segmental motor circuits Flashcards
what are the classes of movement
- Reflex (stereotyped)
- Voluntary (motor cortex)
- Cyclical (programed)
what is a motor unit
alpha motor neuron and the skeletal muscle fibers that it controls and has
NMJs with.
how many fibers do alpha motor neurons innervate
Each alpha motor neuron will innervate a few skeletal muscle fibers. Some neurons control more
skeletal muscle fibers than others.
what are the 3 types of inputs of a LMN
- From local interneurons
- From sensory afferents (periphery)
- From higher centers (descending
tracts)
what are the segmental reflexes
- Monosynaptic, disynaptic, polysynaptic
- Reciprocal inhibition
what is the monosynaptic reflex
- 2 neurons and 1 synapse in between
- stretch reflex
what is a disynaptic reflex
inverse stretch reflex
what are are polysynaptic reflexes
- Withdrawal (flexion) reflex
- Reciprocal (antagonist) inhibition
what does the stretch reflex involve
Muscle spindle pathway
- Sensory axons from the muscle spindle goes to the dorsal root ganglion,
- synapses to the motor
neuron in the ventral horn of grey matter, and causes the alpha motor neuron to fire and contract
the muscle that was stretched.
what is the stretch reflex also called
Stretch reflex has other names: myotatic reflex or deep tendon
reflex.
steps of muscle stretch
step 1) stretching of the muscle stimulates muscle spindles
step2) activation of sensory neuron
step 3) information processing at motor neuron
step4) activation of motor neuron
step5) contraction of the muscle
what happens when during the knee jerk reflex test
- when Dr taps the tendon you stretch the muscle which stretches the spindle and reflex goes through spinal cord which causes alphamotor neuron to fire and contraction of quads and jerk of the leg
what are the functions of the stretch reflex
Holds muscle length constant
Maintenance of erect posture
Responsible for muscle tone
what is muscle tone
- the basal level of contraction of a muscle due to gravity
what is the gamma motor system
- it adds to the reflex story
- controls the spindle itself, its sensitivity
- stetch the muscle spindle itself
what is the gamma motor neuron pathway
-CNS stimulates gamma motor neuron to stretch the muscle spindle
muscle
spindle axon enters the DRG to the spinal cord, where it synapses with the alpha motor
neuron that innervates the muscle alpha motor neuron fires, and the muscle in
contracted
this low level contraction adds to muscle tone
intrafusal vs extrfusal muscle fibers
- extrafusal - bulk of the muscle
- intrafusal - muscle spindle
what does the inverse stretch reflex involve ( Disynaptic reflex)
Golgi tendon organ
Senses muscle tension
- 3 neurons 2 synapses
what is the inverse stretch reflex pathway
GTO measures muscle tension. It synapses with an inhibitory interneuron in the segmental pool
of neurons within the spinal cord. The inhibitory interneuron acts on the alpha motor neuron,
causing the muscle to relax.
- overall decresases contraction of EXTRAFUSAL muscle fibers
example of inverse stretch
- overloading muscle with a weight if its too heavy the inverse stretch reflex inhibits and drop the weight
what does the flexor ( withdrawl) reflex respond to
- respond to painful stimulus
what is the pathway of the flexor reflex
- Free nerve ending in the skin synapse in the gelatinosa.
- Polysynapstic connections of interneurons to the alpha motor neuron.
- There’s also the ascending spinothalamic tract, which goes through the anterior
commissure and up the spinothalamic tract. - Pain afferents synapse with interneurons which converge on an alpha motor neuron
and typically cause reflexive flexion withdrawal.
what is a example of a flexor reflex
- touching a hot stove
- remove the hand even before you feel the pain
what are antagonist muscles
- biceps - reflexer
- tricpes - extencer
- there needs to be reciprocal inhibition
what is reciprocal inhibiton
- if you reflex biceps alpha motor neurons of triceps need to be inhibited and vice versa
what are the 2 routed of reciprocal inhibiton
- Classic muscle spindle reflex. Muscle spindle in the extensor muscle excites the alpha motor
neuron, which goes back to the same muscle and causes it to contract. - Muscle spindle in the extensor muscle branches and connects with an inhibitory interneuron.
This inhibitory interneuron acts on the flexor muscle and causes it to relax.
what are the cranial nerve reflexes
- jaw jerk
- corneal reflex
what is the jaw closing reflex
- If you stretch the masseter muscle by tapping the chin, there’s a reflex that contract
the masseter - usually, this reflex is inhibited
- But if there is some abnormality of the central motor pathway, this reflex may become
apparent.
jaw closing reflex pathway
- masseter muscle has muscle spindle
- spindle afferent goes to mesencephalic nicleus
- The muscle spindle fiber synapses with the alpha motor neuron in the
motor nucleus of V and causes the masseter to contract, closing the jaw.
where is the mesecephalic nucleus of V in the pons
what is the cornal refelx
- if you touch the cornea you blink
what is the corneal reflex pathway
- receptors in the cornea go through V1 and synapse in the brainstem at the chief
sensory nucleus of V. - There is an interneuron that goes to the motor nucleus of 7 (facial muscles), which causes a
contraction of the eye muscles
blinking reflexively
what are central pattern gernators
Neuronal networks in spinal cord or brain stem that generate,
rhythmic alternating activity.
-
examples of central pattern gerneators
Walking
Swimming
Chewing
Breathing
Eye movements
Sneezing & coughing
mastication
- exerts huge forces
- also can execute precise movements such as for speech with speed
- mandible moves rapidly
what reflexes are included in mastication
Central Pattern Generator
Stretch reflexes (spindles in jaw
closing muscles)
Mandibular rest position
Periodontal reflexes
Joint reflexes
