CNS: Reflexes

Polysnaptic Spinal Reflex Arc

Receptor - in skin, muscle, joint or viscera
Afferent (sensory) fiber - process of a pseudounipolar cell
in the dorsal root ganglion
Interneuron in Spinal Cord - (distinguishes polysynaptic from
monosynaptic reflex)
Efferent (motor) fiber - axon of cell i

Afferent neuron

This is a sensory neuron that APPROACHES the CNS; process of a pseudounipolar cell, DRG

Efferent neuron

This is a motor neuron that EXITs the CNS; axon of cell is in Ventral or Lateral Horn.


This is a neuron that, if present, then the reflex is polysynaptic; if absent, then the reflex is monosynaptic.


A simple, automatic, inborn response to a sensory stimulus, such as the knee-jerk response. Four basic types: Flexor, Crossed Extension, Patellar, and Autogenic Inhibition.

Flexor reflex

: Withdrawal from stimulus
: Pain-R (free nerve endings in skin)
Afferent Fiber
: Pain fibers
(small diameter, lightly myelinated, slowly-conducting)
: YES (polysynaptic): to same side ventral horn
Efferent Fiber?Effector
r*: A

Crossed Extension reflex

: Opposite side for balance
: Pain-R
(Free nerve endings in skin)
Afferent Fiber
: Pain fibers
(small d, lightly myelinated, slowly-conducting)
: YES (poly-): to contralateral VH
Efferent Fiber?Effector
r*: Activates Extensors

Patellar reflex

: Muscle Stretch
: Sensory Receptor (muscle spindle)
Afferent Fiber
: Excited Sensory n.
: NO (mono-)
Efferent Fiber?Effector
r*: Activates contraction (same muscle), relieves stretching; Inhibits antagonist muscle causing rela

Autogenic Inhibition reflex

: Too much stress on attachment to bone, so there is a rapid, discrete, protective inhibitory reflex. Keeps you from pulling tendons off bones.
: Golgi Tendon Organ (High tension)
Afferent Fiber
: none
: YES (poly-)
Efferent Fi

Polysynaptic reflex

reflex that has at least one interneuron placed between the sensory afferent and the motor efferent, thus having a longer delay between stimulus and response

Monosynaptic Reflex

sensory neuron synapses on ONE motor neuron, spinal reflex, a stretch reflex, fastest reflex(20-40msec), receptor is muscle spindle, All sensory fibers have their cell bodies in the dorsal root ganflia of the spinal cord.

Detection of muscle stretch

� Spiral sensory n + ? motor n ? contraction of Intrafusal muscle fibers (Muscle spindle)
� Hitting patellar tendon ? Stretch sensation by lengthing intrafusal fiber
� This increases the distance btwn sensory coils; reflex to return length to normal

? neuron

This is a neuron that controls the contraction of extrafusal fibers.

? neuron

This is a neuron that controls the contraction of intrafusal fibers that make up the muscle spindle.

?? coactivation

This neuronal control from the brain keeps all fibers the same length so that the spindle is able to detect stretch as well as keeps the muscle sensitive to changes in length and the rate of change.

Visceral Spinal Reflex Arc

(1) Receptor in visceral organ ? (2) Visceral afferent n. (to DRG) ? (3) Interneuron in secondary visceral gray ? (4) pre-ganglionic visceral Efferent neurons in LATERAL horn, which synapses in paravertebral ganglion onto post-ganglionic visceral efferent

Muscle spindle

This is a small connective tissue capsule containing "intrafusal" muscle fibers. Its afferents respond to stretch on the membrane.

Amyotrophic lateral sclerosis (ALS)

This is a motor neuron disease defined by progressive loss of motor function, fatal.
~ 5-10% inherited. 90-95% are Sporadic, causes unknown but may include environmental toxins.
Currently a stem cell trial, collaboration between a Michigan trained neurosu

Spinal motor atropy

This is a motor neuron disease defined by a mutation of SMN1 (survival motor neuron protein); Age of onset varies and symptoms range from severe to mild. Patients may die as infants or survive to old age

Kennedy's disease (X-linked bulbo-spinal atrophy)

This is a motor neuron disease defined by expansion of a CAG repeat in the first exon of the androgen receptor gene
Affects males, female's homozygous for the trait may have mild symptoms
Progressive but not usually fatal.


This is a motor neuron disease defined by having a cyst or cavity form within the spinal cord. This cyst may expand and elongate over time, destroying the spinal cord. Cavitation of the spinal cord frequently damages anterior white commisure which would a

Spinal cord hemisection (Brown-Sequard Syndrome)

This is a motor neuron disease defined by trauma that results in Ipsilateral LMN paralysis at the level of damage (LMN damage); Ipsilateral UMN paralysis below the level of damage (Lat. corticospinal tract); Ipsilateral loss of fine touch, proprioception

Stroke and Multiple Sclerosis

This is a motor neuron disease defined by damage that may occur in the spinal cord as well as the brain. Symptoms depend on location of damage.

Spinal cord anatomy (review)