A&P Chapter 13

How do a nerve and a neuron differ? Which would most affect motor or sensory function: damage to a neuron or damage to a nerve? Explain.

A nerve is a group of axons and small blood vessels surrounded by connective tissue. Damage to a single motor or sensory neuron would produce far less functional impairment than damage to an entire nerve, which contains multiple sensory and motor neurons.

Which cranial nerves are sensory only, primarily motor, and mixed?

CN I, II, and VIII are sensory; CN III, IV, VI, XI, and XII are motor; CN V, VII, IX, and X are mixed.

The sensory division detects sensory stimuli and delivers them to the __________.

CNS

To which division of the PNS would damage be most life-threatening? Explain.

Damage to the visceral sensory or motor division would be the most life-threatening, because these divisions innervate vital organs such as the heart, blood vessels, and lungs and maintain homeostasis.

Why is it advantageous to our survival that our thermoreceptors and nociceptors are slowly adapting?

Extremes in temperature and painful stimuli can cause serious homeostatic imbalances. If our thermoreceptors and nociceptors were rapidly adapting, we would not be adequately sensitive to these ongoing stimuli.

Predict the effect of damage to proprioceptors and sensory neurons that communicate with the cerebellum.

Information from proprioceptors allows the cerebellum to detect and correct motor error. Damaged proprioceptors (or their sensory neurons) would not effectively communicate motor error to the cerebellum, leading to dyskinesia (difficulty executing smooth movements).

Explain how the arrangement of axons within a nerve resembles the arrangement of muscle fibers within a skeletal muscle.

Like a muscle, a nerve consists of multiple cells bound together in fascicles by connective tissue sheaths. The fascicles, in turn, are surrounded and held together by another connective tissue sheath.

Predict what might happen if a disease caused the axons of the first-order somatic sensory neurons that transmit proprioceptive stimuli to lose their myelin sheaths.

Loss of their myelin sheaths would cause these neurons to conduct action potentials much more slowly than normal. This would delay the arrival at the cerebellum of critical feedback on motor error.

Explain why a lower motor neuron disorder leads to absent or weakened simple stretch reflexes but an upper motor neuron disorder does not.

Lower motor neurons directly trigger muscle contraction, so if they are not functioning, muscles will not contract after the stretch stimulus. Upper motor neurons do not directly contact muscle, so lower motor neurons may continue to stimulate muscles to contract when the stretch receptors are triggered.

Why is visceral pain often perceived as cutaneous pain?

Many spinal nerves carry both somatic and visceral neurons, so visceral sensations travel along the same pathways as somatic sensations. This causes referred pain.

Which types of mechanoreceptors allow one to read Braille (raised dots that enable a blind person to read text)? (Hint: What receptors allow for fine, discriminative touch?)

Reading Braille relies largely on Merkel cell fibers, as they have the finest spatial resolution and are numerous in the fingertips.

Drugs that stimulate the parasympathetic nervous system often lead to the adverse effect of excessive drooling. Which cranial nerve(s) is/are involved in this effect? What would happen if the activity of these nerves was instead inhibited?

The cranial nerves that innervate the salivary glands are CN VII and CN IX. If the activity of these nerves is inhibited, less saliva will be produced, and the individual will experience a dry mouth.

Mr. Pratchett has damage to his facial nerve, but not his trigeminal nerve. What would happen to his blink reflex? Explain.

The damage to the facial nerve would impair the motor arm of the corneal blink reflex, so the eyelid wouldn't close even though the stimulus would be detected.

An experimental drug blocks the body's muscle spindles from detecting stretch. Predict the effects of such a drug on movement, posture, and balance.

The feedback from muscle spindles helps ensure that movement is fluid and prevents damage from excessive stretching of a muscle. Inhibition of muscle spindles would result in jerky movements, difficulty maintaining proper posture and balance, and potential muscle damage from excessive stretching.

Explain why damage to the spinal cord around the level of C4 or higher generally leaves a patient unable to breathe without the assistance of mechanical ventilation.

The phrenic nerve, which innervates the diaphragm and so is the main nerve that drives breathing, is formed of axons from C3, C4, and C5. Damage at or above the level of C4 would render this nerve unable to function.

Which two cranial nerves mediate the gag reflex?

The sensory arm is mediated by the glossopharyngeal nerve, and the motor arm is mediated by the vagus nerve.

How do upper and lower motor neurons differ?

Upper motor neurons are located in the motor areas of the cerebral cortex; they do not directly contact skeletal muscle fibers. They are involved in making the "decision" to move, as well as planning and monitoring movement. Lower motor neurons directly stimulate skeletal muscle fibers to contract, but they are not involved in the planning of movement.

List and describe the basic steps involved in producing movement, beginning with the upper motor neurons in the cerebral cortex.

Upper motor neurons select a motor program. They stimulate the caudate nucleus and putamen, which inhibit the globus pallidus. This allows the thalamus to stimulate the upper motor neurons, which in turn stimulate lower motor neurons. The lower motor neurons then stimulate muscle fibers to contract.

Place the following sequence of events for the detection of somatic sensation in the proper order. Place a 1 by the first event, a 2 by the second event, and so on. a. ____ The central process transmits the action potential to a second-order sensory neuron in the CNS. b. ____ The action potential is transferred to the central process in the posterior root ganglion. c. ____ The stimulus triggers an action potential. d. ____ The signal is transferred to other CNS sensory neurons for eventual perception and interpretation. e. ____ The action potential is propagated along the peripheral process of the neuron.

a. 4 b. 3 c. 1 d. 5 e. 2

Often when the brain is damaged, the cranial nerves are damaged as well. A simple physical exam can be used to test clinically for the function of each cranial nerve. Damage to which cranial nerve or nerves would lead to the following findings? Explain your answer. a. Complete loss of taste sensation b. Inability to move the tongue c. Inability to move the eyes in any direction d. Loss of balance and equilibrium e. Inability to close the jaw

a. CN VII, CN IX, and CN X detect taste. b. CN XII provides motor innervation to most muscles of the tongue. c. CN III, CN IV, and CN VI provide motor innervation to the extrinsic eye muscles that move the eyes. d. CN VIII, specifically the vestibular portion, is responsible for balance and equilibrium. e. CN V innervates the muscles of mastication that close the jaw.

The lower motor neurons that innervate contractile skeletal muscle fibers are called: a. a-motor neurons. b. b-motor neurons. c. upper motor neurons. d. g-motor neurons.

a. a-motor neurons.

Fill in the blanks: A spinal nerve divides into a(n) ________________ that serves the anterior side of the body and the limbs and a(n) __________ that serves the posterior side of the body.

anterior ramus, posterior ramus

Merkel cell fibers, tactile corpuscles, Ruffini endings, and lamellated corpuscles are all types of: a. nociceptors. b. mechanoreceptors. c. photoreceptors. d. chemoreceptors.

b. mechanoreceptors.

Which of the following is the correct order of events of a reflex arc? a. Stimulus detection and delivery S motor response S integration in the CNS b. Motor response S stimulus detection and delivery S integration in the CNS c. Stimulus detection and delivery S integration in the CNS S delivery of motor response d. Integration in the CNS S motor response S stimulus detection and delivery

c. Stimulus detection and delivery S integration in the CNS S delivery of motor response

A receptor potential: a. always leads to an action potential. b. never leads to an action potential. c. causes hyperpolarization of the neuron. d. leads to an action potential if the stimulus is strong enough.

d. leads to an action potential if the stimulus is strong enough

A _________ consists of the axons of many neurons, multiple connective tissue sheaths, and blood vessels.

nerve

The _________ are the main organs of the PNS.

peripheral nerves

The __________________________________ performs the sensory and motor functions of the nervous system and links the CNS with the rest of the body and its external environment.

peripheral nervous system (PNS)

The PNS is classified functionally into the ___________ and the __________.

sensory (afferent) division and the motor (efferent) division.

First, write the Roman numeral that corresponds to each named cranial nerve (after the abbreviation CN). Second, match the cranial nerve with its correct function from the column on the right. CN ____Vestibulocochlear nerve CN ____ Trigeminal nerve CN ____ Hypoglossal nerve CN ____ Abducens nerve CN ____ Vagus nerve CN ____ Olfactory nerve CN ____ Accessory nerve CN ____ Oculomotor nerve CN ____ Facial nerve CN ____ Optic nerve CN ____ Glossopharyngeal nerve CN ____ Trochlear nerve a. Motor to the lateral rectus muscle b. Motor to the muscles of facial expression; lacrimation; salivation; taste to the anterior two-thirds of the tongue c. Sense of smell d. Motor to the muscles for swallowing; salivation; taste to the posterior one-third of the tongue; somatic sensation from the throat e. Senses of hearing and equilibrium f. Motor to the superior oblique muscle g. Motor to the tongue h. Motor to the sternocleidomastoid and trapezius muscles i. Sense of vision j. Motor to muscles of swallowing and speaking; parasympathetic innervation to thoracic and abdominal viscera; sense of taste from the throat k. Sensory to the face; motor to the muscles of mastication l. Motor to four of six extrinsic eye muscles; constricts the pupil; changes the shape of the lens; opens the eyelid

CN VIII Vestibulocochlear nerve e CN V Trigeminal nerve k CN XII Hypoglossal nerve g CN VI Abducens nerve a CN X Vagus nerve j CN I Olfactory nerve c CN XI Accessory nerve h CN III Oculomotor nerve l CN VII Facial nerve b CN II Optic nerve i CN IX Glossopharyngeal nerve d CN IV Trochlear nerve f

Fill in the blanks: _________ detect the degree to which a muscle is stretched, whereas _________ detect the force of a muscle contraction.

Muscle spindles, Golgi tendon organs

Mark the following statements as true or false. If a statement is false, correct it to make a true statement. a. The somatic sensory division of the PNS detects sensory stimuli from the organs in the thoracic and abdominopelvic cavities. b. The somatic motor division of the PNS consists of lower motor neurons that directly innervate skeletal muscle fibers. c. The visceral motor division is also known as the autonomic nervous system and maintains homeostasis of many physiological variables. d. The term nerve is the equivalent of the term neuron. e. There are 31 pairs of spinal nerves and 12 pairs of cranial nerves.

a. False: The visceral sensory division of the PNS detects sensory information from the organs in the thoracic and abdominopelvic cavities. b. True c. True d. False: The term nerve is not the equivalent of the term neuron. (Nerves are composed of the axons of many neurons.) e. True

Predict the symptoms (both sensory and motor) you might experience if you suffered an injury to each of the following nerves: a. Median nerve b. Common fibular nerve c. Femoral nerve d. Sciatic nerve e. Ulnar nerve

a. Inability to flex the wrist and fingers; numbness or pain on the anterior thumb, the anterior second and third digits, and the lateral side of the fourth digit b. Inability to dorsiflex and evert the ankle; numbness or pain around the knee, the anterior and distal leg, and the dorsum of the foot c. Inability to flex the hip and extend the knee; numbness or pain along the anterior thigh and the medial thigh, leg, and foot d. Significant impairment of most muscles of the lower limb except the anterior thigh muscles; numbness or pain on the posterior lower limb and part of the anterior leg e. Reduced ability to flex the forearm and difficulty moving the hand; numbness or pain along the fifth digit and the medial side of the fourth digit

Define each of the following terms in your own words, using 20 or fewer words. a. Peripheral nerve b. Nerve plexus c. Posterior root ganglion

a. Peripheral nerve: a collection of axons, connective tissue sheaths, and blood vessels b. Nerve plexus: a network of nerves that innervates a specific body region c. Posterior root ganglion: a collection of cell bodies of sensory neurons located in the posterior root outside the spinal cord

Mark the following statements as true or false. If a statement is false, correct it to make a true statement. a. The simple stretch reflex is a monosynaptic reflex with only a single synapse in the spinal cord between the sensory and motor neurons. b. The Golgi tendon organs detect stretch in a simple stretch reflex. c. A flexion reflex is a monosynaptic reflex with only one synapse in the spinal cord. d. The crossed-extension reflex occurs simultaneously with the simple stretch reflex.

a. True b. False: Muscle spindles detect stretch in a simple stretch reflex. c. False: A simple stretch reflex is a monosynaptic reflex with only one synapse in the spinal cord. d. False: The crossed-extension reflex occurs simultaneously with the flexion reflex.

Predict the effects of damage to each of the following motor neurons: a. Upper motor neurons of the premotor cortex b. Upper motor neurons of the primary motor cortex c. Lower motor neurons

a. Upper motor neurons of the premotor cortex are involved in planning movement and selecting a motor program, and so damage to them would lead to a loss of specific motor skills. b. Damaged upper motor neurons of the primary motor cortex cannot stimulate lower motor neurons, leading to uncontrolled muscle contractions mediated by those lower motor neurons and the spinal cord. c. A lower motor neuron is the only path for stimulating the muscle fibers of its motor unit, so damage to lower motor neurons produces paralysis or paresis.

First-order somatic sensory neurons are _________ neurons whose cell bodies are located in the _________. a. multipolar, posterior horn b. pseudounipolar, posterior root ganglion c. bipolar, anterior horn d. pseudounipolar, posterior horn

b. pseudounipolar, posterior root ganglion

Match the following nerves with the structures they innervate. ____ Phrenic nerve ____ Median nerve ____ Femoral nerve ____ Tibial nerve ____ Radial nerve ____ Intercostal nerves ____ Common fibular nerve ____ Musculocutaneous nerve a. Motor to the triceps brachii muscle and muscles in the forearm that extend the hand; sensory from the posterior hand b. Motor to the muscles in the anterior arm that flex the forearm; sensory from skin over the lateral forearm c. Motor to the muscles in the anterior and lateral leg that evert and dorsiflex the foot; sensory from the skin of the anteroinferior leg d. Motor to the diaphragm muscle e. Motor to the muscles in the anterior thigh extend the knee; sensory from the skin over the anterior thigh and leg f. Motor to the hamstring muscles that extend the thigh and flex the leg, muscles of the leg that plantarflex the foot, and muscles of the foot; sensory from the skin over the posterior and lateral leg and foot g. Motor to the muscles between the ribs and the abdominal muscles; sensory from the skin over the abdomen h. Motor to the muscles in the forearm that flex the hand, certain intrinsic hand muscles; sensory from the skin of the anterior hand

d - Phrenic nerve h - Median nerve e - Femoral nerve f - Tibial nerve a - Radial nerve g - Intercostal nerves c - Common fibular nerve b - Musculocutaneous nerve