Anatomy & Physiology

Anatomy & Physiology Study Guide a hot stove top and blinking when the eyelashes are touched are two examples. Acquired reflexes consist of complex, learned motor patterns. An experienced driver hit the brakes when trouble appears ahead; a professional skier must make quick adjustments in body position while racing. These motor responses are rapid and automatic, but they were learned rather than pre-established, and thus, these reflexes are enhanced by repetition. Nature of Respons 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 Reflex s 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 R flexes 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 Achieve Page 167 of 368 ©2018