Motor Control: Reflexes Flashcards
Cortical Reflexes
- require cerebrum, but not usual parts involved in voluntary motion
- placing rxn = hold baby in air, contact with surface will cause them to make weight bearing motion with surface
- hopping rxn = unexpected shove > hop on side to keep from falling over
Spinal Reflexes
- stretch/myotatic = muscle contraction in response to stretching; keeps muscle at a constant length
- Golgi tendon = inhibitory effect on muscle resulting from muscle tension stimulating Golgi tendon organs to prevent damage
- crossed extensor = contralateral limb compensates for loss of support when the ipsilateral limb withdraws from painful stimulus in a withdrawal reflex
Brainstem/Medulla Reflexes
vestibular, righting reflex
suckle, yawn, eye/head movements (still seen in anencephalic babies)
Reflexes
Function: protection, correct actions without conscious thought, quick response needed, needed for infants
Characteristics: involuntary, fast, short, precise, direct
Level of organization: any CNS level; don’t need cortex most of the time
Initiation: sensory input
Circuitry: fixed
Speed: fast
Specificity: high
Volitional Motion
Level of organization: needs cortical and subcortical involvement
Purpose: response to stimuli, need, desire
Initiation: higher cognition, sensory input
Circuitry: variable depending on motion
Speed: variable depending on need
Specificity: high
Myotatic Reflex
- stretch reflex
- contraction/shortening of stretched muscle
- protects muscle from tearing due to stretch
- initiated by muscle spindle
- monosynaptic and segmental
- passive stretch of muscle
- contraction of stretched muscle back to normal length
Muscle Spindle
- detects muscle stretch
- within skeletal muscle in fusiform capsule
- parallel to muscle fibers
- contains afferent and efferent parts
Intrafusal Fibers
- fibers within capsule
- motor and sensory parts
Extrafusal Fibers
- muscle fibers making up remainder of muscle
- working part of muscle
Sensory Part of Muscle Spindle/Intrafusal Fiber
- not contractile
- sensitive to length
- two sensors with different afferents: nuclear bag fiber and nuclear chain fiber
1A Fibers
- primary afferents
- dynamic fiber
- innervates both nuclear bag and chain fibers
- large, myelinated to increase conduction velocity and reduce threshold
- sensitive to length of muscle and how quickly muscle length is changing (changes # of APs fired)
1B Fibers
- secondary afferent
- slightly smaller and myelinated
- lesser conduction velocity and sensitivity so increased threshold
- innervates only nuclear chain fiber
- sensitive to length of muscle only
Motor Part of Intrafusal Fibers - Myotatic Reflex
- striated like skeletal muscles
- innervated by gamma motor neuron
- control the length of the sensory portion of the intrafusal fiber (controls sensitivity to additional length changes)
- smaller diameter, less myelin, lower conduction velocity, increased threshold
Contraction of Intrafusal Fibers
stretches sensory portion and makes it more sensitive to superimposed stretch
Gamma Motoneuron Control of Sensitivity
- contraction of intrafusal contractile fibers stretches sensory portion
- increases sensitivity of 1a and II fibers to stretch
- overall length of muscle spindle stays same
Alpha Motoneurons
- large, heavy myelination
- innervates skeletal muscle extrafusal fibers via NMJ
- activates muscle
- activity leads directly to motion
Gamma Motoneurons
- slightly smaller, slower than alpha
- still fast
- innervates contractile component (intrafusal fibers) of muscle spindle via NMJ
- activity causes contraction
- controls sensitivity of muscle spindle
- activity doesn’t directly lead to motion
Golgi Tendon Reflex
- reverse myotatic
- autogenic inhibition
- initiated by golgi tendon organ
- active contraction of muscle
- polysynaptic reflex (inhibitory)
- segmental
- abrupt relaxation of contracted muscle to prevent damage from excess force
Synapses of Myotatic Reflex
- within spinal cord, 1A afferents from muscle spindle synapses directly onto alpha-motoneuron innervating the stretched muscle
- releases EAA > triggers EPSP in alpha motoneuron > more action potentials > contraction in stretched muscle > 1a discharge rate returns back to normal
- 1A afferent from muscle spindle also synapses on interneuron that releases GABA or glycine
- causes IPSP in alpha motoneuron of antagonist muscle and thus fewer action potentials (relaxation and lengthening)
Golgi Tendon Organs
- innervate tendon
- bare nerve endings with many branches
- action potentials increase with tension
- tension generated by contraction of attached muscles
- 1B fiber to spinal cord
Synapses of Golgi Tendon Reflex
- 1b afferent from golgi tendon releases EAA at interneuron
- spinal interneuron releases GABA
- causes IPSP in alpha motoneuron of contracting muscle
- fewer APs causes less tension in muscles
- abrupt relaxation of muscle occurs returning golgi tendon organ discharge rate back to normal
- high threshold needed for reflex to kick in because don’t want it to kick in until tendon is compromised
Central Modification of Reflexes
higher centers of brain often inhibitory of reflexes
some regions provide excitation
Spinal Shock/Transection
- loss of reflexes bc excitatory inputs have been lost although circuitry/neurons is still intact
- spinal neurons become hyperpolarized
- reflex recovery comes from axonal sprouting below level of transection (looking for new input) and from expression of receptor phenotypes that are self-activating such as 5HTC receptor
Brainstem Facilitatory Region
- associated with reticular activation system
- activates gamma moto neurons and makes muscle spindles more sensitive
- spontaneously active
Brainstem Inhibitory Region
- inhibit gamma motoneurons, making muscle spindles less sensitive
- requires activation from cortical regions
Spasticity
-with loss of cortex, brainstem inhibitory region is not activated
-brainstem facilitatory region dominates = cont. activation of gamma-motoneurons which contracts intrafusal muscle and lengthens the nuclear bag/chain fibers
-uncontrolled facilitatory region causes stretch reflexes that fight any passive motion = spasticity and hyperactive reflexes
-resists motion in given direction
Presentation: pt resists passive stretch of muscles; contraction doesn’t start until stretch occurs
Decerebrate Posturing
- all anti-gravity muscles in extension, usually bilat
- caused by loss of input from all structures rostral to pons / caudal to red nucleus
- indicative of severe brain injury
Rigidity
-resists motion in all directions
-results from maintained muscle contraction
-cont. activation of alpha-motoneurons by brainstem
-loss of cortical influence that inhibits medullary input to alpha motoneurons
Presentation: contraction of muscles in absence of other stimuli
Decorticate Posture
- rigidity
- flexion of upper limb joints = disinhibition of red nucleus and its control of UE flexors
- extension of lower limbs = disinhibition of reticulospinal and vestibulospinal pathways
- depending on head position left or right will change a
- internal rotation of legs in extended position; flexion of arms dependent on head position
- bilat or unilat
- loss of cortical inputs caused by lesion of internal capsule
Stroke
unilateral decorticate posturing results from strokes in vicinity of internal capsule
