We have seen that the nervous system is involved in various types of reflex activity. The basic pathways for such reflexes have been considered. Many of these reflexes can be demonstrated clinically, and constitute a valuable aid in establishing the integrity of various levels of the nervous system. Some of the reflexes that are used for this purpose are described below.
Sudden stretching of a muscle (by tapping its tendon) produces reflex contraction of the muscle. The pathway for this reflex involves two neurons only. Stretching of the muscle stimulates proprioceptive nerve endings located in muscle spindles and other receptors. These impulses are carried to the spinal cord by neurons that synapse with motor neurons in the ventral grey columns (Fig. 3.5). Fibres arising from these motor neurons reach the muscle and produce contraction. Stretch reflexes are abolished if any part of the pathway for it (i.e., the reflex arc) is interrupted. Under certain conditions these reflexes may be exaggerated. From a clinical point of view it is important to know the level of the spinal cord at which each reflex is mediated. Some of the important stretch reflexes are described below. The spinal segments concerned are given in brackets.
1. The knee jerk or patellar tendon reflex consists of extension of the leg by contraction of the quadriceps when the ligamentum patellae is tapped (L2, L3, L4).
2. The ankle jerk or Achilles tendon reflex consists of plantar flexion of the foot on tapping the tendo calcaneus (L5, S1, S2).
3. The biceps tendon reflex consists of flexion of the forearm on tapping the biceps tendon (C5, C6).
4. The triceps tendon reflex consists of extension of the forearm on tapping the triceps tendon (C6, C7, C8).
5. The supinator jerk (or radial periosteal reflex) consists of flexion of the forearm when the distal end of the radius is tapped (C7, C8). Note that the muscle responsible for this reflex is the brachioradialis, not the supinator. It is called the supinator jerk because the brachioradialis was at one time called the supinator longus. This is a periosteal reflex, not a tendon reflex. According to some authorities the spinal segments responsible for the reflex are C5, 6,7.
6. The wrist tendon reflex consists of flexion of the fingers on percussion on wrist tendons (C8, T1).
7. The jaw or masseter reflex is a myotatic reflex mediated through the trigeminal nerve (and not through the spinal cord). To elicit this reflex the patient is asked to open the mouth slightly. The examiner places his index finger over the middle of the patient’s chin and taps it. This results in bilateral contraction of the masseter and temporalis muscles. Both afferent and efferent components of the reflex arc pass through the mandibular division of the trigeminal nerve, the nuclei concerned being located in the pons.
Stimulation of skin in certain regions of the body causes contraction of underlying muscles. This occurs reflexly, the reflex being mediated through the spinal cord. Some of these superficial reflexes are described below.
1. The abdominal reflexes consist of contraction of underlying muscles on stroking the skin of the abdomen in its upper (T6,7), middle (T8,9) and lower (T10 to T12) parts.
2. The cremasteric reflex consists of elevation of the scrotum on stroking the skin of the medial side of the thigh (T12 to L2).
3. The gluteal reflex consists of contraction of the glutei on stroking the overlying skin (L4 to S1).
4. The normal plantar reflex consists of plantar flexion of the toes on stroking the skin of the sole (L5 to S2). When there is an injury to the corticospinal system an abnormal response is obtained. There is extension (dorsiflexion) of the great toe and fanning out of other toes. This response is referred to as Babinski’s sign. Such a response may also be seen in newborn infants, and sometimes in sleeping or intoxicated adults.
5. The anal reflex consists of contraction of the external anal sphincter on stroking the perianal region (S4,5, coccygeal).
Movements of the head (through neck muscles) depend on segments C1 to C4; those of the diaphragm on segments C3 to C5; those of the upper extremity on segments C5 to T1; and those of the lower extremity on segments L1 to S2. Filling of the urinary bladder is mediated by segments T12 to L2, and evacuation by segments S3 to S5. Erection of the penis depends on segments S2 to S4, and ejaculation on segments L1 and L2 (smooth muscle), and also on S3 and S4 (striated muscle).
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