B7.017 Spinal Cord Reflexes Flashcards
function of reflexes in a neuro exam
can provide info about both sensory and motor components
do not involve cognitive processes
undergo specific changes following nervous system injury
mixed nerves
contain both sensory and motor axons
function of sensory receptors present in mixed nerves
provide info about proprioception
are the afferent limb of reflexes
types of sensory receptors in mixed nerves
muscle spindles
-length and velocity
Golgi tendon organs
-stress and force at the tendons
what type of sensory fiber is not present in the skin
Aa (Ia and Ib)
only present in muscle spindles / GTO
Aa fiber
Ia and Ib largest fastest primary muscle spindles GTO
AB fiber
II 2nd largest 2nd fastest secondary muscle spindles skin mechanoreceptors
Ad fiber
III 2nd smallest 2nd slowest skin mechanoreceptors thermal receptors nociceptors
C fiber
IV smallest slowest skin mechanoreceptors thermal receptors nociceptors
how is proprioception achieved
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)
function of muscle spindles
provide information about muscle stretch
composition of muscle spindles
muscle fibers (intrafusal) innervated by afferent neurons located throughout muscle bed in parallel with extrafusal fibers (300-400 muscle spindles per limb muscle)
characteristics of muscle spindles
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
function of GTO
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
composition of GTO
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)
2 unconscious pathways that receive muscle spindle info
reflex pathway (all occurs within the spinal cord) spinocerebellar pathways (smooth movements)
monosynaptic reflex pathway
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
polysynaptic reflex pathway
same as monosynaptic but interneuron bridges the sensory affector in the dorsal horn and the motor effector in the ventral horn
motor unit
alpha motor neuron and all the muscle cells it innervates
final common pathways
lower motor neurons
secure transmission
muscle contract every time a motor neuron fires
how many neurons innervates a given muscle cell
only one!!!
innervation ratio
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)
motor neuron pool
all motor neurons that innervate a single muscle
characteristics of motor neuron pools
- 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)
2 ways to increase motor neuron force
- increase firing rate of an individual motor unit
2. increase number of motor units firing
what is a motor neuron axons response to stimulation
twitch tension response (rise and fall in muscle tension)
why does an increased firing rate increase force?
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
how are new motor units recruited?
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
recruitment threshold
tension at which a motor unit becomes active
characterize slow (type 1) motor units
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
characterize fast (type 2) motor units
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
types of effector organ reflexes
somatic reflexes - skeletal muscle
autonomic reflexes - smooth muscle and glands
monosynaptic alpha motor neurons that act in reflexes
all homonymous motoneurons (innervate muscle from which spindle arises)
about 50% of heteronymous motoneurons (opposing muscles)
function of a stretch reflex
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
sequence of events in a stretch reflex
- tendon of a muscle is tapped, causing the muscle to stretch
- sensory endings (Ia and II axons) in the muscle spindles are stimulated
- central afferent processes synapse directly on the motoneurons of the muscle being stretched
- these excitatory synapses excite the motoneurons, leading to contraction of that muscle
- inhibition of the opposing muscle occurs via an inhibitory interneuron (reciprocal innervation)
dynamic phase of the reflex
- Ia afferents detect both velocity and length
- tendon tap is a powerful velocity stimulus to the spindle (small length change but at high velocity)
- deep tendon reflexes are mediated largely by Ia afferent input to the spinal cord
tonic phase of the reflex
important in maintaining muscle tone
cutting the Ia afferent fibers (removing muscle spindle input) greatly reduces tension
what do y motoneurons do?
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
where do y motoneurons innervate
contractile portion of the intrafusal fibers (striated)
describe the inverse stretch reflex
opposite of the stretch reflex, muscle stretch decreases muscle contraction
activated by stretching a spastic or rigid muscle
mediated by the GTO / Ib afferent
phases of the inverse stretch reflex
- contraction initially increases because of stretch reflex (lengthening muscle, increases Ia activation)
- at threshold, rigidity “melts away” and the limb extends freely
- inverse stretch reflex occurs as the rigidity melts away (shuts off alpha motoneuron to release tension)
what is a flexion reflex (withdrawal reflex)
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
what is the crossed extensor reflex
contralateral component of the flexion (withdrawal) reflex
helps maintain balance
sensory input crosses the midline to stabilize other limb
muscle tone
state of contraction of muscle
dependent on & a measure of the excitability of alpha motoneurons
assessed clinically using deep tendon reflexes
2 categories of muscle tone disorders
hypotonia / hyporeflexia
hypertonia / hyperreflexia
hypotonia
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
hypertonia
heightened deep tendon reflexes, muscle rigidity (spasticity, clonus)
occurs following upper motoneuron lesion
reflexes post spinal cord injury
2 stages
- spinal shock- 1-2 weeks following injury most reflexes are diminished below the level of the lesion
- heightened reflex activity - over a period of time most reflexes become heightened due to the loss of descending inhibition of lower motoneurons
give an overview of the spinocerebellar tract
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)
function of the spinocerebellar tract
smooth out movements
damage to this pathway results in ataxia
