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Classification of Neural Reflexes: Efferent division that controls the effector

a. Somatic motor neurons - control skeletal muscles

b. Autonomic neurons - control smooth and cardiac muscle, glands, and adipose tissue.


Classification of Neural Reflexes: Integrating region within the Nervous system

a. Spinal reflexes do not require input from the brain

b. Cranial reflexes are integrated within the brain


Classification of Neural Reflexes: Time at which the reflex develops

a. Innate (inborn) reflexes are genetically determined.

b Learned (conditioned) reflexes are acquired through experience


Classification of Neural Reflexes: The number of neuron in the reflex pathway

a. Monosynaptic reflexes have only two neurons: one afferent (sensory) and one efferent. Only somatic motor reflexes can be monosynaptic.

b. Polysynaptic reflexes in crude one or more interneurons between the afferent and efferent neurons. All autonomic reflexes are polysynaptic because they have three neurons: one afferent and two efferent.


- also known as patellar tendon-tap reflex, knee-jerk reflex or myotactic reflex
- stretching of a muscle stimulates the muscle spindle afferents
plays an important role in the control of posture

Muscle Stretch Reflex


Components of a Muscle Stretch Reflex

1. Dynamic Stretch Reflex
2. Static Stretch Reflex


- caused by rapid stretch or unstretch
- transmitted from primary sensory or annulospiral endings of the muscle spindles
- oppose sudden changes in muscle length
- lasts within a fraction of a second only

Dynamic Stretch Reflex


- elicited by the continuous static receptor signals
- transmitted by both primary and secondary endings
- causes the degree of muscle contraction to remain reasonably constant
- continues for a prolonged period

Static Stretch Reflex


Damping Function of Stretch Reflexes

- muscle spindles prevent oscillation or jerkiness of body movements
- ensure that contraction is relatively smooth, even though the motor nerve to the muscle is excited at a slow frequency


- reinforcement technique for eliciting deep tendon reflexes
o fingers are locked together and one hand pulls against the other while reflex is evoked

- physiologic basis
o when one muscle is stretched, it facilitates a substantial number of alpha motor neurons
o transient increase of gamma motor neuron activity

Jendrassik's Maneuver


Jendrassik’s maneuver facilitates multiple alpha motor neurons. What does this mean?

easier recruitment


If they are being facilitated, where are they located in the neuronal pool?

facilitated pool (it's easier to excite)


- oscillation of a stretch reflex
- ordinarily occurs only when the stretch reflex is highly sensitized by facilitatory impulses from the brain



- elicited by noxious stimuli
- transmitted by group II, III, IV fibers
- possesses at least one interneuron, and so the most basic flexion reflex is disynaptic
- usually many muscles are involved through polysynaptic pathways
- to achieve withdrawal of a limb:
o flexor muscles in the limb must contract while the extensor muscles relax

Flexor Withdrawal Reflex


What type of neuronal circuit is exemplified by flexor withdrawal reflex?



Receptor that senses pain



- supports the body as the weight shifts away form the painful stimuli
- stimulation of the flexion reflex frequently elicits extension of the contralateral limb about 250 ms later
- long latency between flexion and crossed extension represents the time taken to recruit interneurons
- helps to maintain posture and balance

Crossed extensor reflex


- ensures that the extensor muscles acting on a joint will relax while flexor muscles contract
- neuronal circuit that causes this reciprocal relation is called reciprocal innervation

Reciprocal Inhibition


Components of Flexor Withdrawal Reflex

- diverging circuits to spread the reflex to the necessary muscles for withdrawal
- reciprocal inhibition circuits to inhibit the antagonist muscles
- circuits to cause afterdischarge lasting many fractions of a second after the stimulus is over


What type of neuronal circuit is exemplified by prolonged afterdischarge in crossed extensor reflex?



- Golgi tendon organs monitor muscle tension
- negative feedback mechanism that prevents development of too much tension on muscles
- when tension becomes extreme, reflex inhibitory effects lead to instantaneous relaxation of the entire muscle (lengthening reaction)

Inverse Myotactic Reflex


What are the four major spinal cord reflexes?

1. Muscle Stretch Reflex
2. Golgi Tendon reflex
3. Flexor withdrawal reflex
4. Crossed extension reflex


# of synapses: Monosynaptic
stimulus: Muscle stretch
afferent fibers: Group Ia fibers
efferent response: Muscle contraction

Muscle Stretch Reflex


# of synapses: Di/polysynaptic
stimulus: Muscle tension
afferent fibers: Group Ib fibers
efferent response: Muscle relaxation

Golgi Tendon reflex


# of synapses: Polysynaptic
stimulus: Pain
afferent fibers: Group II, III, IV fibers
efferent response: Ipsilateral muscle flexion

Flexor withdrawal reflex


# of synapses: Polysynaptic
stimulus: Pain
afferent fibers: Group II, III, IV fibers
efferent response: Contralateral muscle extension

Crossed extension reflex


- caused by transection of the spinal cord
- loss of spinal reflexes (areflexia) and flaccid paralysis below the level of the injury
- over the ensuing weeks, spinal cord activity below the level of the lesion returns as the excitability of undamaged neurons increases
- may give rise to spasticity of the paralyzed muscle groups

Spinal Shock


Events in Spinal Shock

- Neurogenic Shock
- Areflexia
- Incontinence


- arterial blood pressure falls instantly
- demonstrates that sympathetic nervous system activity becomes blocked almost to extinction

Neurogenic Shock


- may last 2 weeks to several months
- order of return: stretch reflexes, flexor reflexes, postural antigravity reflexes, remnants of stepping reflexes