Reflexes Flashcards
define motor unit
motor neuron and all fibers it innervates
define motor neuron pool
all axons that innervate a muscle
what are lower motor neurons, and what do they include?
motor neurons of the brainstem & spinal cord that innervate skeletal muscle:
- ventral horn cells & processes
- motor nuclei of CNs & processes
name the 2 types of ventral horn cells
alpha motor neurons
gamma motor neurons
what do alpha (a) motor neurons innervate?
extrafusal skeletal muscle fibers
what do gamma (y) motor neurons innervate?
intrafusal fibers (specialized skeletal muscle fibers enclosed in a fibrous capsule called a muscle spindle)
symptoms of LMN lesion
- paresis/paralysis/plegia
- atrophy
- fasciculations
- hypotonia and hyporeflexia
- fibrillations
what are fasciculations?
visible muscle twitching resulting from involuntary synchronous contraction of all muscle fibers in a motor unit
fasciculations are characteristic of…
slowly progressive diseases of the motor unit
what are fibrillations?
spontaneous activity of one muscle fiber detected only with electromyography
fibrillations are a sign of…
progressed muscle denervation or myopathy
what is a muscle spindle?
- specialized skeletal muscle fibers in fibrous capsule situated in parallel with the extrafusal muscle fibers
- composed of intrafusal m fibers, sensory (afferent) axons, and motor axons (efferents which regulate sensitivity)
what is the motor innervation of muscle spindles?
gamma (y) motor neurons
describe alpha and gamma co-activation
- stretch-> 1a proprioceptors in muscle spindle send info in spinal cord about stretch
- contraction-> intrafusal fibers become slack
- y-motor neurons cause them to tighten back up so sensitivity is not lost
why is alpha and gamma co-activation important?
ensures extrafusal and intrafusal fibers are in sync
what is the sensory innervation of muscle spindles?
Ia (annulospiral endings) fibers and II (flower spray) fibers that detect muscle stretch
name the 3 types of intrafusal fibers
- nuclear chain fibers
- nuclear bag fibers: dynamic & static
what are nuclear chain fibers?
- thin intrafusal fibers, with nuclei arranged in a single file
- afferents include group Ia and II
what are nuclear bag fibers?
thicker intrafusal fibers, with nuclei grouped in the central region
what are dynamic nuclear bag fibers?
- sensitive to rate of change (rapidly adapting)
- afferents include group Ia only
what are static nuclear bag fibers?
- slowly adapting stretch receptors
- afferents include both group Ia and II
what is the stretch (deep tendon) reflex?
monosynaptic (no interneuron) connection between central axon of Ia fiber and the a-motor neuron, causing muscle contraction in response to stretch
in the stretch reflex, the ____ muscle is excited via the ____ neuron
agonist
alpha motor
what is the sensory receptor of the stretch reflex?
muscle spindle
what is the Ia inhibitory reflex?
central Ia axon –< Ia inhibitory interneuron –< alpha motor neuron innervating antagonist muscle
when testing the biceps deep tendon reflex, the biceps muscle is excited by the ____, while the triceps are inhibited by the ____
stretch reflex: alpha motor neuron
Ia inhibitory reflex: Ia inhibitory interneuron
what is the function of recurrent (renshaw cell) inhibition?
- to avoid overexcitation of agonist motor neuron pool
- negative feedback system allows for control of excitability of all muscles around a joint
explain recurrent (renshaw cell) inhibition
- agonist a-motor neuron (or descending tracts) –< renshaw inhibitory cell
- renshaw cell withdraws inhibition of Ia interneuron so that antagonist is excited while agonist is inhibited
what is the overall effect of renshaw cell reflex?
- directly inhibits agonist
- withdraws inhibition of antagonist (inhibits Ia inhibitory interneuron)
what is a golgi tendon organ?
encapsulated sensory receptors located in the junction between muscle & tendon, in series with the extrafusal muscle fibers
what is the sensory innervation of golgi tendon organs?
Ib proprioceptors sensitive to muscle tension
explain the Ib reflex
in response to tension in the muscle:
central axons of Ib fibers from golgi tendon organ, cutaneous receptors, & joint receptors –< Ib inhibitory interneuron inhibits a-motor neuron pool of agonist
what is the overall effect of the Ib reflex?
inhibition of the agonist muscle
what is the pain withdrawal reflex called?
flexion & crossed extension reflex
what is the flexion & crossed extension reflex?
polysynaptic pathways where central axons of A-delta nociceptors facilitate flexion (withdrawal) of the hurt lower limb, while facilitating extension of the contralateral lower limb for support
explain the flexion & crossed extension reflex
A-delta central axons synapse on:
- inhibitory interneurons inhibit a-motor neuron pool of ipsilateral extensors
- excitatory interneurons that facilitate a-motor neuron pool of ipsilateral flexors
- inhibitory interneurons that inhibit a-motor neuron pool of contralateral flexors
- excitatory interneurons that facilitate a-motor neuron pool of contralateral extensors
what information do fasciculus gracilis & cuneatus carry?
ipsilateral:
proprioception
vibration
2-point discrimination
pressure
touch
nucleus gracilis and cuneatus give rise to ____ that decussate to form the ____ which ends in the ____
internal arcuate fibers
medial lemniscus
ventral posterior lateral thalamic nucleus
from the thalamus, fibers of the DCML then run through the ____ and end in the ____
internal capsule
somatosensory cortex (3,1,2)
examination of DCML lesion
- vibration tuning fork on dermatomes
- position sense mainly of upper extremities
- romberg’s test
what tract is responsible for proprioception from the lower extremity?
spinocerebellar tract
what tracts make up the anterolateral system, and what is their function?
spinothalamic
spinoreticular
spinomesencephalic
pain & temperature perception
pain & temperature fibers enter the tract of ____ and end in the ____
Lissauer
dorsal horn
when pain and temperature fiber cross at the ____, they form the ____ and end in the ____
ventral white commissure
spinothalamic tract
ventral posterior lateral thalamic nucleus
spinothalamic fibers that end in the thalamus then travel through the ____ to end in the ____
internal capsule
somatosensory cortex (3,1,2)
examination of spinothalamic tract lesion
pinwheel (prick) over dermatomes
