B7.017 Spinal Cord Reflexes

Question 1

function of reflexes in a neuro exam

Answer 1

can provide info about both sensory and motor components
do not involve cognitive processes
undergo specific changes following nervous system injury

Question 2

mixed nerves

Answer 2

contain both sensory and motor axons

Question 3

function of sensory receptors present in mixed nerves

Answer 3

provide info about proprioception

are the afferent limb of reflexes

Question 4

types of sensory receptors in mixed nerves

Answer 4

muscle spindles
-length and velocity
Golgi tendon organs
-stress and force at the tendons

Question 5

what type of sensory fiber is not present in the skin

Answer 5

Aa (Ia and Ib)

only present in muscle spindles / GTO

Question 6

Aa fiber

Answer 6

Ia and Ib
largest
fastest
primary muscle spindles
GTO

Question 7

AB fiber

Answer 7

II
2nd largest
2nd fastest
secondary muscle spindles
skin mechanoreceptors

Question 8

Ad fiber

Answer 8

III
2nd smallest
2nd slowest
skin mechanoreceptors
thermal receptors
nociceptors

Question 9

C fiber

Answer 9

IV
smallest
slowest
skin mechanoreceptors
thermal receptors
nociceptors

Question 10

how is proprioception achieved

Answer 10

requires information about degree of muscle stretch (length) and the velocity of stretch during movement (spindles)
also requires information about forces and tensions at tendons (GTO)

Question 11

function of muscle spindles

Answer 11

provide information about muscle stretch

Question 12

composition of muscle spindles

Answer 12

muscle fibers (intrafusal) innervated by afferent neurons
located throughout muscle bed in parallel with extrafusal fibers (300-400 muscle spindles per limb muscle)

Question 13

characteristics of muscle spindles

Answer 13

2 components
-rapidly-adapting (dynamic) component that provides info about velocity of stretch
-slowly-adapting (static) components provide information about the length of the fiber
muscle spindle innervation contributes to muscle tone

Question 14

function of GTO

Answer 14

monitor stresses and forces (tension) at the tendon
contraction of the muscle stretches the GTO fibers, deforming and depolarizing the nerve endings which increases Ib firing

Question 15

composition of GTO

Answer 15

encapsulated structure with elastic fibers that insert on 1% of extrafusal fibers at the tendon
in series with extrafusal fibers
no motor component (Ib afferents only)

Question 16

2 unconscious pathways that receive muscle spindle info

Answer 16

reflex pathway (all occurs within the spinal cord)
spinocerebellar pathways (smooth movements)

Question 17

monosynaptic reflex pathway

Answer 17

stimulus acts on a sensory receptor which transmits the information via sensory neuron to the dorsal horn
synapses with somatic motor neuron in the ventral horn which then acts on a skeletal muscle

Question 18

polysynaptic reflex pathway

Answer 18

same as monosynaptic but interneuron bridges the sensory affector in the dorsal horn and the motor effector in the ventral horn

Question 19

motor unit

Answer 19

alpha motor neuron and all the muscle cells it innervates
final common pathways
lower motor neurons

Question 20

secure transmission

Answer 20

muscle contract every time a motor neuron fires

Question 21

how many neurons innervates a given muscle cell

Answer 21

only one!!!

Question 22

innervation ratio

Answer 22

of fibers innervates / alpha motor neuron
different ratios for different types of muscle
-extra ocular 9:1 (fine control, small loads)
-postural muscle 2000:1 (coarser control, large loads)

Question 23

motor neuron pool

Answer 23

all motor neurons that innervate a single muscle

Question 24

characteristics of motor neuron pools

Answer 24

• occupy localized longitudinal columns in the ventral horn
• form clusters that extend for several spinal segments
• exhibit a medial to lateral organization in the ventral horn (distal muscles are dorsolateral, proximal muscles are ventromedial)

Question 25

2 ways to increase motor neuron force

Answer 25

1. increase firing rate of an individual motor unit | 2. increase number of motor units firing

Question 26

what is a motor neuron axons response to stimulation

Answer 26

twitch tension response (rise and fall in muscle tension)

Question 27

why does an increased firing rate increase force?

Answer 27

twitch tension does not reach baseline before the next twitch occurs (temporal summation) gradually increasing firing rate increases temporal summation which, in turn, increases mean muscle force

Question 28

how are new motor units recruited?

Answer 28

more active motor units = greater muscle force recruited by size during voluntary movements -low muscle tension, small motor units activated -high muscle tension, large motor units activated

Question 29

recruitment threshold

Answer 29

tension at which a motor unit becomes active

Question 30

characterize slow (type 1) motor units

Answer 30

``` small alpha motoneuron slow conduction velocity few, small diameter muscle cells small twitch tension slow contraction speed small extracellular spike size oxidative metabolism (many mitochondria) high capillary supply high fatigue resistance red muscle color small forces for a long time ```

Question 31

characterize fast (type 2) motor units

Answer 31

``` large alpha motoneuron fast conduction velocity more, large diameter muscle cells large twitch tension fast contraction speed large extracellular spike size glycolytic metabolism (few mitochondria) low capillary supply low fatigue resistance white muscle color large forces for a short time ```

Question 32

types of effector organ reflexes

Answer 32

somatic reflexes - skeletal muscle | autonomic reflexes - smooth muscle and glands

Question 33

monosynaptic alpha motor neurons that act in reflexes

Answer 33

all homonymous motoneurons (innervate muscle from which spindle arises) about 50% of heteronymous motoneurons (opposing muscles)

Question 34

function of a stretch reflex

Answer 34

help maintain tone and set the background tension of postural muscles needed for the performance of voluntary movements has two components: -phasic (knee jerk) -tonic

Question 35

sequence of events in a stretch reflex

Answer 35

1. tendon of a muscle is tapped, causing the muscle to stretch 2. sensory endings (Ia and II axons) in the muscle spindles are stimulated 3. central afferent processes synapse directly on the motoneurons of the muscle being stretched 4. these excitatory synapses excite the motoneurons, leading to contraction of that muscle 5. inhibition of the opposing muscle occurs via an inhibitory interneuron (reciprocal innervation)

Question 36

dynamic phase of the reflex

Answer 36

1. Ia afferents detect both velocity and length 2. tendon tap is a powerful velocity stimulus to the spindle (small length change but at high velocity) 3. deep tendon reflexes are mediated largely by Ia afferent input to the spinal cord

Question 37

tonic phase of the reflex

Answer 37

important in maintaining muscle tone | cutting the Ia afferent fibers (removing muscle spindle input) greatly reduces tension

Question 38

what do y motoneurons do?

Answer 38

adjust the length of muscle spindles so they can continuously provide input to the spinal cord as the extrafusal fibers shorted and lengthen adjust the gain of the muscle spindles

Question 39

where do y motoneurons innervate

Answer 39

contractile portion of the intrafusal fibers (striated)

Question 40

describe the inverse stretch reflex

Answer 40

opposite of the stretch reflex, muscle stretch decreases muscle contraction activated by stretching a spastic or rigid muscle mediated by the GTO / Ib afferent

Question 41

phases of the inverse stretch reflex

Answer 41

1. contraction initially increases because of stretch reflex (lengthening muscle, increases Ia activation) 2. at threshold, rigidity "melts away" and the limb extends freely 3. inverse stretch reflex occurs as the rigidity melts away (shuts off alpha motoneuron to release tension)

Question 42

what is a flexion reflex (withdrawal reflex)

Answer 42

initiated by cutaneous stimuli (hot stove, stepping on glass) flexor reflex afferents (free nerve endings, Ad and C fibers) act via polysynaptic reflex on relevant flexor muscles and withdraw them

Question 43

what is the crossed extensor reflex

Answer 43

contralateral component of the flexion (withdrawal) reflex helps maintain balance sensory input crosses the midline to stabilize other limb

Question 44

muscle tone

Answer 44

state of contraction of muscle dependent on & a measure of the excitability of alpha motoneurons assessed clinically using deep tendon reflexes

Question 45

2 categories of muscle tone disorders

Answer 45

hypotonia / hyporeflexia | hypertonia / hyperreflexia

Question 46

hypotonia

Answer 46

diminished deep tendon reflexes, flaccid muscle early stage spinal cord transection, where alpha motoneuron excitability is damaged due to spinal shock / trauma following peripheral nerve injury, ventral horn injury

Question 47

hypertonia

Answer 47

heightened deep tendon reflexes, muscle rigidity (spasticity, clonus) occurs following upper motoneuron lesion

Question 48

reflexes post spinal cord injury

Answer 48

2 stages 1. spinal shock- 1-2 weeks following injury most reflexes are diminished below the level of the lesion 2. heightened reflex activity - over a period of time most reflexes become heightened due to the loss of descending inhibition of lower motoneurons

Question 49

give an overview of the spinocerebellar tract

Answer 49

gets feedback from periphery in the thoracic cord, a DCML neuron will leave the cuneate fasciculus and exit the cord prior to the cuneate nucleus INSTEAD enters Clarke's nucleus (an accessory cuneate nucleus) and synapses travels to cerebellum from Clarke's nucleus (dorsal spinocerebellar tract)

Question 50

function of the spinocerebellar tract

Answer 50

smooth out movements | damage to this pathway results in ataxia