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BMB Test 1 > B7.017 Spinal Cord Reflexes > Flashcards

B7.017 Spinal Cord Reflexes Flashcards

1
Q

function of reflexes in a neuro exam

A

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

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2
Q

mixed nerves

A

contain both sensory and motor axons

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3
Q

function of sensory receptors present in mixed nerves

A

provide info about proprioception

are the afferent limb of reflexes

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4
Q

types of sensory receptors in mixed nerves

A

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

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5
Q

what type of sensory fiber is not present in the skin

A

Aa (Ia and Ib)

only present in muscle spindles / GTO

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6
Q

Aa fiber

A 
Ia and Ib
largest
fastest
primary muscle spindles
GTO
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7
Q

AB fiber

A 
II
2nd largest
2nd fastest
secondary muscle spindles
skin mechanoreceptors
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8
Q

Ad fiber

A 
III
2nd smallest
2nd slowest
skin mechanoreceptors
thermal receptors
nociceptors
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9
Q

C fiber

A 
IV
smallest
slowest
skin mechanoreceptors
thermal receptors
nociceptors
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10
Q

how is proprioception achieved

A

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)

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11
Q

function of muscle spindles

A

provide information about muscle stretch

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12
Q

composition of muscle spindles

A 
muscle fibers (intrafusal) innervated by afferent neurons
located throughout muscle bed in parallel with extrafusal fibers (300-400 muscle spindles per limb muscle)
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13
Q

characteristics of muscle spindles

A

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

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14
Q

function of GTO

A

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

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15
Q

composition of GTO

A

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)

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16
Q

2 unconscious pathways that receive muscle spindle info

A 
reflex pathway (all occurs within the spinal cord)
spinocerebellar pathways (smooth movements)
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17
Q

monosynaptic reflex pathway

A

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

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18
Q

polysynaptic reflex pathway

A

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

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19
Q

motor unit

A

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

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20
Q

secure transmission

A

muscle contract every time a motor neuron fires

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21
Q

how many neurons innervates a given muscle cell

A

only one!!!

22
Q

innervation ratio

A

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)

23
Q

motor neuron pool

A

all motor neurons that innervate a single muscle

24
Q

characteristics of motor neuron pools

A
  • 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)
25
Q

2 ways to increase motor neuron force

A
  1. increase firing rate of an individual motor unit

2. increase number of motor units firing

26
Q

what is a motor neuron axons response to stimulation

A

twitch tension response (rise and fall in muscle tension)

27
Q

why does an increased firing rate increase force?

A

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

28
Q

how are new motor units recruited?

A

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

29
Q

recruitment threshold

A

tension at which a motor unit becomes active

30
Q

characterize slow (type 1) motor units

A 
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
31
Q

characterize fast (type 2) motor units

A 
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
32
Q

types of effector organ reflexes

A

somatic reflexes - skeletal muscle

autonomic reflexes - smooth muscle and glands

33
Q

monosynaptic alpha motor neurons that act in reflexes

A

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

34
Q

function of a stretch reflex

A

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

35
Q

sequence of events in a stretch reflex

A
  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)
36
Q

dynamic phase of the reflex

A
  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
37
Q

tonic phase of the reflex

A

important in maintaining muscle tone

cutting the Ia afferent fibers (removing muscle spindle input) greatly reduces tension

38
Q

what do y motoneurons do?

A

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

39
Q

where do y motoneurons innervate

A

contractile portion of the intrafusal fibers (striated)

40
Q

describe the inverse stretch reflex

A

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

41
Q

phases of the inverse stretch reflex

A
  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)
42
Q

what is a flexion reflex (withdrawal reflex)

A

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

43
Q

what is the crossed extensor reflex

A

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

44
Q

muscle tone

A

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

45
Q

2 categories of muscle tone disorders

A

hypotonia / hyporeflexia

hypertonia / hyperreflexia

46
Q

hypotonia

A

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

47
Q

hypertonia

A

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

48
Q

reflexes post spinal cord injury

A

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
49
Q

give an overview of the spinocerebellar tract

A

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)

50
Q

function of the spinocerebellar tract

A

smooth out movements

damage to this pathway results in ataxia

BMB Test 1 (29 decks)

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