Anatomy & Physiology

Anatomy & Physiology Study Guide Spinal reflexes vary in complexity: • Spinal reflexes range from simple monosynaptic reflexes to more complex polysynaptic and intersegmental reflexes, in which many segments interact to produce a coordinated motor response. • The stretch reflex (such as the patellar, or knee-jerk, reflex) is a monosynaptic reflex that automatically regulates skeletal muscle length and muscle tone. The sensory receptors involved are muscle spindles. • A postural reflex maintains one's normal upright posture. • Polysynaptic reflexes can produce more complicated responses than monosynaptic reflexes. Examples include the tendon reflex (which monitors the tension produced during muscular contractions and prevents damage to tendons) and withdrawal reflexes (which move affected portions of the body away from a source of stimulation). The flexor reflex is a withdrawal reflex affecting the muscles of a limb. The crossed extensor reflex complements withdrawal reflexes. • All polysynaptic reflexes involve pools of interneurons, are intersegmental in distribution, involve reciprocal inhibition, and have reverberating circuits, which prolong the reflexive motor response. Several reflexes may cooperate to produce a coordinated response. The brain can affect spinal cord–based reflexes: • The brain can facilitate or inhibit reflex motor patterns based in the spinal cord. • Motor control involves a series of interacting levels. Monosynaptic reflexes form the lowest level; at the highest level are the centers in the brain that can modulate or build on reflexive motor patterns. • Facilitation can produce an enhancement of spinal reflexes known as reinforcement. Spinal reflexes may also be inhibited, as when the plantar reflex in adults replaces the Babinski sign in infants.

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