Info

Part

Description

Function

Receptor

The receptor end of a dendrite or a specialized receptor cell in a sensory organ

Sensitive to a specific type of internal or external change

Sensory neuron

Dendrite, cell body, and axon of a sensory neuron

Transmits nerve impulse from the receptor into the brain or spinal cord

Interneuron

Dendrite, cell body, and axon of a neuron within the brain or spinal cord

Serves as processing center; conducts nerve impulse from the sensory neuron to a motor neuron

Motor neuron

Dendrite, cell body, and axon of a motor neuron

Transmits nerve impulse from the brain or spinal cord out to an effector

Effector

A muscle or gland

Responds to stimulation by the motor neuron and produces the reflex or behavioral action

1. Fasciculus gracilis (fah-sik'u-lus gras'il-is) and fasciculus cuneatus (ku'ne-at-us). These tracts are located in the posterior funiculi of the spinal cord. Their fibers conduct sensory impulses from the skin, muscles, tendons, and joints to the brain, where they are interpreted as sensations of touch, pressure, and body movement.

At the base of the brain in an area called the medulla oblongata (me-dul'ah ob"long-ga'tah) most of the fasciculus gracilis and fasciculus cuneatus

Shier-Butler-Lewis: I III. Integration and I 11. Nervous System II: I I © The McGraw-Hill

Human Anatomy and Coordination Divisions of the Nervous Companies, 2001

Physiology, Ninth Edition System

Uses of Reflexes

Since normal reflexes depend on normal neuron functions, reflexes are commonly used to obtain information concerning the condition of the nervous system. An anesthesiologist, for instance, may try to initiate a reflex in a patient who is being anesthetized in order to determine how the anesthetic drug is affecting nerve functions. Also, in the case of injury to some part of the nervous system, observing reflexes may reveal the location and extent of damage.

Injury to any component of a reflex arc alters its function. For example, a plantar reflex is normally initiated by stroking the sole of the foot, and the usual response is flexion of the foot and toes. However, damage to certain nerve pathways (corticospinal tract) may trigger an abnormal response called the Babin-ski reflex, which is a dorsiflexion, extending the great toe upward and fanning apart the smaller toes. If the injury is minor, the response may consist of plantar flexion with failure of the great toe to flex, or plantar flexion followed by dorsiflexion. The Babinski reflex is normally present in infants up to the age of twelve months and is thought to reflect immaturity in their corticospinal tracts.

Other reflexes that may be tested during a neurological examination include the following:

1. Biceps-jerk reflex. Extending a person's forearm at the elbow elicits this reflex. The examiner's finger is placed on the inside of the extended elbow over the tendon of the biceps muscle, and the finger is tapped. The biceps contracts in response, and the forearm flexes at the elbow.

2. Triceps-jerk reflex. Flexing a person's forearm at the elbow and tapping the short tendon of the triceps muscle close to its insertion near the tip of the elbow elicits this reflex. The muscle contracts in response, and the forearm extends slightly.

3. Abdominal reflexes. These reflexes occur when the examiner strokes the skin of the abdomen. For example, a dull pin drawn from the sides of the abdomen upward toward the midline and above the umbilicus causes the abdominal muscles underlying the skin to contract, and the umbilicus moves toward the stimulated region.

4. Ankle-jerk reflex. Tapping the calcaneal tendon just above its insertion on the calcaneus elicits this reflex. The response is plantar flexion, produced by contraction of the gastrocnemius and soleus muscles.

5. Cremasteric reflex. This reflex is elicited in males by stroking the upper inside of the thigh. In response, the testis on the same side is elevated by contracting muscles. ■

Fasciculus gracilis

Posterior spinocerebellar tract

Lateral corticospinal tract

Lateral reticulospinal tract Rubrospinal tract

Anterior spinocerebellar tract Lateral spinothalamic tract

Anterior reticulospinal tract Medial reticulospinal tract Anterior spinothalamic tract

Fasciculus cuneatus

Fasciculus gracilis

Fasciculus cuneatus

Anterior reticulospinal tract Medial reticulospinal tract Anterior spinothalamic tract

Brain Transverse View And Labeled

Anterior corticospinal tract

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

Get My Free Ebook


Post a comment