Human Physiology Chapter 13

have afferent neurons that bring information to the CNS

-Muscle spindles -Golgi tendon organs

A polysynaptic reflex has at least __________ in the reflex pathway.

-Two synapses -Three Neurons

The control of voluntary movement can be divided into what three steps?

1. decision making and planning 2. initiating the movement 3. executing the movement

What are the three levels of nervous system that control movement? Briefly describe their roles.

1. the spinal cord, which integrates spinal reflexes and contains the central pattern generators 2. the brain stem and cerebellum, which control postural reflexes and hand and eye movements 3. the cerebral cortex and basal ganglia, which are responsible for voluntary movements

Which reflex is NOT controlled by the brainstem? A) urination B) vomiting C) sneezing D) swallowing E) salivating

A

Upon testing the knee-jerk reflex during a routine physical, a nurse notices that the reflex is hyperactive. What could be the explanation for this? If instead the reflex was hypoactive, what may the nurse conclude?

A hyperactive knee-jerk reflex suggests a lack of descending inhibition. This could indicate injury or disease in the spinal cord above the reflex arc, or in a brain area involved in motor control. A hypoactive reflex could indicate damage to any of the components of the local reflex arc, including the muscle, the spinal nerve, or that level of the spinal cord. Alternatively it is possible the patient is deliberately inhibiting the reflex by descending inhibition. Reflexes can also be adversely affected by metabolic disorders.

our experiences tell us how to behave; bracing yourself in anticipation would be an example

Acquired

The motor neurons that innervate the normal contractile fibers of the muscle are the __________.

Alpha motor neurons

Explain alpha-gamma coactivation.

Alpha motor neurons control the contraction of skeletal muscle fibers. Gamma motor neurons adjust the stretch sensitivity of the muscle spindle, so that the spindle is active even when the muscle shortens. Coactivation of both sets of fibers causes the tension on the muscle spindles to be maintained as the muscle shortens; thus sensitivity to stretch is maintained.

List three ways that neural reflex pathways can be classified. Name one of the reflexes in each category. What additional information may be gained from the name of the reflex? Name a specific reflex and identify its classification in all of the categories you listed.

Any three of the following four answers are correct classifications. Further, the effector or action is sometimes evident by the reflex name, e.g., flexor reflex. Answers to specific reflexes will vary, e.g., knee-jerk reflex is somatic, spinal, innate, and monosynaptic. 1. by the efferent division of the nervous system that controls the response: somatic reflexes and autonomic reflexes 2. by the CNS location where the reflex is integrated: spinal reflexes and cranial reflexes 3. by whether the reflex is innate or learned: innate reflexes and learned or conditioned reflexes 4. by the number of neurons in the reflex pathway: monosynaptic reflexes and polysynaptic reflexes

Reflex that is an example would be slowing or speeding the heart and other internal organs that are not consciously controlled

Autonomic

Identify the FALSE statement. A) Adipose tissue is controlled by autonomic efferents. B) All reflexes require input from the brain. C) Some reflexes are genetically determined. D) Muscle spindles are stretch receptors. E) Proprioceptors detect limb position and movement.

B

The structure whose abnormal function is associated with Parkinson's disease is the

Basal ganglia

Which class of movement can be considered a combination of the other two? A) reflex B) voluntary C) rhythmic

C

Which is NOT true regarding autonomic reflexes? A) Vomiting, sneezing, and coughing are all examples of autonomic reflexes. B) Integrating centers for autonomic reflexes in the brain include the hypothalamus, brain stem, and limbic system. C) Autonomic reflexes are all monosynaptic, with their synapse in the central nervous system. D) Many autonomic reflexes are characterized by tonic activity, a continuous stream of action potentials.

C

Rhythmic reflexive motor activities, such as breathing or walking, are controlled by networks of neurons in the central nervous system called __________.

Central pattern generators

Reflex integrated in the brain

Cranial

A muscle at rest exerts no tension. Is this statement true or false? Explain your answer.

False. Normal muscles maintain a resting tension known as muscle tone.

__________ allows the body to anticipate a stimulus and begin the response, whereas negative feedback results in the __________ of a response.

Feedforward, cessation

The motor neurons associated with intrafusal muscle fibers are referred to as __________ motor neurons.

Gamma

found at the junction of tendons and muscle fibers

Golgi tendon organs

Motor neurons are sometimes inhibited by

Golgi tendon organs.

Explain how an animal with a paralyzing spinal injury can be induced to walk, though it can't walk on its own. Diagram and label the structures involved (Hint: Keep it simple; for example, you could draw a rectangle and label it "right leg muscle" and another for the left, and don't worry about drawing all neurons involved, just enough to show you understand the basics. You may wish to start with a simple sketch of a muscle reflex then add onto it as necessary.)

If the animal is supported on a moving treadmill, the central pattern generators involved in walking will become active and will produce coordinated contraction and relaxation of limb muscles. Diagram should include a cross section of the spinal cord with an area labeled "central pattern generator," a sensory afferent, and a motor efferent as in any sketch of a reflex.

In a __________ reflex, a sensory neuron synapses directly on a motor neuron.

Monosynaptic

eventually synapses with alpha motor neurons that innervate extrafusal muscle fibers

Muscle spindles

excite both alpha and gamma motor neurons

Muscle spindles

have gamma motor neurons that innervate internal fibers

Muscle spindles

maintain muscle tone at rest through tonic activity

Muscle spindles

Are relaxation of a muscle and inhibition of a muscle the same thing? Explain your answer.

No. Relaxation is a passive process that normally follows contraction and occurs in the absence of further activity in the motor neurons. Inhibition involves prevention of muscle stimulation and consequent contraction by activity in interneurons that inhibit motor neurons. While in both cases the motor neuron is not producing signals, the reason is lack of stimulation in one case, inhibition in the other.

Reflex with several neurons involved in a pathway; can be complex

Polysynaptic

__________ monitor the position of skeletal muscles and joints.

Proprioceptors

Make a map of the following terms: alpha motor neurons, CNS, contractile fibers, extrafusual muscle fibers, Golgi tendon organs,integrating center, joint receptors, muscle spindles, proprioreceptors, sensory neurons, sensory receptors and somatic motor neurons.

Skeletal muscle reflexes have the following components: Sensory receptors known as proprioreceptors—the three types are: muscle spindles, Golgi tendon organs, and joint receptors. Sensory neurons carry input from proprioreceptors to the CNS, the integrating center. Somatic motor neurons carry the output signal from the CNS; alpha motor neurons are somatic motor neurons that innervate skeletal muscle contractile fibers, known as extrafusual muscle fibers.

Reflex that involves skeletal muscles

Somatic

__________ reflexes involve skeletal muscles.

Somatic

Reflex that is integrated in the spinal cord, often modulated by the brain

Spinal

How does the stretch reflex protect a muscle?

Stretching activates the stretch reflex, which in turn contracts the muscle, thereby preventing overstretching.

Most reflex movements are integrated by

The spinal cord

Diagram and label the knee-jerk reflex. What is the physiological function of this reflex? Explain how this reflex may be important during walking, if you didn't notice a dip in the sidewalk and stepped into it. What is the role of reciprocal inhibition? How would the reflex be affected if reciprocal inhibition failed? Describe the effects on the reflex of severing each structure involved in the reflex, considering one structure at a time. Describe the effects of damaging the opposite side of the spinal cord, or areas higher or lower on the spinal cord.

The diagram should resemble Fig. 13-7 in the chapter. The function of the reflex is to control movement at the knee joint. If you stepped down farther than you expected, your opposite knee may bend more than it would have, activating the reflex and straightening that leg to prevent you from falling. Reciprocal inhibition allows muscles opposing extension of the leg to be inhibited. If this inhibition failed, leg extension would not occur; instead, the knee would be "locked." Severing the afferent or efferent nerve, the spinal cord at the level of the reflex, or the muscle would all prevent the reflex from occurring. Damage to the opposite side of the spinal cord or areas above or below the cells involved in the reflex should have no effect on the basic reflex.

Bowel and bladder control involve spinal reflex pathways that are located in the sacral region of the spinal cord. In both instances, two sphincter muscles, an inner sphincter of smooth muscle and an outer sphincter of skeletal muscle, control the passage of waste out of the body. How would completely severing the spinal cord at the lumbar level affect an individual's bowel and bladder control?

The person would still exhibit a defecation (bowel) and micturition (bladder) reflex because the spinal reflex is processed at the local sacral level of the spinal cord, which is intact at that level. Efferent impulses from the organs would stimulate specific interneurons in the sacral region that would synapse with the motor neurons controlling the sphincters, thus bringing about emptying when organs began to fill. This is the same situation that exists in a newborn infant who has not yet fully developed the descending tracts necessary for conscious control. The individual with the spinal cord transection would lose voluntary control of the bowel and bladder because these functions rely on impulses carried by motor neurons in the brain that must travel down the cord and synapse with the interneurons and motor neurons that are involved in the reflex. With a lumbar transection, these impulses no longer reach the reflex control center.

You are a technician in a neuroanatomy lab, and it's your first day of training to learn a technique of labeling neurons. Your boss tells you that you will fill individual motor neurons, by injecting horseradish peroxidase (HRP) conjugated to tetanus toxin, into a skeletal muscle. How could this possibly label the neurons?

The process of labeling neurons by HRP was introduced in Ch. 9; this enzyme is taken up by axon terminals and transported retrogradely, filling most or perhaps all dendrites, as well as the axon and soma. When the substrate of the enzyme is added to tissue slices, a dark reaction product is formed, and this is how the details of the cell are visualized. Toxins such as tetanus and cholera toxin, which are also taken up and transported by axon terminals, enhance the uptake of HRP, if they are attached (conjugated) to the HRP.

List and describe the three categories of movement. Explain why these categories are NOT always distinct.

The three categories are reflex, voluntary, and rhythmic. Reflex movements are initiated by sensory input and are integrated primarily in the spinal cord. Voluntary movements are integrated in the cerebral cortex and require no external stimuli. Rhythmic movements are a combination of reflex and voluntary. Voluntary movements improve with practice as do reflexive, voluntary movements can become reflexive once learned, and voluntary movements depend upon input from postural reflexes.

Ten-year-old Tina falls while climbing a tree and lands on her back. Her frightened parents take her to the local emergency room where she is examined. Her knee-jerk reflex is normal and she exhibits a negative Babinski reflex. These results suggest that

Tina suffered no damage to her spinal cord.

Autonomic reflexes are also called __________ reflexes.

Visceral

While watching Olympic weight lifters on television, Carl notices that on several occasions an athlete would lift the weight to his chest and then suddenly drop it. Can you offer an explanation for what might be happening?

What Carl is observing is an example of the stretch reflex in action. As the weight lifter lifts the weight, he is contracting the biceps brachii and brachialis muscles and at the same time stretching the triceps brachii muscle. If the mass is great, the amount of flexion necessary to move the mass could begin to overstretch the triceps that is being relaxed so as not to interfere with flexion. Stimulation of the muscle spindles in the triceps produces a reflex arc that brings about the relaxation of some of the motor units in the muscles involved in flexion, like the biceps. When the tension produced by the flexing muscles decreases to a point where they can no longer work against the resistance of the mass, the weight lifter will drop the weights involuntarily.

You are studying for your physiology test with your lab partner. She insists that the purpose of the knee-jerk reflex is to test for neural or muscular damage. State whether or not you agree with her, and why.

While reflexes can be exploited by medical personnel as a means to assess neural and muscular function, that is not their "purpose" in the sense of why they occur. Each reflex has a function; for example, the knee-jerk reflex helps control movement at the knee joint.

The sensory fibers of the muscle spindle organs synapse onto

alpha motor neurons.

After stretching an intrafusal fiber, the next event is

an increase in action potentials along the associated sensory neuron.

The __________ is essentially a group of interneurons that run from the motor cortex to the spinal cord.

corticospinal tract

The reflex that complements a withdrawal reflex by making compensatory adjustments on the opposite side of the body receiving the stimulus is the

crossed extensor reflex.

The "normal" contractile fibers of the muscle are also called the

extrafusal fibers.

reflexes that one is born with; genetically determined

innate

The flexion reflex

moves a limb away from a painful stimulus.

The effector in a reflex is the

muscle or gland

The tension maintained in a muscle at rest is known as __________.

muscle tone

When there is a resistance to the movement produced by a given limb muscle,

muscle tone is increased and more motor units are recruited to the contraction.

The collection of pathways controlling a single joint is known as __________. The simplest reflex in this collection of pathways is the __________, which involves only two neurons: __________ neuron from the __________ and the __________ neuron to the muscle.

myotatic unit; monosynaptic stretch reflex, sensory, muscle spindle, somatic

Most reflexes are regulated by

negative feedback

Most reflexes are regulated by __________.

negative feedback

A __________ reflex has at least one interneuron placed between the sensory and motor neuron.

polysynaptic

In order to flex a limb, the extensors must first be relaxed and vice versa. This is done by a process called __________.

reciprocal innervation

Interneurons of the corticospinal tract synapse onto

somatic efferents

Muscle spindles are __________ receptors and cause reflex __________, whereas Golgi tendon organs respond primarily to __________ and cause a __________ reflex.

stretch, contraction, tension, relaxation (Golgi tendon organs are relatively insensitive to muscle stretch.)

The reflex that prevents a muscle from exerting too much tension is the

tendon reflex.

Relaxation of skeletal muscle results from __________ input by the somatic motor neuron.

the absence of excitatory

Spinal interneurons prevent muscle antagonists from interfering with an intended movement by

the process of reciprocal inhibition.