Anatomy & Physiology I and II

Anatomy & Physiology Study Guide

©2018 Achieve Test Prep Page 167 of 367 In monosynaptic reflexes, there is little delay between sensory input and motor output. These reflexes control the most rapid, stereotyped motor responses of the nervous system to specific stimuli. The best-known monosynaptic reflex is the stretch reflex, which provides automatic regulation of skeletal muscle length. The patellar reflex is an example of this type. The sensory receptors involved in the stretch reflex are muscle spindles . Each contains a bundle of small, specialized skeletal muscle fibers called intrafusal muscle fibers . Larger extrafusal muscle fibers surround the muscle spindle and are responsible for the resting muscle tone and, at greater levels of stimulation, the contraction of the entire muscle. Polysynaptic Reflexes Polysynaptic reflexes can produce far more complicated responses than monosynaptic reflexes. One reason for this intricacy is that the interneurons involved control multiple muscle groups. Additionally, excitatory or inhibitory postsynaptic potentials (EPSPs or IPSPs) are produced by these interneurons at CNS motor nuclei. Therefore, the response can involve the stimulation of some muscles and the inhibition of others. The tendon reflex is an example. The tendon reflex monitors the external tension produced during a muscular contraction and prevents tearing or breaking of the tendons. Withdrawal Reflexes Withdrawal reflexes shift affected parts of the body away from a stimulus. Painful stimuli create the strongest withdrawal reflexes, but these reflexes are sometimes initiated by the stimulation of Nature of the Response Somatic reflexes create a process for the involuntary control of the muscular system. Superficial reflexes are activated by stimuli at the skin or mucous membranes. Stretch reflexes are activated by the sudden elongation of a tendon, and thus of the muscle to which it attaches. These reflexes are also known as deep tendon reflexes or myotatic reflexes . Visceral reflexes, or autonomic reflexes, control the activities of other systems. Complexity of the Circuit In the easiest reflex arc, the sensory neuron synapses on a motor neuron, which processes the information. This category of reflex is a monosynaptic reflex . A synaptic delay always occurs with transmission across a chemical synapse, but with only one synapse, the interruption between the stimulus and the response is minimized. Most reflexes, however, have at least one interneuron between the sensory neuron and the motor neuron. Such polysynaptic reflexes have a longer delay between stimulus and response. The length of the delay is proportional to the number of synapses involved. Spinal Reflexes Spinal reflexes vary in complexity. They range in complexity from simple monosynaptic reflexes, involving a single segment of the spinal cord, to polysynaptic reflexes that involve many segments. In the most complicated spinal reflexes, called intersegmental reflex arcs, many segments interact to produce a coordinated, highly variable motor response. M nosyn ptic Reflexes