Motor Control: Reflexes Flashcards
What are brainstem/midbrain reflexes?
Vestibular Righting reflex Suckle Yawn Eye/head movements
What are spinal reflexes?
Stretch (myotactic)
Golgi tendon reflex
Crossed extensor
What is an importance difference between a reflex movement and voluntary movement?
Reflex movement occurs faster than the fastest voluntary motion
Distinguish between reflex activity and volitional movement
Reflex activity:
Precise motions in response to afferent stimuli
Mediated at all levels of CNS
Rapid initiation
Many elicited even during unconsciousness
Volitional activity
Originates in cortical areas associated with judgement, initiative, and motor control
Longer onset latency due to processing
Require conscious awareness
Describe myotatic reflex. Appearnce, purpose, and characteristics
Appearance: contraction (shortening) of a stretched muscle
Purpose: protect muscle from tearing due to stretch
Characteristics: initiated by muscle spindle. Monosynaptic (afferent fibers synapse directly onto efferent), segmental reflex
Describe muscle spindle
Parallel to extrafusal fibers
Several different types of fibers within muscle spindle
Afferent (sensory) and efferent (muscle
Intrafusal fibers of the muscle spindle have both motor and sensory components
Describe the sensory portion of a muscle spindle
Is not contractile
Is the portion sensitive to length
Is actually two sensors with different afferents: nuclear bag fiber and nuclear chain fiber
Describe the primary afferent of a muscle spindle
Innervates both nuclear bag and nuclear chain
Large, myelinated Ia fiber
Sensitive to both:
-length of muscle (increases in length cause increases in firing rate of neurons)
-how fast length is changing (faster the change, the greater the firing rate)
At rest, there is some discharging with the neurons
How are action potentials generated for primary afferents in muscle spindle?
They are generated when deformation (stretching) opens a member of the epithelial sodium channel (ENaC/amiloride sensitive degenerin family). Colocalization of this channel with a neuronal form of the acid-sensitive sodium channel (ASIC2) has also been found.
Describe the secondary afferent for a muscle spindle
Smaller, myelinated group II fiber
Innervates only nuclear chain fiber
Sensitive only to length of muscle
Compare and contrast the functions of the primary and secondary afferents associated with the muscle spindle
Primary:
Large Ia fiber (heavily myelinated, fast velocity, low threshold)
Innervate both nuclear bag and nuclear chain fiber
Detects length and change in length
Secondary
Group II fiber (smaller diameter, less myelin, still pretty fast)
Innervates only nuclear chain fiber
Detects only length
Describe the motor portion of the muscle spindle
Intrafusal contractile elements
Same as skeletal muscle
Innervated by a gamma motorneuron
Control the length of the sensory portion
By contracting the intrafusal muscles, the sensory portion stretches. This renders the sensory portion more sensitive to a superimposed stretch
Compare and contrast the functions of the alpha and gamma motorneurons
Alpha: Large, heavily myelinated fiber Innervates (via NMJ) skeletal muscle Responsible for activating muscle Activity directly leads to motion
Gamma:
Slightly smaller, slower than alpha. Still fast overall
Innervates contractile component of the muscle spindle via NMJ
Activity causes contraction
Controls sensitivity of muscle spindle
Activity does NOT directly lead to motion
What channel/receptor is found on the motor end plate?
Nicotinic cholinergic receptors
What ion directly produces Ach release from alpha-motoneurons?
Calcium
An axon enters spinal cord via dorsal root. Its cell body is located in dorsal root ganglion. What is its most likely function?
Afferent neuron relaying information about muscle length to brain
A patient accidentally ingests a substance that enzymatically cleaves VAMP, a protein that is required for vesicular fusion in alpha motoneurons. What is the effect of this cleavage?
Decreased Ach release from terminal
You accidentally touch a hot iron with your right hand. The withdrawal reflex is activated. When do you feel pain: before or after withdrawing hand? What would be the consequence if you had to think about the act of withdrawing your hand?
Most people feel pain concurrently with withdrawing the hand or slightly after the hand is withdrawn.
Fibers conveying the thermal/pain information reaches the spinal cord, while afferent information continues up to thalamus and cortex in order to be processed into sensation of pain.
Since motor output causing the withdrawal is conveyed using large neurons, it beats signal conveying sensation to brain
If you had to think about withdrawing your hand, there would be more damage to the hand
What is the difference between alpha and gamma motoneurons? Group Ia and Ib or Group II?
alpha motoneuron is bigger (and has more myelin) than gamma.
Group I>II>III>IV
Ia>Ib
IV not myelinated. III only sparsely
Touch left thumb to left shoulder. Agonist muscle is biceps. As muscle is contracting, what happens to length of muscle spindle? What is the reflex effect? What effect will this have on ability to touch shoulder?
As muscle length shortens, muscle spindle also shortens.
By shortening muscle spindle, action potential number traveling up Ia afferent decreases. This will lead to less activation of alpha motoneuron.
With less activation of the alpha motoneuron, biceps are less contracted, slowing or preventing me from touching my shoulder.
Describe reciprocal inhibition
Full reflex contains a second efferent limb.
In order for muscle length of stretched muscle to change, the antagonist muscle must also chnage.
Ia afferent also activates as interneuron in spinal cord that inhibits alpha motoneuron innervating antagonist muscle.
Describe control of muscle spindle sensitivity.
One function of gamma motoneurons (they also innervate intrafusal contractile elements)
When intrafusal fibers contract, they stretch muscle spindle.
By stretching, nuclear bag and nuclear chain are sensitized to additional stretching.
Describe golgi tendon reflex. Appearnce, purpose, characteristics
Appearance: sudden (abrupt) relaxation of a contracted muscle
Purpose: protect muscle from damage due to excessive force
Characteristics: initiated by golgi tendon organ. Polysynaptic, segmental reflex
Describe golgi tendon organs
Innervate tendon
Bare nerve ending with lots of branches
Action potentials increase with tension (mech to open sodium channels not known)
Ib fiber to spinal cord
Compare and contrast functions of the muscle spindle and golgi tendon reflexes
Myotactic reflex Initiated by muscle spindle Passive stretch of muscle Monosynaptic reflex Contraction of stretched muscle back to normal length
Golgi tendon (reverse myotatic) Initiated by golgi tendon Active contraction of muscle Polysynaptic reflex Abrupt relaxation of contracted muscle to prevent damage
Describe spinal shock and recovery from spinal shock
Even though neurons producing reflex are below level of transection (and are intact), the reflex fails to occur
Recovery can result from:
Axonal sprouting below level of transection
Expression of receptor phenotypes that are self-activating (5HTC receptor). This causes persistent influx of calcium into motoneuron. If input from the brain is reestablished, this aids in motor recovery. If input cannot be reestablished, muscle spasms result.
What causes decerebrate posturing?
Results from loss of all structures rostral to the pons
Describe rigidity
Resists motion in all directions
Results from maintained muscle contraction
Continual activation of alpha-motoneurons
Not due to reflexes
Brainstem is activating alpha-motoneurons, but not clear why
Cutting afferents does not affect rigidity
Describe spasticity
Resists motion in a given passive direction
Myotatic reflex hyperactive
Continual activation of gamma-motoneurons
Contracts intrafusal muscle
Lengthens nuclear bag/chain fibers
Brainstem
This leads to increased sensitivity to stretch reflex
Describe brainstem facilitatory region, brainstem inhibitory region, and spasticity.
Facilitatory region: by activating gamma-motoneurons, makes muscle spindle more sensitive. This region is spontaneously active.
Inhibitory region: inhibits gamma motoneurons, making muscle spindles less sensitive. This region requires activation from cortical regions.
With the loss of the cortex, the brainstem (-) region is not activated, leaving the (+) region to dominate
The end result of the uncontrolled brainstem facilitatory region is stretch reflexes that fight any passive motion -> spasticity
Identify occurrence and cause of spasticity or rigidity
Spasticity
Patient resists a passive stretch of their muscles
Contraction doesn’t start until stretch occurs
Hyperactive myotatic reflex due to increased gamma motoneuron firing
Cause: damage to cortex that abolishes activation of brainstem inhibitory region (brain arousal systems)
Rigidity
Contraction of muscles in absence of other stimuli
Alpha-motoneurons continually active
Cause: loss of cortical influence that inhibits a medullary input to alpha motoneurons
Describe decorticate posturing
Flexion of upper limb joints
Extension of lower limbs
Dependent on head position
Unlike decerebrate posturing, decorticate posturing is dependent on head position and will change as the head is moved passively
Describe unilateral decorticate posturing
Results from strokes in vicinity of internal capsule
Relatively common site for a stroke
Identify spinal shock, decorticate, and decerebrate posturing and their lesions
Spinal shock
Transection of spinal cord
All reflexes abolished, even if circuit is intact
Cause: hyperpolarization of spinal neurons due to loss of excitatory input from cortex
Decorticate posture
Internal rotation of legs in extended position, flexion of arms, dependent on head position
May be bilateral or unilateral
Loss of cortical inputs -> most commonly caused by lesion of internal capsule in humans
Decerebrate posture
Contraction of all anti-gravity muscles (arms in extension)
Loss of input from all structures rostral to pons
Usually bilateral and indicative of severe brain injury
Which of the following reflexes requires cortical input? A. Withdrawing hand from hot stove B. Turning head towards a sound C. Hopping reaction D. Turning eyes towards a motion E. The righting response
C. Hopping reaction
What innervates intrafusal muscle fibers (contractile elements?)
Gamma-motoneuron
Dr. Karius tries to lift 47 cases of diet Pepsi into her car in one lift. A reflex is elicited, resulting in complete relaxation of her muscles. Activation of what fiber type produced this response? How many synapses in the spinal cord are required for this reflex? What receptor is most likely found on the alpha-motoneuron as part of this reflex?
Ib fiber
2
Glycine
What are cortical reflexes?
Placing reaction
Hopping reaction
