CH 4: Spinal reflexes & Muscle tone Flashcards
a reflex action consists of:
a specific, sterotyped response to an adequate stimulus
the adequate stimuli for somatic spinal reflexes involves:
input to the SC from sensory receptors in muscles, skin and joints
response of a somatic spinal reflex involves:
contraction of striated skeletal muscle fibers
monosynaptic reflex:
a reflex response mediated by as few as two neurons- 1 afferent and 1 efferent
What is an example of a monosynaptic reflex?
stretch reflex or deep tendon reflex (important in clinical neurology)
polysynaptic reflex:
a reflex mediated by more than 2 neurons
most reflexes involve severeal interneurons in addition ot the afferent and efferent neurons
What is an example of a polysynaptic reflex?
flexor reflex
crossed extensor reflex
segmental reflexes=
those restricted to a single SC segment
intersegmental reflexes=
neurons in several to many SC levels
supraspinal reflexes=
neurons in structures of the brain that influence the SC
what are muscle spindles?
receptor organs within striated muscles
encapsulated structures found in most skeletal muscles of the body
varying in length
vary in number or density in different muscles
what do muscle spindles provide?
provide the CNS with information about the length and rate of change in length of muscles
provide afferent input for stretch reflexes
each muscle spindle contains:
2-12 thin muscle fibers of modified striated muscle
Because the fibers are enclosed within the capsule of the spindle-shaped or ______ they are termed _____ muscle fibers
fusiform bag
intrafusal muscle fibers
*to diferentiate them from the large extrafusal fibers
muscle spindles are attached to:
the CT septa that run between extrafusal fibers
consequently, the entire muscle spindle structure is connected to the muscle’s tendons in parallel with the extrafusal fibers **important in ms spindle function
what does the parallel alignment of intrafusal and extrafusal muscle fibers mean?
the parallel arrangement means that increases in force during muscle contraction induce modest lengthening of the tendon, accompanied by a relative reduction of spindle fiber length and a decline in spindle afferent firing
what are the types of intrafusal fibers?
NUCLEAR BAG
NUCLEAR CHAIN
BAG2
what is a nuclear bag?
the longest and largest type of intrafusal fiber
contains numerous large nuclei closely packed in a central bag
what is a nuclear chain?
shorter and thinner intrafusal fibers
contains a single row or chain of central nuclei
what is bag2?
the third type of intrafusal fibers
intermediary in structure
what motoneurons innervate the contractile ends of both bag and chain fibers?
gamma or beta motoneurons
bag and chain fibers terminate into what 2 types of efferent endings?
1: PLATES (chiefly on nuclear bag fibers and rarely on chain fibers)
2: TRAILS: (mostly on nuclear chain fibers but are frequently found on bag fibers too)
muscle spindles contain abundant _____nerve endings that generate________
sensory nerve endings
generate action potentials with stretching of the intrafusal fibers
what are the 2 types of afferent nerve endings?
1: PRIMARY
2: SECONDARY
primary nerve endings are derived from?
group Ia nerve fibers
the Ia fibers from these endings make monosynaptic contact w/ alpha motoneurons innervating the same (homonymous) muscle ad synergistic (heteronymous) muscles
respond to both the length of muscle and the rate of change in length of muscle
where do primary afferents arise?
the central region of both bag and chain fibers
where do secondary endings arise?
predominantly on nuclear chain fibers and lie to either side of the primary endings
secondary nerve endings are derived from?
group II fibers- that excite homonymous alpha motoneurons monosynaptically
respond to chiefly the length of muscle
alpha, beta and gamma motoneurons of lamina IX in the SC regulate…
skeletal muscle activity
what are alpha motoneurons?
the largest of the anterior horn cells
alpha motoneurons can be stimulated monosynaptically by:
1: the Ia (primary) afferents and II (secondary) afferents of the muscle spindles
2: corticospinal tract fibers in primates
3: lateral vestibulospinal tract fibers
4: recticulospinal and raphe spinal tract fibers
ongoing alpha motoneuron activity is primarily influenced by?
interneurons in the SC gray matter
these interneurons mediate polysynaptic segmental, intersegmental and supraspinal reflexes as well as the activity from additional descending tracts
each alpha motoneuron innervates…
a group of extrafusal muscle fibers within a specific muscle
what is a motor unit?
the functional unit defined by this neuron and its muscle fibers
where are larger motor units?
in proximal muscles- which are used for postural control– many muscle fibers are controlled by a SINGLE motoneuron
where are small motor units?
in distal limb muscles – which are used for finely controlled movements
ex: intrinsic muscle of the hand- each alpha motoneuron may innervate fewer than 10 muscle cells
what are Renshaw cells?
special interneurons in the ventral horn
influence alpha motoneurons
collateral fibers emerging from the axons of the alpha motoneurons excite Renshaw cells.
Renshaw cells, in turn, inhibit alpha motoneurons and can thereby modify the probability of firing these neurons
gamma motoneurons innervate?
intrafusal fibers
gamma motoneurons are also called?
fusimotor neurons
gamma motoneurons differ from alpha motoneurons in 4 ways:
1: don’t innervate extrafusal fibers
2: are regulated rimarily by intersegmental and supraspinal pathways and little by segmental sensory inputs
3: don’t receive inhibitory feedback through Renshaw cells
4: tend to discharge spontaneously often at high frequencies
beta motoneurons innervate..
both intrafusal and extrafusal muscle fibers
- provide a significant fraction of the innervation of muscle spindles
beta motoneurons are comparable in diameter to alpha motoneurons
what is the sensory function of the spindle receptors?
to inform the NS of the length and rate of change in length of the extrafusal fibers
the primary afferents respond to..
both the length of muscle and the rate of change in the length of muscle
the secondary endings respond to..
chiefly muscle length
because muscle spindles are located in parallel with extrafusal fibers, muscle contraction can…
decrease the effective stretch on the spindles and thereby can decrease the sensitivity of the spindle
activation of the gamma motoneurons makes..
the intrafusal muscle fibers contract –> can make the sensory portion of the muscle spindles more responsive to stretch and can maintain the sensitivity of the spindle
in addition to modulating the stretch reflex, gamma motor neurons participate in…
producing and controlling complex movements
what are the 2 types of gamma motoneurons?
1: dynamic gamma motoneuron
2: static gamma motoneuron
what are dynamic gamma motoneurons?
affects the afferent responses to phasic extension more than the responses to static tension
terminate in plate endings solely on nuclear bag fibers
what are static gamma motoneurons?
increases the spindle response to static extension
terminate in trail endings on both bag and chain fibers
information from muscle spindles reaches higher levels of the NS, particularly the..
cerebellum and cerebral cortex
these areas control the descending pathways that facilitate and inhibit both alpha and gamma motoneuron activity.
what is alpha-gamma coactivation?
most supraspinal and intersegmental systems influence the discharge of both alpha and gamma motoneurons innervating a particular muscle
muscle spindles convey information needed for the..
conscious perception of limb position and movement
what is the stretch (myotatic) reflex?
a segmental reflex
stretching a single muscle spindle induces contraction limited to the part of the muscle containing the spindle.
stretching a single or group of muscles therefore produces a discrete response, limited to the same muscle or group of synergistic muscles
this imposes an orderly relation between muscle length change and the resultant force- causes the muscle to act as a compliant spring, an important attribute for posture and gait
what is the deep tendon reflex?
a fractional manifestation of the stretch reflex that can be tested by tapping the tendon of a muscle.
simultaneous stretching of the intrafusal fibers (as the extrafusal fibers are being stretched) stimulates the sensroy endings on the intrafusal fibers, particularly the primary afferents (Ia).
The Ia afferents monosynaptically stimulate the alpha motoneurons that innervate the (quad) muscle and polysynaptically inhibit the alpha mtoneurons of the antagonist muscle group (hamstrings). the quads then suddently contract and the hamstrings relax, causing the leg to extend at the knee
what are GTOs?
golgi tendon organs
consist of encapsulated structures attached in series with the large, collagenous fibers of tendons at the insertions of muscles and along the fascial covering of muscles
within the capsule, sensory nerve endings (Ib) terminate on small bundles of collagenous fibers of tenders.
what afferent fibers come from GTOs?
Ib afferents
what do GTOs do?
when muscle contraction occurs, shortening of the contractile part of the muscle stretches the noncontractile region containing the tendon organs.
this leads to vigorous firing of the afferent fibers innervating the GTOs
these receptors respond primarily to muscle contraction
Their Ib afferent fibers project to the SC, where they polysynaptically inhiit the alpha motoneurons innervating the agonist muscle and facilitate motoneurons of the antagonist muscle.
GTOs provide continuous and accurate information about _____?
muscle force
which both segmental and supraspinal pathways use to control movement
what is muscle tone?
the resistance that an examiner perceives when passively manipulating the limbs of a patient
“resistance to passive manipulation”
not related to conscious effort
what are the 2 general abnormalities of muscle tone?
hypotonia
hypertonia
what is hypotonia?
a decrease of resistance to passive manipulation of the limbs
when does hypotonia occur?
1: immediately if the ventral roots containing the motor neurve fibers to the muscle are cut
2: transection of the dorsal roots that contrain sensory fibers from the muscle
3: disease that affects certain parts of the NS, particularly the cerebellum
reflex activity of the NS maintains and regulates:
muscle tone
muscle tone is NOT a property of isolated muscle
what is hypertonia?
an increase of resistance to passive manipulation
what are the 2 general forms that hypertonia appears?
spasticity
rigidity
what is spasticity?
an increased resistance to externally imposed stretch
sometimes accompanied with a “clasp–knife” inhibition
when does spasticity occur?
disease of the corticospinal and corticobulbar pathways
what is the clasp-knife phenomenon?
begins with a marked increase in resistance to passive manipulation during the initial portion
as manipulation proceeds, the resistance suddenly decreases and disapperas.
bc of activation of GTO and group III and IV afferents
what is rigidity?
a plastic or “cogwheel” type of resistance to passive
often without changes in muscle stretch reflexes
when does rigidity occur?
disease of the basal ganglia
the sensory receptors in the skin and subcutaneous tissue respond to:
touch
pressure
temperature
tissue damage
skin sensory receptors generate signals that alter the activity of spinal motoneurons through _____ and produce _______
through interneurons
produce polysynaptic reflexes
what are 2 examples of polysynaptic responses to aversive stimuli that cause a limb to be withdrawn from a source of injury and a postural compensation to prevent falling?
flexor reflex
crossed extensor reflex
how do the flexor and crossed extensor reflexes work?
example:
a noxious stimulus to a limb results in flexion of the ipsilateral limb and extension of the contralateral limb
when a person steps on something hot, the stimulated extremity withdraws in reflex fashion. this results from the polysynaptic facilitation of alpha motoneurons innervating the ipsilateral flexor muscles and inhibition of motoneurons innervating the extensor muscles of the same leg (FLEXOR REFLEX)
Simultaneously, the opposite limb extends to support the weight of the body. this extension results from facilitation of motoneurons innervating extensor muscles and inhibition of motoneurons innervating flexor muscles (CROSSED EXTENSOR REFLEX)
