6. Motor System I Flashcards
*five groups on neurons involved in movement
- voluntary muscles (intrafusal/extrafusal fibers)
- lower motor neurons
- basal ganglia
- cerebellum
- brainstem centers
motor unit: define
a motor neuron & all of the muscle cells it innervates
lower motor neurons:
location, action
- found in spinal cord and brainstem
- in the spinal cord, they are in anterior/ventral gray horn
- medial column - axial muscles
- lateral column - more distal muscles of extremities (only at cervical and lumbosacral enlargements)
lower motor neurons: organization in spinal cord
-
somatotopic organization:
- more medial motor neurons innervate axial and proximal limb muscles
- more lateral motor neurons innervate distal limb muscles
where are lower motor neurons found
(other than in brainstem)?
- eye muscles - CN 3,4,6
- mastication muscles - CN 5
- facial expression muscles - CN 7
- palate/pharynx/larynx/upper esophagus muscles - CN 9, 10
- SCM and trapezius muscles - CN 11
- tongue muscles - CN 12
2 key receptors of movement, & function
- muscle spindles - provide info on (1) muscle length, & (2) changes in muscle length
- golgi tendon organs - provides info on the tension generated by active muscles
explain how golgi tendon organ responds to increased tension?
- as muscle elongates –>
- alpha motor neurons are under more tension –>
- conveys more afferent activity via 1b afferent –>
- transduces applied tension
- acts to contract/shorten the muscle
reciprocal inhibition
the automatic antagonist alpha motor neuron inhibition which is evoked by contraction of the agonist muscle
(sending inhibitory afferent to inhibit a diff’t neuron)
stretch (deep tendon, myotatic) reflex:
pathway
- hammer to patellar tendon
- muscle spindle detects elongation of the muscle
- sends info on 1A afferent fiber
- to interneuron
- to motor neurons which contract the quadriceps (agonist), and release/lengthen the hamstring (antagonist)
- causes the leg to kick
purpose of deep tendon reflex
- clinical and classic example of the monosynaptic reflex arc
- This is a reflex of proprioception which helps maintain posture and balance, allowing to keep one’s balance with little effort or conscious thought.
key difference between Ia and II motor neurons?
- Group Ia conveys change in length w/ a dynamic response in response to linear stretch to show muscle length change (more firing to indicate longer length of muscle; and there is NO firing when muscle shortens/relaxes)
- Group II only has a static response; in which as muscle length changes, it is the new steady state – just muscle length
example of muscle spindle as length detector:
holding a mug, and having something poured into it
- Given a command to hold bicep (elbow) at flexion at 90 degrees against the weight of the mug
- You need to change a certain amount of the extension to counter the mug
- If increasing load in mug, elbow will extend (bc not enough motor fibers to support) –> stretching biceps
- Compensatory response: so you need to increase the stretch/contraction of biceps/flexors to support the heavier mug (biceps contract, triceps lengthen)
(stretch-induced muscle contraction)
which type of neuron is only capable of a static response?
type II neurons
which type of motor neurons enable muscles to remain sensitive to muscle length change during muscle shortening?
gamma motor neurons
- (the gamma neurons keep the muscle (intrafusal muscle fibers) from going slack –> to be able to provide length, and length change information to the CNS)
- Regulates the sensitivity of the spindle during muscle shortening
CC: lower motor neuron syndrome
symptoms
- paralysis (enduring flaccid paralysis)
- hypotonia (or atonia)
- no superficial reflexes
- reduced/absent deep tendon (stretch) reflexes
- fasiculations (brief, spontaneous contraction affecting small number of muscle fibers; can cause flicker of movement under the skin)
- atrophy (denervation atrophy)
