Appendix G

A-34

8. First, what is the absolute refractory peri-od? Recall that it is the time during whicha neuron cannot fire an action potential nomatter how strong the stimulus. This isdue to all the voltage-gated Na + channelsalready being open. Thus, if the axon isnonresponsive for 1 ms, action potentialscan be generated no faster than everymillisecond. Because there are 1000 msin 1 second, the maximal frequency is1000 action potentials/second.9. The question asks us to compare signaltransmission rates of electrical and chemi-cal synapses. The first thing we need to dois consider the mechanism for each typeof synapse. Electrical synapses requireonly ion flow from one neuron to the next.Chemical synapses rely on diffusion of theneurotransmitter across the synaptic cleft,binding to the receptor and then triggering asignal transduction pathway. Thus, electricalsynapses take no more time to be transmit-ted than an individual action potential takestraveling from node to node in a myelinatedaxon, or down the axon in an unmyelin-ated axon. On the other hand, the chemicalsynapse is more complex and reliant onmolecular conformation change, enzymeactivity, and so on, all of which are slowerprocesses.10. The key piece of information in the questionis the fact that neuron B releases both a neu-rotransmitter and a neuromodulator, whichis excitatory (produces EPSPs). EPSPsdepolarize neuron C and bring it closer tothreshold than it is when stimulated by onlyneuron A. Neuron C does not require asmuch time for temporal summation toreach threshold to fire an action potentialwhen stimulated by neuron B vs. neuron A.Therefore, neuron C fires more action poten-tials with stimulation by neuron B alone thanwith temporal summation of neuron A alone.In other words, the neurotransmitter fromneuron B is more effective when releasedwith the neuromodulator from neuron Bthan it is when released alone from neuron A.

Chapter 12

2. In chapter 11, we learned that a ganglion isa cluster of neuron cell bodies. Similarly, inthis chapter we learned that the dorsal rootganglia are clusters of sensory cell bodiesand the dorsal roots are bundles of sensoryaxons. The ganglia are larger in diameterthan the roots because of the size differencebetween the cell bodies and axons. To answerthe second question, refer to figure 12.4 tosee the direction of action potential propa-gation in sensory axons and motor axons.Sensory axons carry action potentials from

peripheral tissues to the central nervoussystem (CNS), which includes the brainand spinal cord. Motor axons carry actionpotentials from the CNS to peripheral tissues.Now, identify the types of axons in eachstructure listed. Recall that spinal nerveshave both sensory and motor axons, soaction potentials are propagated both to thespinal cord and away from the spinal cord.Dorsal roots contain only sensory axons, soaction potentials are conducted to the spinalcord only. Finally, ventral roots containonly motor axons, so action potentials areconducted away from the spinal cord.3. The body areas in figure 12.14 is color-coded and labeled for the nerve innervation.To answer this question, look at the leftarm, forearm, and hand to see which nervesare indicated. In the figure, it is apparentthat nerves C5–T1 are damaged. C5 and T1innervate the left arm, C6 and T1 innervate theforearm, and C7 and C8 innervate the hand.4. Recall that the phrenic nerve innervates thediaphragm, allowing for the contractionnecessary for breathing. If the right phrenicnerve were damaged, then we would expectlack of muscle contraction in the right halfof the diaphragm, affecting breathing. Toanswer the second part of the question, weneed to consider the location of spinal cordinjury to predict the effect it would have onthe diaphragm. Remember that the phrenicnerve is part of the cervical plexus, whichincludes spinal nerves C1–C4. If the spinalcord were severed at the level of C2, thephrenic nerve would be damaged, and thecontractions of the diaphragm would notoccur, eliminating the person’s ability tobreath. Death would likely occur if medicalassistance were not administered quickly.On the other hand, if the spinal cord werecompletely severed at the level of C6, thephrenic nerve would not be damaged, andthe diaphragm would not be affected.5. a. Looking at figure 12.14, we can see thatnerves C7, C8, and T1 innervate the skin ofthe hand. Also after reviewing the descrip-tion of the brachial plexus and viewingfigures 12.19, 12.21, and 12.22, we canconclude that the radial, ulnar, and mediannerves innervate the skin of the hand.b. Figures 12.19 and 12.21 indicate thatthe symptoms of pain, tingling, andnumbness in the ring finger and little fin-ger of the right hand that radiated downthe posteromedial portion of the forearmand hand most likely involve damage tothe ulnar nerve, althoughthis does not explain the radiation ofsymptoms into the forearm and elbow.c. When the physician carried out a carefulexamination of Sarah’s right upper limb to

map the extent of the pain and numbness,he most likely was trying to develop adifferential diagnosis between the ulnarnerve damage and damage to the C8-T1brachial plexus root. Cervical ribs com-press the roots of the brachial plexus,which might be the cause of Sarah’ssymptoms. An x-ray of the neck can indi-cate the presence of extra rib, which mostcommonly affects the roots C8–T1.d. Based on our answers for parts a andb, we can assume that the ulnar nerveis most likely affected, so the musclesinnervated by the ulnar nerve may beaffected. These muscles are listed infigure 12.21. However, the radial andmedian nerves are also associated withthe C8–T1 root and may be affected;therefore, the muscles innervated by thesemight also be affected. These muscles arelisted in figures 12.19 and 12.22.6. Figure 12.25 indicates that the femoralnerve innervates several muscles involvedin hip flexion and knee extension, bothactivities that were difficult for Carl. Thisindicates that the femoral nerve is involved.The sources of nerve fibers in the femoralnerve are L2, L3, and L4; therefore, theintervertebral disk involved compresses L2,L3, or L4 on the left side of the vertebralcolumn. Recall that figure 12.14 includes adermatomal map. We can see from the mapthat L3 innervates the dermatome of themedial thigh and is the most likely spinalnerve involved. Carl’s motor movementswere affected because the reduced controlof action potentials from the femoral nerveto the muscles of the thigh caused muscleweakness. The referred pain results fromcompression of the spinal nerve that inner-vates the medial thigh and the knee. Thecompression stimulates action potentials inthe nerves, and the pain is referred to thesite of the sensory receptors for that nerve.

Chapter 13

2. We learned in this chapter that reflexes thatmaintain blood pressure are integrated bythe medulla oblongata. In response to bloodloss, the reflexes increase the heart rate.Similarly, the reflexes cause the constric-tion of blood vessels in the skin and viscerato increase blood volume and thereforeblood pressure. The lack of blood flowthrough the skin results in pallor. Recallthat respiratory reflexes are integrated inthe medulla oblongata and the pons.3. a. The Clinical Impact, “Traumatic BrainInjuries and Hematomas,” explains that,often, when one part of the head suffersa heavy blow, the brain moves within