Reflex Activity I. Definition and Components of a Reflex

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Lecture: Reflex Activity
I. Definition and Components of a Reflex
A. reflex - a rapid,"wired" motor response to a specific stimulus; involuntary, subconscious
1. somatic reflex - skeletal muscle

2. visceral (autonomic) reflex - smooth muscle, cardiac muscle, visceral organs, glands

B. Components of a Typical Reflex Arc
1. Receptor - site where stimulus is received
a. mechanoreceptors - sensitive to touch, pressure, vibration and stretch
I. Meissner's corpuscles - receptors for discriminative touch, below skin epidermis

ii. Merkel's discs - light touch receptors in deep layers of epidermis

iii. Pacinian corpuscles - respond to skin pressure/vibration, deep in the dermis

iv. Ruffini's corpuscles - respond to stretch of dense connective tissues

v. Krause's End Bulbs - respond to pressure and touch, in mucous membranes

vi. muscle spindles - respond to stretch in "intrafusal fibers" of skeletal muscle

vii. Golgi tendon organs - respond to stretch in tendon, close to muscle insertion point

viii. joint kinesthetic receptors - monitor position of synovial joints (position)

b. thermoreceptors - respond to temperature change

c. photoreceptors - respond to light (the retina)

d. chemoreceptors - respond to different molecules in the blood, kidney, tongue, nose, etc.

e. nociceptors - respond to harmful stimuli (painful); heat, cold, pressure, chemical

2. Sensory Neuron - transmits the signal from receptors AFFERENT nerve fiber to the CNS
3. Integration center - point where sensory and motor neurons integrate information to respond
a. monosynaptic pathway - sensory neurons synapses directly with the motor neuron

b. polysynaptic pathway - one or more "interneurons" between sensory and motor neurons

4. Motor Neuron - transmits motor signal from CNS to the effector organ via EFFERENT nerve
5. The Effector - the muscle or organ that responds to signal of the motor neuron

II. Skeletal Muscle Reflex (Spinal Reflex)
A. The Classic Stretch Reflex
1. Anatomy of Skeletal Muscle Stretch Reflex
a. intrafusal muscle fiber - special muscle fiber that responds to stretching; center is non- contractile; ends have contractile property
i. type Ia sensory fiber - wraps around CENTER of intrafusal fiber; respond to rate and degree of stretching of the muscle

ii. type II sensory fiber - wrap around END intrafusal fiber; respond only to degree of stretch of the muscle [records “resting length” of the intrafusal fiber]

iii. gamma motor fiber - synapses on contractile end of intrafusal fiber; maintains level of sensitivity of this stretch receptor, depending on the length of the muscle at any given time [sets the “resting length” of intrafusal fiber]
b. extrafusal fiber - normal contractile muscle fibers of a skeletal muscle
i. alpha motor fiber - synapses on normal extrafusal fibers to allow a response to a stretched muscle
2. Mechanism of Skeletal Muscle Stretch Reflex
a. Excitation of Muscle Spindle (Stretching)
1. external stretch - external force (weight) on muscle or contraction of antagonistic muscle (eg. quadriceps - hamstrings)
2. internal stretch - activated gamma motor neurons cause contraction of intrafusal fiber, stretching midsection of spindle
b. Type Ia and II Sensory Fiber Activation
1. AFFERENT (sensory) fiber synapses directly on alpha motor neuron, causing contraction of extrafusal muscle fibers of the stretched muscle
2. Reciprocal Inhibition - AFFERENT (sensory) fiber synapses on interneurons, which then INHIBIT motor neurons to the antagonstic muscle (eg. quadriceps - hamstrings)
3. Relay of information to brain (cerebellum) also occurs via "dorsal white columns" of the spinal cord
c. Regulation of Muscle Spindle Length
1. The length of the muscle spindle organ, and therfore its sensitivity to stretch, is regulated by activation/inhibition of the gamma motor neuron reflex arc.

3. Example: The Patellar Tendon (Knee-Jerk) Reflex

a. reflex hammer hits patellar tendon, stretching the quadriceps muscle

b. intrafusal fibers of muscle spindle are stretched, type Ia and II sensory (afferent) fibers are activated

c. alpha motor neuron to quadriceps activated, resulting in quadriceps contraction

d. alpha motor neuron to hamstrings inhibited (via interneuron), resulting in hamstring relaxation

e. gamma motor neuron to end of muscle spindle regulates sensitivity to stretch
4. Clinical Significance of Patellar Tendon Reflex
a. test integrity of sensory neurons, motor neurons, and the spinal cord

b. hypoactive - when damage to peripheral nerves, dorsal root ganglion (sensory cell bodies), or ventral horn of spinal cord (motor cell bodies)

c. hyperactive - when inhibitory influence from brain (via corticospinal tracts) is reduced; e.g. polio and stroke patients

B. Deep Tendon Reflexes

1. Opposite of Stretch Reflex - the stretched or contracting muscle responds by relaxing
a. Golgi tendon organs are activated by stretched or contracted muscle
b. Reciprocal Activation: the AFFERENT (sensory) fibers from the Golgi tendon organs INHIBIT motor neurons to the muscle, allowing relaxation to occur
c. the AFFERENT (sensory) fibers from the Golgi tendon organs ACTIVATE motor neuron to the antagonistic muscle, causing it to contract
2. The deep tendon reflexes COUNTERBALANCE the stretch reflexes to allow smooth motion to occur, especially during complex activity (eg. running, swimming,etc.)
III. Special Reflex Systems
A. The Flexor (Withdrawal) Reflex - allows the withdrawal of a limb from a painful stimulus
1. ipsilateral, polysynaptic reflex - this occurs on the same side of the body as the stimulus, and involves at least one interneuron in the pathway
B. The Crossed Extensor Reflex - allows one limb to act in opposite way as other limb
1. Lifting foot from a hot coal, other leg must react to support the weight of the body

2. contralateral reflex - stimulus on one side of the body affects action of opposite (contra) side

C. Superficial (Cutaneous) Reflexes - gentle stroking of the skin can cause a variety of responses
1. Plantar Reflex - gently stroking the lateral aspect of the bottom of the foot (test L4 - S2)
a. normal adult - the toes will flex (curl down)

b. Babinski sign - in infant (up to 6 months) and in adults with damaged corticospinal tract, toes will fan out

2. Abdominal reflexes - gently stroke skin on lateral aspects of the abdomen (test T8 - T12)

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