Motor Reflexes Flashcards
What are examples of a cortical reflexes?
Placing reaction and hopping reaction
What are examples of spinal cord reflexes?
Stretch/myotactic, golgi tendon reflex and crossed extensor
What are examples of a brainstem/midbrain reflex?
Vestibular, righting reflex, yawn and eye/head movements
Last three can still occur in anencephalic infants
What is the purpose of a reflex?
Protection, correct action without conscious thought, the need to respond quickly
Infants - need to do something but don’t have voluntary ability
What are some characteristics of reflexes?
Involuntary, fast/quick, short acting, precise
Describe the precision of reflexes
Required in order to be effective
Can make reflexes look like volitional movement
Difference between the two is reflexes are faster than volitional movement
Compare the level of organization between a reflex and volitional motion
Reflex: any CNS level and doesn’t require presence of cortex for most
Volitional motion: requires cortical and subcortical involvement
Compare the purpose for a reflex compared to volitional movement
Reflex: direct and rapid response to sensory stimuli
Volitional motion: response to stimuli, need, desire
Compare the initiation of a reflex to volitional motion
Reflex: sensory input
Volitional: high cognition, sensory input
Compare the circuitry of a reflex to volitional motion
Reflex: fixed
Volitional: variable depending on motion
Compare the speed of a reflex to volitional motion
Reflex: faster than volitional
Volitional: variable depending on need
What is the specificity for a reflex and volitional motion?
Both have high specificity
What is the appearance of a myotactic (stretch) reflex?
Contraction/shortening of a stretched muscle back to its normal length
What is the purpose of the myotactic reflex?
Protect muscles from tearing due to stretch
What are the characteristics of a myotactic reflex?
Monosynaptic segmental reflex
Requires passive stretch of the muscle
Initiated by muscle spindle
Describe a muscle spindle
Found within skeletal muscle embedded in a fusiform capsule
In parallel to the muscle fibers
Contains both afferent (sensory) and efferent (contractile) components
The capsule of the muscle spindle is fusiform, the fibers within the capsule are called what?
Intrafusal fibers
Describe characteristics of the sensory component of the muscle spindle
Is not contractile
Is the portion sensitive to length
Is actually two sensors with different afferents including nuclear bag fiber and nuclear chain fiber
Innervated by primary and secondary afferents
What are the primary afferents to the sensory component of the muscle spindle?
Via Ia fiber which innervates both the nuclear bag and nuclear chain
Large myelinated Ia fiber (high conduction velocity, low threshold)
Sensitive to both the length of the muscle and how fast the length is changing
Describe the secondary afferents to the sensory component of the muscle spindle
Via smaller myelinated group II fiber which innervates the nuclear chain fiber only
Sensitive to only the length of the muscle
Within the SC the Ia afferent from the muscle spindle synapse directly onto what?
The alpha motor neuron innervating the stretched muscle
The motor neuron is excited by the activation of the Ia afferents -> contraction relieves the stretch returning the Ia discharge rate back to normal
What occurs in the antagonist muscle during the myotactic reflex?
Stretches during this reflex
Ia afferent branches and synapses onto an interneuron of the same segment which is inhibitory (GABA or glycine) —> causes ipsp’s in the antagonist muscle (relaxation) while the agonist contracts
Describe alpha motor neurons
Large heavily myelinated fibers
Innervate skeletal muscle via NMJ (extrafusal fibers)
Responsible for activating muscle
Activity directly leads to motion
Describe gamma motor neurons
Slightly smaller and slower than alpha but still fast overall
Innervates the contractile component (intrafusal fibers) of the muscle spindle via NMJ
Activity causes contraction but does not directly lead to motion
Controls sensitivity of muscle spindle
What is the appearance of the golgi tendon reflex?
Sudden/abrupt relaxation of contracted muscles
What is the purpose of the golgi tendon reflex?
Protect muscle from damage due to excessive force
What are some characteristics for the golgi tendon reflex?
Requires active contraction of muscle
Initiated by the golgi tendon organ
Polysynaptic segmental reflex
Describe the golgi tendon organ
Innervates tendons
Bare nerve ending with lots of branches
APs increase with tendon (generated by contraction of muscle)
Ib fiber taking into the spinal cord (slightly smaller diameter than Ia, slight decrease in myelin, slower conduction velocity but higher threshold)
Describe the pathway for the golgi tendon reflex
Ib afferent from golgi tendon release EAA —> synapses on GABA-nergic interneuron -> synapse with alpha motor neuron of contracting muscles -> motor neuron is inhibited by the activation of the interneuron -> abrupt relaxation of the muscle occurs returning the golgi tendon organ discharge rate back to normal
Describe central modification
The higher centers of the brain are often seen to be inhibitory to the reflexes but certain regions do provide an excitatory input
+ = alpha motor neuron
- = nociceptors and mechanoreceptors
Describe the brainstem facilitatory region
Is spontaneously active
By activating the gamma motor neurons the muscle spindle becomes more sensitive
Describe the brainstem inhibitory region
Requires activation from cortical regions
Inhibits gamma motor neurons making the muscle spindle less sensitive
Describe spasticity
With the loss of the cortex, the brainstem inhibitory regions are not activated leaving the brainstem facilitatory region to dominate
The end result of the uncontrolled brainstem facilitatory region is stretch reflexes that fight any passive motion (spasticity)
When a spinal transection occurs, even though the neurons producing the reflex are below the level of the transection (and therefore intact) what happens to the reflex?
The reflex fails to occur (spinal shock)
What causes spinal shock?
Hyperpolarization of spinal neurons due to loss of excitatory input from cortex
Recovery of reflexes can sometimes occur after a spinal transection and this is believed to result from what?
Axonal sprouting below the level of the transection
Expression of receptor phenotypes that are self activating (5HTC receptor)
Decerebate posturing is the result from what?
Loss of all structures rostral to the pons (caudal to red nucleus)
Describe the rigidity that occurs in decerebate posturing
Resists motion in all directions
Results from maintained muscle contraction
Continual activation of alpha motor neurons By the brainstem
Describe spasticity during decerebate posturing
Resists motion in a given direction
Myotactic reflex hyperactive
Continual activation of gamma motor neurons via brainstem which contracts intrafusal muscle and lengthens muscle bag and chain
Describe decorticate posturing
Flexion of the upper limb joints (disinhibition of red nucleus and its control of UE flexors)
Extension of the lower limbs (disinhibition of reticulospinal and vestibulospinal pathways)
Dependent on head position (release of postural reflexes)
Describe unilateral decorticate posturing
Results from stroke in the vicinity of the internal capsule
A relatively common site for a stroke
