerebrum, the brain’s largest part, controls consciousness among other advanced functions.
The cerebrum is divided into left and right halves, called the left and right cerebral hemispheres, and each half has four lobes: frontal, parietal, temporal, and occipital. The names of the lobes come from the skull bones that overlay them (see Chapter 5). Table 7-2 shows you what each lobe controls.
TABLE 7-2 Functions of Lobes within Cerebral Hemispheres
Lobe
Functions
Frontal lobe
Speech production, concentration, problem solving, planning, and voluntary muscle control
Parietal lobe
General interpretation area, understanding speech, ability to use words, and sensations including heat/cold, pressure, touch, and pain
Temporal lobe
Interpretation of sensations, remembering visually, remembering through sounds, hearing, and learning
Occipital lobe
Vision, recognizing objects visually, and combining images received visually with other senses
The cerebral cortex is the brain’s outer layer of gray matter. It covers the entire surface of the cerebrum and overlays the deeper white matter. The brain’s elevations are called gyri (singular gyrus). The shallow grooves that separate the elevations are called sulci (singular sulcus). Deep grooves in the brain are called fissures.
When you look at the top of a brain, you notice a deep groove running down the middle of the cerebrum. This groove is the longitudinal fissure, and it incompletely divides the cerebrum into the left and right hemispheres. The corpus callosum, located within the brain at the bottom of the longitudinal fissure, contains myelinated fibers that connect the left and right hemispheres.
Making your moves smooth: The cerebellum
The cerebellum lies just below the back half of the cerebrum. A narrow stalk (vermis) connects the cerebellum’s left and right hemispheres. The outside is gray matter, and the inside is white matter.
The cerebellum coordinates your skeletal muscle movements, making them smooth and graceful instead of stiff and jerky. The cerebellum also maintains normal muscle tone and posture, utilizing sensory information from the eyes, inner ear, and muscles.
Coming up roses: Your brain stem
The brain stem consists of the midbrain, pons, and medulla oblongata. The medulla oblongata is continuous with the spinal cord after it passes through the hole called the foramen magnum at the bottom of the skull (see Chapter 5 for more on the skeletal system).
Inside your brain, just in front of (anterior to) the cerebellum, lie the midbrain and pons. The midbrain serves as a “station” for information that passes between the spinal cord and the cerebrum or between the cerebrum and the cerebellum. Impulses pass through the midbrain, which has centers for reflexes based on vision, hearing, and touch. If you see, hear, or feel something that scares you, alarms you, or hurts you, your midbrain immediately responds by sending out impulses to generate the appropriate type of scream, jump, or exclamation.
Reflex arcs sometimes create immediate, unconscious responses. Reflex arcs happen unconsciously whenever you touch something really hot or sharp. Sensory neurons detect pain, temperature, pressure, and the like. If sensory neurons detect something that could harm your body, such as heat that may cause a burn or a sharp object that may puncture the skin, an impulse passes from the receptor in the skin through the sensory neuron to the spinal cord, and then to motor neurons that cause a muscle to contract and pull the body part at risk of injury away from the heat or sharp object.
Reflexes occur so fast that you don’t have time to think about how to react. By the time the impulse gets to your brain, the spinal cord has already taken care of the problem! In normal processes of the CNS, impulses travel to the brain for interpretation and production of the proper response. However, using the spinal cord rather than the brain to produce a response, reflex arcs save time and possibly damaging consequences.
If the midbrain is a station for impulses, the pons is the bridge that joins the cerebellum with the cerebrum’s left and right hemispheres, allowing the cerebrum to influence the cerebellum. Axon bundles fill the pons and respond quickly to information it receives through the eyes and ears. (See the “Neurons” section earlier in the chapter for more on axons.)
The medulla oblongata, which transitions into the spinal cord, is responsible for several important functions, such as your breathing, the beating of your heart, and the regulation of your blood pressure. The medulla oblongata also contains the axons that send out the signals for coughing, vomiting, sneezing, and swallowing, based on information it receives from the respiratory or digestive systems. And whenever you get those annoying hiccups, blame your medulla oblongata.
Regulating systems: The diencephalon
Right smack in the middle of the brain, the hypothalamus and the thalamus form the diencephalon. The hypothalamus regulates sleep, hunger, thirst, body temperature, blood pressure, and fluid level to maintain homeostasis. (Flip to Chapter 2 for an overview of homeostasis.)
The thalamus is the gateway to the cerebrum. Whenever a sensory impulse travels from somewhere in your body (except from the nose — sensations of smell are sent directly to the brain by the olfactory nerve), it passes through the thalamus. The thalamus then relays the impulse to the proper location in the cerebral cortex, which then interprets the message. Think of the thalamus as an email server, routing your message through the correct lines.
Following fluid through the ventricles
Each cerebral hemisphere contains a lateral ventricle (the first and second ventricles). The other two ventricles are, believe it or not, the third and fourth ventricles. (Recall that a ventricle is a connecting, fluid-filled cavity.) The third ventricle lies just about in the center of your brain; the fourth ventricle lies at the top of the brain stem. The cerebral aqueduct (also called the mesencephalic aqueduct) connects the third and fourth ventricles. From the inferior portion of the fourth ventricle, a narrow channel called the central canal continues down into the spinal cord.
When you think of an aqueduct, you may think of Rome. The Romans built aqueducts as a system for distributing water. Well, in your central nervous system, the ventricles and aqueducts serve as a system to circulate cerebrospinal fluid (CSF).
A clear fluid made in the brain, CSF is contained in the brain’s four ventricles, the subarachnoid space (the space between the arachnoid and the pia mater), and the central canal of the spinal cord. The CSF picks up waste products from the CNS cells and delivers them to the bloodstream for disposal. The CSF also cushions the CNS. Along with your skull and vertebrae, CSF adds a protective layer around your brain and spinal cord.
Perhaps the most important function of CSF is to keep the ions in balance and thus stabilize membrane potentials (more on that in the “Across the neuron” section). CSF circulates from the lateral ventricles to the third ventricle, through the cerebral aqueduct into the fourth ventricle, and then down through the central canal of the spinal cord. From the fourth ventricle, CSF oozes into the subarachnoid space just under the arachnoid membrane, which continuously covers the spinal cord and brain. In the subarachnoid space, CSF can seep through tiny spaces to get to the bloodstream.
In a procedure known as a spinal tap, CSF is drawn through a needle from the subarachnoid space. Doctors can test the CSF for the presence of bacteria that cause meningitis or for the presence of proteins that can indicate other diseases, such as Alzheimer’s.
Blood-brain barrier
Blood entering the CNS must pass through the blood-brain barrier, which restricts the entry of certain blood molecules into the CNS. The barrier consists of intercellular connections, called tight junctions, between the endothelial cells of the capillaries and the processes of the astrocytes that surround them. Endothelial cells restrict the diffusion of bacteria, including many common pathogens, thereby protecting the brain from infection. They also block large or hydrophilic molecules from entering the CSF, including some toxins and some drugs. The blood-brain barrier permits the diffusion of small hydrophobic molecules (oxygen, hormones, and carbon dioxide). Other molecules, such as glucose, are carried across the barrier by facilitated diffusion. This works in conjunction with the meninges to tightly control the contents of the fluid that puts it into contact with the nervous tissue of the spinal cord and brain.
Transmitting the Impulse
To move a message from one part of your environment (whether internal or external) to your brain or spinal cord, an impulse must pass through each neuron and continue on its path. Through a chain of chemical events, the dendrites receive the stimulus, which generates an impulse that travels through the cell to the end of an axon, where a neurotransmitter is released, generating an impulse in the next neuron. The entire impulse passes through a neuron in about seven milliseconds, faster than a lightning strike. We take a look at this lickety-split transmission in detail in the following sections. Refer to Figure 7-4 to visualize this process.
Illustration by Kathryn Born, MA
FIGURE 7-4: Impulse transmission: stimulus of dendrite (a), generation and spread of action potential (b), and recovery wave (c).
Across the neuron
When a neuron isn’t stimulated (when it’s “resting,” that is — just sitting with no impulse to transmit), its membrane is polarized: The electrical charge on the outside of the membrane is positive while the electrical charge on the inside is negative. The fluid outside the cell contains excess Na+ (sodium ions); the cytoplasm contains excess K+ (potassium ions). This gradient is maintained by Na+/K+ pumps on the membrane. When the neuron is inactive and polarized, it’s said to be at its resting potential, which is about –70mV.
In addition to the K+, negatively charged protein and nucleic acid molecules also inhabit the cell; therefore, the inside is negative relative to the outside.
When a stimulus reaches a dendrite, it open ions channels in the cell membrane. The stimulus could be a neurotransmitter from another neuron or it could be an initiating stimulus on a receptor (for example, temperature increase on a thermoreceptor). When the ion channels open, positive ions (usually Na+) rush in; they’re attracted to the negative charge inside the cell. The ions move through the cytoplasm toward the trigger zone. In motor neurons, this is where the axon meets the cell body. In sensory neurons, this is where the singular axon meets up with the dendrites (refer to Figure 7-1).
The occurrence of a stimulus does not mean the neuron will generate an impulse. Neurons can only “fire” (create an impulse) or not. For example, the neuron attached to a thermoreceptor can’t say “I’m a little warm” versus “I’m burning up!” It either says “Yes, I’m hot” or nothing at all. As a result, the influx of ions must create enough of a voltage change to warrant a response being sent. This is the role of the trigger zone; it determines if the threshold stimulus has been met. For most neurons, this is –55mV. If a neuron is stimulated, and positive ions move in increasing the charge to –60mV, then nothing is going to happen. The cell will pump the ions back out to reestablish its resting potential (–70mV). However, the bigger the stimulus (greater force or temperature change, for example), the greater number of ion channels open. Even more positive ions rush in. When the voltage increase hits –55mV at the trigger zone, the axon begins its task of generating impulse.
Neurons can’t send messages of value to the brain. That information comes from the number of neurons sending the same message.
There are two types of ion channels found along the axon: one for sodium (Na+) and the other for potassium (K+). Both of these are voltage gated, meaning a specific electrical charge must be reached and then they will pop right open. For the sodium channels, this is –55mV. However, this voltage change is localized so doors only open in the first segment of the axon (see Figure 7-4b). Na+ ions rush in, depolarizing that segment (making it more positive). Enough ions enter the axon to increase the charge to about 30mV, and because ions can flow down the axon (toward the synaptic knob) the next section quickly reaches –55mV and opens more doors. Each time a set of sodium channels opens on the axon is referred to as an action potential. These occur one right after the other until the synaptic knob is reached (see Figure 7-4c), which is the definition of the term impulse.
So the impulse spreading down the axon is really several action potentials, each one triggering the next. As such, a neuron can’t directly pass impulse to another neuron. When the final action potential reaches the synaptic knob, a slightly different process will trigger the release of neurotransmitters. More on that in the “Across the synapse” section because we’re not done with the axon.
We spread the impulse to the end, which is great because that was our goal (and we even did it without the input of energy). However, it’d be nice if we could use that neuron again in the very near future. (Remember: We’re talking milliseconds here.) Unfortunately, we have a problem. All those Na+ ions have depolarized our cell; we have to get them out. Okay, let’s open those doors again and let them leave. Nope. As long as the doors are open, Na+ will move in because it wasn’t just the charge difference attracting them; there’s also a concentration gradient (see Chapter 3). This is where the potassium ions come in handy.
When the charge reaches 30mV in an axon segment, the sodium doors are snapped shut and the potassium ones open. K+ ions rush out, taking their positive charge with them. This repolarizes the membrane. When resting potential (–70mV) is reestablished, the potassium channels close. We still have a problem, though: Our ions are in the wrong place. At this point, the cell will use energy to pump Na+ back out and K+ back in. Only then will the neuron be capable of firing again. The time it takes for this recovery is known as the neuron’s refractory period.
Many neurons are myelinated, having axons wrapped in insulating lipids created by glial cells: oligodendrocytes in the CNS and Schwann cells in the PNS. (The neurons illustrated in Figure 7-1 are both myelinated.) The myelin is said to speed up impulse transmission. What it does is allow the Na+ to be shuttled further down the axon before generating the next action potential. This decreases the number of action potentials required to travel the length of the axon, thereby decreasing the time of impulse transmission.
Across the synapse
Most neurons don’t touch. A gap called a synapse or synaptic cleft separates the axon of one neuron from the dendrite of the next. To traverse this gap, a neuron releases a chemical called a neurotransmitter that may or may not cause the next neuron to generate an electrical impulse.
When the impulse reaches the synaptic knob, it again triggers ion channels to open. Only this time, calcium ions rush in. The Ca2+ that enter effectively push the packets (vesicles) of neurotransmitters toward the membrane. The neurotransmitters are then released into the synapse where they can bind with any dendrites in the area that have the matching receptor. Figure 7-5 illustrates this process.
Each type of neurotransmitter has its own type of receptor. Whether the postsynaptic (receiving) neuron is excited or inhibited depends on the neurotransmitter and its effect. For example, if the neurotransmitter is excitatory, the Na+ channels open, the neuron membrane becomes depolarized, and the impulse is carried through that neuron. If the neurotransmitter is inhibitory, the K+ channels open, the neuron membrane becomes hyperpolarized as the ions exit, and any ongoing impulse is stopped.
After the neurotransmitter produces its effect (excitation or inhibition), the receptor releases it, and the neurotransmitter goes back into the synapse. In the synapse, one of three things happens to the neurotransmitter. It may
Be broken down by an enzyme
Be taken back up by the cell that released it (a form of recycling)
Simply float away into the surrounding fluid
Making Sense of Your Senses
Some anatomists consider the sensory system to be part of the peripheral nervous system. Others regard it as a separate system. Either way, there are as many as 21 different senses (depending on how they’re grouped). The senses all utilize varieties of five categories of receptor. See Table 7-3 for their description.
Found in skin; separate receptors for warm and cold
Photoreceptor
Light
Rods and cones in retina of eye
Mechanoreceptor
Physical force
Touch receptors, blood pressure, stretch (in bladder and lungs)
You may be saying to yourself, “Wait, I thought we had five senses! Have I been taught wrong my whole life?” The answer is: Not really. The five senses you’re familiar with — touch, hearing, sight, smell, and taste — are your perceived senses. That is, you’re aware that your brain has interpreted the sensory input. You say “Ouch, that’s hot!” or “Wow that smells nice!” The other sensory input is, of course, processed, but that occurs outside of your conscious thought. Otherwise, you’d be constantly bombarded with thoughts about pH and blood pressure monitoring. Read on to learn about how your five perceived senses work.
Touch
The sensory receptors all over the skin perceive at least five different types of sensation: pain, heat, cold, touch, and pressure. The five are usually grouped together as the single sense of touch.
Receptors vary in terms of their overall abundance (pain receptors are far more numerous than cold receptors) and their distribution over the body’s surface (the fingertips have far more touch receptors than the skin of the back). Areas where the touch sensors are packed in are especially sensitive.
The structure of the sensory receptors varies with their function. There are free nerve endings (dendrites) that are responsible for itching and pain. Both the cold and warm thermoreceptors are free nerve endings as well. Other receptors are modified, where the dendrites are wrapped in connective tissue fibers forming mechanoreceptors. There are two varieties of these found in your skin. Meissner’s (or tactile) corpuscles respond to light touch and Pacinian (or lamellated) corpuscles respond to a heavier, more forceful touch (pressure).
Nerve fibers that are attached to different types of skin receptors either continue to discharge during a stimulus or respond only when the stimulus starts and, sometimes, when it ends. That’s why you’re aware of your shoes on your feet when you first put them on, but the stimulus fades within a minute or two. In other words, slowly adapting nerve fibers send information about ongoing stimulation; rapidly adapting nerve fibers send information related to changing stimuli.
Hearing and balance
The ear changes sound waves into nerve impulses sent to the brain (see Figure 7-6). Sound moves through air in waves of pressure. Your outer ear acts as a funnel to channel sound waves to the eardrum, causing the tympanic membrane (eardrum) to vibrate. (The outer ear isn’t necessary for hearing, but it helps.) The ear bones, called ossicles, receive and amplify the vibration and transmit it to the inner ear. The vibrations create tiny ripples in the inner ear’s fluid. The hollow channels of the inner ear’s cochlea are filled with liquid, and it is lined with sensory epithelium studded with hair cells — mechanoreceptors that release a neurotransmitter when stimulated.
The nerve impulses travel from the left and right ears through the eighth cranial nerve to both sides of the brain stem and up to the temporal lobe — the portion of the cerebral cortex dedicated to sound.
Your inner ears also transmit information to your brain regarding what position your head is in; that is, whether you’re horizontal or vertical, spinning or still, moving forward or backward. Therefore, your ears are the key organ of balance. The process of transmitting information to the brain about your body position is basically the same as that of hearing. When you’re moving, the inner ear’s fluid moves and causes hair cells in the semicircular canals to bend, sending impulses to the brain. Your brain then processes the information about where you are spatially and initiates movements to help you keep your balance.
Sight
Vision is probably the most complex of the senses. Your eye’s pupil, the dot in the center of your eye that’s usually black in color, allows light in. The iris, the pretty, colored part of your eye, contains smooth muscle that controls the pupil’s size and thus the amount of light that enters the eye. The iris muscle contracts to dilate the pupil and allow more light in, such as when you’re in a dark room or outside at night. The cornea lies anterior to the iris and pupil and merges with the sclera, which is the outer wall of the eye. (You can kind of see a clear area if you look at an eyeball from the side.) Both are covered by the conjunctiva, a mucus-like membrane. The lens is behind the iris and pupil (see Figure 7-7). Both the cornea and the lens help to bend light rays to aid in focusing.
A clear, gelatinous material fills the vitreous body, which lies behind the lens. And we’re not joking when we tell you that the gelatinous material is vitreous humor. The transparent vitreous humor gives the eyeball its rounded shape, and it also lets light pass through it to the back of the eyeball.
The retina is the innermost layer of the eyeball. The retina contains two types of photoreceptors — rods, which detect dim light and are sensitive to motion, and cones, which detect color and fine details. Three types of cones detect color — one each for detecting red, blue, and green. A missing or damaged cone (regardless of the type) results in color blindness. The macula is the region of the retina with a high concentration of cones, providing the sharpest vision.
When light strikes the rods and cones, nerve impulses are generated and sent to cells that form the optic nerve. The optic nerve joins your eyeball directly to your brain and sends impulses to the brain for interpretation in the occipital lobe.
The optic disc is where the nerve fibers from the retina merge into the optic nerve. Thus, at this point on the retina, there are no photoreceptors — giving you a blind spot in each eye.
Olfaction
The nose knows, but the olfactory cells know better. The nose is the sense organ for smell (olfaction is the proper term): The olfactory cells that line the top of your nasal cavity detect molecules in the air as you breathe. As you take in air through your nostrils, those molecules waft right up to your olfactory cells, where the chemicals bind to the cilia-like “hairs” that line your nasal cavity. That action initiates a nerve impulse that’s sent through the olfactory cell, into the olfactory nerve fiber, up to the olfactory bulb, and right to your brain. (The olfactory bulb is the expanded area at the end of the olfactory tract where the olfactory nerve fibers enter the brain.) The brain then “knows” what the chemical odors are from, and you know what you’re smelling.
Taste
Your sense of taste (gustation) has a simple goal: to help you decide whether to swallow or spit out whatever is in your mouth. This extremely important decision can be made based on a few taste qualities. The tongue, the sense organ for taste, has chemoreceptors to detect certain aspects of the chemistry of food, certain minerals, and some toxins, especially poisons made by plants to deter predation by animals.
The human tongue has about 10,000 taste buds, each with between 50 to 150 chemoreceptor cells.
A taste bud’s gustatory cells generate nerve impulses that are transmitted through the sensory nerve fiber to the gustatory areas of the brain via the seventh, ninth, and tenth cranial nerves. The brain then interprets the impulse and causes the release of the digestive enzymes needed to break down that food. So the sense of taste is tied to the endocrine system as well as to the digestive system.
Taste buds have receptors for sweet, sour, bitter, and salty sensations, as well as a fifth sensation called umami (“savoriness” — the flavor associated with meat, mushrooms, and many other protein-rich foods). Salty and sour detection is needed to control salt and acid balance. Bitter detection warns of foods that may contain poisons — many of the poisonous compounds that plants produce for defense are bitter. Sweet detection provides a guide to calorie-rich foods, and umami detection to protein-rich foods.
Each receptor cell in a taste bud responds best to one of the five basic tastes. A receptor can respond to the other tastes, but it responds strongest to a particular taste. And the taste buds detect only the rather unsubtle aspects of flavor. The nose detects more complex and subtler flavors.
The structure and function of taste buds is an area of active research and controversy, especially in the food industry. Even the exact number of basic tastes and their characterization is controversial, with new “basic tastes” being proposed from time to time. Regardless, the majority of what we perceive as flavor (good or bad) comes from olfactory input rather than gustatory.
Pathophysiology of the Nervous System
The discussion in this section is limited to anatomical disorders of nervous tissue affecting the “lower level” functions. Major brain disorders and physiology-based psychopathologies such as schizophrenia are beyond the scope of this book.
Chronic pain syndrome
Chronic pain is a disorder of the nervous system as much as it is a disorder of the organ causing the pain. Chronic pain has many sources (infection, trauma, and so on), but to be perceptible, the pain signal must be transmitted to the spinal cord and up to pain receptors in the brain. Analgesics (pain meds) work by blocking these receptors. Another approach that has proved effective in relieving chronic pain involves using a device to generate electrical impulses directly to the spinal cord to interrupt or cancel out the incoming pain impulses at the point where the spinal nerve transmitting the pain arrives at the spinal cord.
Multiple sclerosis
Multiple sclerosis (MS) affects the myelin sheath that covers myelinated axons. Lesions in the myelin sheath become inflamed and irritated. When the lesions heal, scar tissue (sclerosis) on the sheath interferes with the transmission of impulses through the axon, thus blocking movement or response in the innervated muscle. As the disease progresses, movement becomes increasingly difficult.
Macular degeneration
Macular degeneration is a vision disorder that’s now a leading cause of blindness among older individuals. In macular degeneration, the macula lutea — a small area of the retina with a large concentration of cone photoreceptors that detect color and fine details — weakens and degenerates. Objects look smaller or larger than they really are, and colors appear faded.
One cause of macular degeneration is overgrowth of new blood vessels around the macula lutea. New growth sounds healthy, but often it’s not. The new vessels leak, and as they ooze blood into the macula lutea, its delicate photoreceptors are destroyed. Macular degeneration can also result from excessive sun exposure, especially among people with blue or green eyes.
Summarizing the functions of the nervous system
The PNS is the route of communication between the brain and spinal cord — the central nervous system (CNS) — and the rest of the body. The sensory information coming in does so via the PNS just as the outgoing messages do. When discussing the PNS, important distinctions are most relevant to the motor output, but don’t forget that the classification does include sensory input.
Axon means “away.” You can remember this because they both start with a.
In addition to the K+, negatively charged protein and nucleic acid molecules also inhabit the cell; therefore, the inside is negative relative to the outside.
