13.3: Somatic Reflexes

Nature of Reflexes

Quick, involuntary, stereotyped reactions of glands or muscle to stimulation

Reflexes Require Stimulation

Not spontaneous actions, but responses to sensory input

Reflexes are Quick

Involve few, if any, interneurons and minimum synaptic delay

Reflexes are Involuntary

Occur without intent, often without awareness and are difficult to suppress

Reflexes are Stereotyped

Occur essentially the same way every time; predictable response

Somatic Reflexes

Reflexes involving the somatic nervous system, or spinal reflexes; involuntary contractions of a muscle

Reflex Arc

Signals travel along a given pathway; 1: somatic receptors in the skin muscles, and tendons; 2: afferent nerve fibers, which carry info from these receptors to the posterior horns of the spina; cord or to the brainstem; 3: an integrating center, a point of synaptic contact between neurons in the gray matter of the cord or brainstem; 4: efferent nerve fibers, which carry motor impulses to the muscles and; 5: effectors, the muscles that carry out the response

Muscle Spindle

Stretch receptors embedded in skeletal muscles; somatic reflexes; large number in muscle that require fine control (less in muscles that control is more course movement); inform the brain of muscle length and body movement; 4-10mm long and tapered at the ends; distributed throughout the muscle belly; contains 3-12 modified muscle fibers and a few nerve fibers, all wrapped in a fibrous capsule; contain 7-8 intrafusal muscles and the rest are extrafusal fibers

Primary Afferent Fibers

Monitor fiber length and speed of the length changes; end in annulospiral endings that coil around the middle of nuclear chain and nuclear bag fibers

Secondary Afferent Fibers

Monitor length only; flower-spray endings wrapped primarily around the ends of the nuclear chain fibers

Gamma (y) Motor Neurons

Innervates the ends of an intrafusal fiber and keeps it taut (adjusts tension in muscle spindle); originate in the ventral horn of spinal cord, the contractile ends of the intrafusal fibers

Alpha (a) Motor Neurons

Innervates the extrafusal fibers

Proprioceptors

Specialized sense organs to monitor position and movement of body parts

Stretch Reflex

When a muscle is stretches, it “fights back” and contracts (protecting itself); mediated mostly by brain; a quick stretch occurs, muscle contracts to protect and simultaneously sends inhibitory message to the antagonist muscle to allow contraction to occur (relaxing the antagonist muscle)

Stretch Reflex Function

Helps maintain equilibrium and posture; stabilizes joints by balancing tension in extensors and flexors; dampen muscle action

Stretch Reflex Feedback System

To a set of synergists and antagonist (muscles), if flexor muscles contract, then extensor muscles get stretches, stimulates a stretch reflex in flexor

Stretch Reflex Tendon Reflex

When a muscle is suddenly stretched, the reflexive contraction of a muscle when its tendon it tapped

Monosynaptic Reflex Arc

Afferent information synapses directly with the alpha motor neurons that return to muscle (only 1 synapse between afferent and efferent neuron, little synaptic delay and very quick response); knee-jerk (patellar) reflex

Reciprocal Inhibition

A reflex phenomenon that prevents muscles from working against each other by inhibiting antagonist when agonist is excited

Flexor Reflex

The quick contraction of flexor muscles resulting in the withdrawal of a limb from an injurious stimulus (and relaxation of antagonist muscle); ipsilateral reflex; (ex: touching a hot stove)

Polysynaptic Reflex Arc

Pathway in which signals travel over many synapses on their way to the muscle; some signals follow routes with only a few synapses and return to the flexor muscle quickly or others follow route with more synapses and have more delays, reaching flexor muscles later; this allows flexor muscle to be stimulated for a prolonged interval (usually so long that voluntary control takes over)

Crossed Extensor Reflex

Maintained by cerebellum and cerebral cortex; contralateral reflex; contraction of extensor muscles in limb opposite of the one that is withdrawn, maintains balance by extending other leg; function is to balance out the reactions from the flexor reflex, to maintain position; afferent branches of the nerve fibers cross from the stimulated aide of the body to the contralateral side of the spinal cord, synapses with interneurons and excites or inhibits alpha motor neurons to the muscles of the contralateral limb, contraction of extensor muscle in the limb opposite from the one that is withdrawn

Intersegmental Reflex Arc

The input and output occur at different levels (segments) of the spinal cord

Golgi Tendon Reflexes

A response to excessive tension on the tendon; function is to inhibit alpha motor neurons to the muscle to the muscle does not contract as strongly (therefore moderate muscle contraction occurs so that injury does not occur), and inhibits fibers connected with excessively stimulated tendon organs (therefore equals all the muscle contractions out, comparatively to one another)

Tendon Organs

Proprioceptors in a tendon near its junction with a muscle; 1 mm long; consists of an encapsulated tangle of knobby nerve endings entwined in the collagen fibers of the tendon

Relaxed State

Collagen fibers are slightly spread out and therefore put little pressure on the nerve endings among the collagen fibers

Contracted State

Muscle pulls on tendon, the collagen fibers come closer and squeeze the nerve endings between them, sends message back to spinal cord (CNS)/feedback on the degree of muscle tension at the joint