spinal reflexes - jullet Flashcards
What are alpha motor neurons controlled by?
interneurons tha pattern particular types of movements
How is the gray matter of the spinal cord segregated functionally?
LATERAL horn: autonomic nervous system. DORSAL horn: sensory neurons. VENTRAL horn: motor neurons (Aa, Ag). INTERMEDIATE GM: interneurons that activate a-motor neuronsLATERAL horn: autonomic nervous system. DORSAL horn: sensory neurons. VENTRAL horn: motor neurons (Aa, Ag). INTERMEDIATE GM: interneurons that activate a-motor neuronsLATERAL horn: autonomic nervous system. DORSAL horn: sensory neurons. VENTRAL horn: motor neurons (Aa, Ag). INTERMEDIATE GM: interneurons that activate a-motor neuronsLATERAL horn: autonomic nervous system. DORSAL horn: sensory neurons. VENTRAL horn: motor neurons (Aa, Ag). INTERMEDIATE GM: interneurons that activate a-motor neurons
How does the motor cortex, brainstem, basal ganglia, and cerebellum contribute to movement?
MOTOR CORTEX: planning, initiating, directing voluntary movements. BRAINSTEM: basic movements and postural control. BASAL GANGLIA: gating proper initiation of movement. CEREBELLUM: sensory motor coordination
What is the lumbar cistern?
space surrounding the dural sac - where CSF is collected during lumbar puncture
Where is the gray matter of the spinal cord most prominent?
in regions connecting to the limbs - because all of the motor/sensory machinery for the limbs are located in the gray matter of these regions
Whata are the cervical and lumbar enlargements?
regions in which the gray matter of the spinal cord is most prominent (also where the limbs are connected) because all of the motor/sensory machinery for the limbs are located in the gray matter of these regions
How does the white matter tracts change as you descend the spinal cord? Why do you get this patterning?
it gradually DECREASES because 1) the ASCENDING SENSORY TRACTS are added at at each successive level as one ascends the cord, and 2) the MOTOR tracts are terminating as one descends the cord
What combines to form the spinal nerve?
ventral and dorsal roots
What does the ventral root of the spinal cord contain?
1) a/g motor neurons, 2) preganglionic sympathetic fibers that arise from the lateral horn of the T1-L2 spinal cord
What type of nerves are motor fibers? Sensory?
MOTOR: Aa, Ag. SENSORY: IA, IB
Where would you normally find Aa fibers? Ag fibers?
Aa: extrafusal (on muscle fibers). Ag: intrafusal (on muscle spindle)
Where would you normally find IA fibers? IB fibers?
IA: muscle spindle. IB: golgi-tendon organ
What is the difference between IA and IB fibers?
IA: faster, on muscle spindles. IB: slower, on golgi-tendon organ
What type of neurons are responsible for most of the activation/control of a-motor neurons? Where can they be found?
interneurons - located in the intermediate gray matter of the spinal cord
T/F Muscles receive innervation from a single spinal segment and nerve root.
False! Most muscles receive innervation from several spinal segments and nerve roots
What is the somatotopic organization of the lower motor neurons in the ventral horn at the cervical level of the spinal cord? (think proximal vs distal and flexor vs extensor muscles)
PROXIMAL muscles are located more medially while DISTAL muslces are located more laterally on the ventral horn. FLEXOR muscles are located more dorsally while EXTENSOR muscles are located more ventrally.
What is a motor unit?
Motor neuron and all of the muscle fibers that it innervates (each motor neuron branches to supply multiple muscle fibers distributed over significant portions of the muscle)
What happens when you gradually increase the force of contraction in a muscle?
motor units are recruited in sequence (rather than all of it activated at once), which allows the muscle contraction to slowly build up to a maximum where all motor units within the muscle are firing at a relatively high rate
What are the three types of muscle fibers?
Type I (red), and Type II (white), and Intermediate (white in appearance)
What muscles predominate when you’re doing things that require little strength, but much endurance? What muscles predominate for powerful movements?
Little strength/high endurance: Type I (Red). Powerful: Type II (White)
How is type I and type II muscle fibers different in terms of size, speed, aerobic metabolism, and fatigue?
TYPE I (RED): small, moderate speed, high metabolism, and fatigue-resistant. TYPE II (WHITE): large, very fast speed, low metabolism, fatigues easily
What are intermediate fibers?
Like the type II muscle fiber, but is fatigue-RESISTANT (type II fibers fatigue easily)
As one goes from standing to jumping, what type of muscle fibers are recruited?
recruited in sequence: TYPE I, INTERMEDIATE - fatigue resistant, TYPE II - fatigue easily
What are 3 classic spinal cord reflexes? What patterns the response of these reflexes?
1) Myotactic, 2) Inverse Myotactic, 3) Flexion reflex. Patterned by interneurons, which control the motor neurons
What is the myotactic reflex?
muscle stretch reflex (aka deep tendon reflex) because it entails striking a tendon to stretch the muscle, which results in CONTRACTION of that muscle
What is the inverse myotactic reflex?
reflex that occurs only when the muscles are stretched to the point of injury, results in RELAXATION of the muscle
What is the flexion reflex?
withdrawal reflex to a painful stimulus
What are the two types of muscle receptors?
1) muscle spindle, 2) golgi-tendon organ
What are muscle spindles?
sensory receptors within the belly of a muscle, which primarily detects changes in the length of the muscle; aligned parallel to extrafusal muscle fibers
What are golgi-tendon organs?
located at the origins/insertion of skeletal muscle fibers into the tendons of skeletal muscles.
What type of fibers are found in muscle spindles?
intrafusal fibers
What type of fibers are found in the bulk of muscle fibers?
extrafusal fibers
What are the intrafusal fibers innervated by?
gamma motor neurons
What are the extrafusal fibers innervated by?
alpha motor neurons
What happens if the intrafusal fibers are contracted?
no overall movement because the intrafusal fibers are far too weak
What happens if the ends of the intrafusal fibers are contracted?
the central portion of the intrafusal fiber is stretched, and since this central portion is wrapped by spindle afferent 1A fibers, this these fibers are deformed and the muscle spindle afferent nerve fibers are activated.
What two events will cause the muscle spindle afferent 1A nerve fibers to be activated?
1) stretch of the muscle itself (extrafusal fibers) and 2) activation of the gamma motor neurons, which will cause the muscle spindle to contract
In the myotactic reflex, what happens when you strike the patellar tendon?
1) quadriceps stretches and causes the annulospiral ending in the spindle to be activated. 2) signal is sent along the 1A afferent nerve fiber back to the dorsal root of the lumbar spinal cord. 3) nerve synapses directly onto motor neurons going back to the quadriceps, activating it. 3) nerve also synapses onto interneurons, which inhibit antagonist muscles & activate other muscles that are agonist to the movement.
What is the purpose of the myotactic reflex?
to stabilize the limb in a particular position and to prevent unintended movement
What is the effect of an active stretch on a muscle? (consider both extrafusal and intrafusal fibers)
The intrafusal and extrafusal fibers are stretched, which results in a BURST OF activity (APs) in the annulospiral endings in the spindle afferent nerve, followed by a new level of stretch with a NEW BASELINE firing pattern
What is the effect of contraction on a muscle? (consider both extrafusal and intrafusal fibers)
The intrafusal and extrafusal fibers are shortened. The muscle spindle is “UNLOADED” and this lack of stretch decreases the firing in the spindle afferent nerve and this makes the muscle spindle UNABLE TO DETECT any further stretch of the muscle from this new position. BUT the gamma motor neurons are activated, which can contract the ends of the intrafusal fibers such that the muscle spindle is aligned with the new position of the muscle, thus restoring its ability to detect any stretch from this new position
During muscle contraction, both the extrafusal and intrafusal fibers shorten, which renders the spindle unable to detect any further stretch. How does the body compensate for this?
gamma motor neurons are activated, which can contract the ends of the intrafusal fibers such that the muscle spindle is aligned with the new position of the muscle, thus restoring its ability to detect any stretch from this new position
What is alpha-gamma coactivation?
activation of both ALPHA motor neuron to the extrafusal fiber and gamma motor neuron to the intrafusal fibers so that the muscle spindle tension is aligned with the new muscle length (allows the muscle spindle to respond to any deviations from this new length)
What is the purpose of spindles?
convey info about the state of the extrafusal fibers to the CNS to inofrm our brain of how our body is moving in space
What is the purpose of the gamma loop?
an efficient, but slow mechanism by which movements or changes in position occur
How does the gamma loop work?
1) gamma motor neurons are activated and causes the intrafusal fibers to contract, resulting in activation of th annulospiral endings/afferent nerve 1A fibers 2) motor neurons to extrafusal fibers activated - causes contraction of agonist muscles (takes tension off intrafusal fibers) and relaxation of antagonist muscles
What does spasticity result from?
overactivity of gamma motor neurons to the muscle spindles; may be due to damage to discending projections from the brainstem that normally inhibit gamma motor neurons.
What happens if there is overactivity of the gamma motor neurons to the muscle spindles?
hyper-sensitive stretch reflex: greater resistance to passive movement and even greater resistance to faster movements
What purpose do golgi-tendon organs serve? Where are they normally found?
GTOs signal muscle tension; more contraction in the muscle = more signal generated in the afferent IB fiber. Exists in the CT that attaches the muscle fiber to the muscle tendon.
When are golgi-tendon organs normally activated?
when the limit of muscle compliance is reached (ie when the CT of the muscle is about to tear), the GTOs are massively activated
What happens to the golgi-tendon organs when muscle fibers contract to the limit of compliance?
tension in placed on the CT, mechanically activating the 1B afferent nerve fiber
What reflex is the golgi-tendon organ involved in?
inverse myotactic reflex
What activates the inverse myotactic reflex?
GTO activation of IB afferent fibers (usually near the point of muscle tear) sends information back to the spinal cord, which causes inhibition of motor neurons to that muscle (agonist, relaxation) and activation of antagonist muscles (contraction)
What happens if someone dropped a 50 pound bar onto your right arm?
Biceps would initially flex to hold onto the 50lb bar, but after a while it can’t sustain the 50lb bar, the GTOs get activated, causing a net inhibition of agonist muscles (biceps, brachialis) and activation of antagonist muscles (triceps), causing a net extension of the arm.
What is the flexion reflex? What does it cause?
withdrawal reflex to a painful stimulus. Causes physiologic FLEXION of ipsilateral limb nand physiologic EXTENSION of contralateral limb
What is a possible cause for an overactive flexion reflex? What happens in the case of an overactive flexion reflex? What is one test for this?
Spasticity. CAUSE: disturbances in corticospinal tract. TEST: babinski sign: stroking the lateral side of the sole of the foot will tend to result in extension of the great toe)
What is responsible for patterning reflexes and routine/repetitive movements? Where are these normally found?
interneurons. LOCATION: 1) intermediate gray matter of the spinal cord, 2) brain stem (reticular formation)
