Chapter 21: Reflexes

Effector

A muscle fiber or a gland cell, responds to efferent impulses by contracting or secreting, respectively

Integration center

A single synapse between a sensory neuron & a motor neuron

Plantar reflex

An important neurological test Is elicited by stimulating the cutaneous receptors in the sole of the foot Cause the toes to flex & move close together Opposite response could indicate Babinski's sign (abnormal, only in babies)

Ciliospinal reflex

Another example of reflex activity in which pupillary responses can be observed Pupil reacts with body stimulation

Motor neuron

Conducts efferent impulses from the integration center to an effector organ

Fatigue also influences the reflex response. The subject should jog in position until she or he is very fatigued. Test the patellar reflex again, & record whether it is more or less vigorous than the first response. Would you say that nervous system activity or muscle function is responsible for the changes you have just observed?

Fatigue results in a less vigorous response. Muscle function is responsible. Fatigue likely results from a problem with excitation-contraction coupling, or possibly a problem at the neuromuscular junction. Either could reduce the response of muscle to nervous stimulation.

Stretch reflexes

Important for maintaining & adjusting muscle tone for posture, balance, & locomotion Initiated by tapping a tendon or ligament

Superficial reflexes

Initiated by stimulation of receptors in the skin & mucosae Abdominal, cremasteric, & plantar reflexes

Corneal reflex

Mediated through the trigeminal nerve

Crossed-extensor reflex

More complex than the stretch reflex Consists of a flexor, or withdrawal, reflex followed by extension of the opposite limb

The calcaneal tendon (ankle-jerk) reflex assesses the first two sacral segments of the spinal cord. With your shoe removed & your foot dorsiflexed slightly to increase the tension of the gastrocnemius muscle, have your partner sharply tap your calcaneal tendon with the broad side of the reflex hammer. During walking what is the action of the gastrocnemius?

Plantar flexion due to the contraction of the triceps surae(gastrocnemius and soleus muscles) is the result. Contraction of the gastrocnemius muscle usually results in plantar flexion of the foot.

Autonomic reflexes

Pupillary, ciliospinal, & salivary reflexes

Salivary reflex

Secrete varying amounts of saliva in response to reflex activation

Receptor

Site of stimulation action

Gag reflex

Tests the somatic motor responses of cranial nerves IX & X

Patellar (knee-jerk) reflex: Observations, muscles contracted, & what nerve is carrying the afferent & efferent impulses

The leg swings forward as the quadriceps muscles contract (hamstrings are reciprocally inhibited). The femoral nerve is carrying the impulses

Have the subject remove a shoe & lie on the cot or lab bench, w/ knees slightly bent & thighs rotated so that the posterolateral side of the foot rests on the cot. Alternatively, the subject may sit up & rest the lateral surface of the foot on a chair. Draw the handle of the reflex hammer firmly along the lateral side of the exposed sole from the heel to the base of the great toe. What is the response?

The normal response is downward flexion (curling) and adduction of the toes.

Gently stroke the oral mucosa on each side of the subject's uvula with a tongue depressor. What happens?

The posterior pharyngeal walls elevate as pharyngeal muscles contract, and the subject gags.

Now test the effect of muscular activity occurring simultaneously in other areas of the body. Have the subject clasp the edge of the lab bench & vigorously attempt to pull it upward w/ both hands. At the same time, test the patellar reflex again. Is the response more or less vigorous than the first response?

The response during other muscle activity is usually more vigorous due to increased facilitation in the spinal cord.

Test the effect of mental distraction on the patellar reflex by having the subject add a column of 3-digit numbers while you test the reflex again. Its the response more or less vigorous than the first response? What are the conclusions about the effect of mental distraction on reflex activity?

The response is usually greater than the first response. Mental distraction seems to increase the reflex response.

Stand to one side of the subject; the subject should look away from you toward the opposite wall. Wait a few seconds & then quickly, but gently, touch the subject's cornea w/ a wisp of absorbent cotton. What reflexive reaction occurs when something touches the cornea? What is the function of this reflex?

The subject blinks. The function is to protect the eye. The subject experiences discomfort (if not pain) because the cornea lacks pressure receptors but is richly supplied with pain receptors.

The subject should sit with eyes closed & with the back of one hand resting on the lab bench. Obtain a sharp pencil, & suddenly prick the subject's index finger. What are the results? Did the extensor part of this reflex occur simultaneously or more slowly than the other reflexes you have observed? What are the reasons for this?

The subject withdraws the pricked hand by flexion of the elbow. Then the other elbow extends. The extensor part of the reflex is relatively slow, probably because many association neurons are involved.

Sensory neuron

Transits afferent impulses to the CNS

Pupillary light reflex

the retina of the eye is the receptor, the optic nerve contains the afferent fibers, the oculomotor nerve contains the efferent fibers, & the smooth muscle of the iris is the effector