Kenyon: Motor Neurons Flashcards
Cell bodies in the brainstem or spinal cord
The axons end in neuromuscular junctions with skeletal muscle fibers.
Excitatory
Lower motor neurons
Cell bodies in the brainstem or cerebral cortex
The axons end in synapses on other upper motor neurons, local circuit neurons, lower motor neurons, or some combination.
Excitatory or inhibitory or both!
Upper motor neurons
Describe the major pathway from upper motor neurons to skeletal muscles.
upper motor neurons in the motor cortex (for planning, initiating, directing voluntary movements) or in the brainstem (basic movements and posture) –> local circuit neurons (lower motor neurons) –> motor neuron pool (lower motor neurons)–> skeletal muscles
Describe the minor pathway from upper motor neurons to skeletal muscles.
upper motor neurons in the motor cortex or brainstem centers –> motor neuron pools (lower motor neurons) –> skeletal muscles
What is the final common pathway in the organization of motor control?
motor neuron pools –> skeletal muscles
Where is there sensory input in the organization of motor control?
to local circuit neurons
to motor neuron pools
to the cerebellum
Many spots along the motor control pathway may be excitatory or inhibitory. Which spot is only excitatory?
motor neuron pool –> skeletal muscle
Lower motor neurons can synapse in what two places?
spinal cord
brainstem
What do lower motor neurons that synapse in the ventral horn of the spinal cord do?
control the skeletal muscles of the body
What do lower motor neurons that synapse in the somatic and branchial motor nuclei do?
control the muscles of the head
What are the somatic motor nuclei that lower motor neurons may synapse in?
oculomotor
trochlear
abducens
hypoglossal
What are the branchiomotor nuclei that lower motor neurons can synapse in?
trigeminal
facial
ambiguus
spinal accessory
What are upper motor neurons in the primary motor cortex and premotor cortex for?
planning, initiating, and directing series of movements in limbs and eyes
What are upper motor neurons in the cingulate gyrus for?
expressions of emotions especially facial muscles
Where are upper motor neurons found in the brainstem?
- vestibular nuclei
- reticular formation
- superior colliculus
What do upper motor neurons in the brainstem do?
regulate muscle tone
orient eyes, head, and body with respect to vestibular, somatic, auditory, and visual sensory information
Compares actual movement (proprioceptive input) with intended movement and commands corrections. Modulates ongoing movements
cerebellum
The basal ganglia is made up of the forebrain nuclei. What are they?
caudate putamen globus pallidus substantia nigra subthalamic nucleus
What are the basal ganglia (forebrain nuclei) responsible for?
initiation of movements
suppressing unwanted movements
starting wanted movements
Receive input from upper motor neurons and sensory neurons.
Excite or inhibit lower motor neurons
local circuit neurons
In the spinal cord, the lower motor neurons for the axial (postural) muscles are located (blank), while the lower motor neurons for distal muscles (limbs) are located (blank)
medially; axially
In the spinal cord, where are local circuit neurons controlling POSTURE located? Are these projections long or short? What is unique about them?
medially; long; they contract OR relax muscles over a large part of the body as needed in posture
In the spinal cord, where are local circuit neurons controlling LIMBS located? Are their projections long or short?
laterally (near the motor neurons); projections are short **local circuit neurons controlling limbs control a more limited set of muscles
Each lower motor neuron innervates from 3 to 3000 muscle fibers. The motor neuron and its fibers are called a (blank). The lower motor neurons for a given muscle are clustered in (blank) in one or more spinal cord segments.
motor unit; ventral horns
The muscle fibers innervated by a single motor neuron are spread throughout the muscle. Compare the lower motor neurons of the gastroc and the eye.
the gastroc has 1 motor neuron for about 2000 muscle fibers, while the eye muscles have 1 motor neuron for every 3 muscle fibers (more fine movement)
What are the different types of motor units?
slow
fast fatigable
fast fatigue-resistant
red muscle, resist fatigue
Tend to be continuously active (e.g. posture).
slow motor units
paler
large forces for short bursts
fast fatigable motor units
easier to depolarize to threshold (needs fewer EPSPs), slow and steady firing rate, good for slow postural muscles
small motor unit
harder to depolarize to threshold (needs more EPSPs), fast firing rate, good for fast fatigable muscles
large motor unit
As the synaptic activity driving muscle contraction increases, which units are activated first?
slow units, then fast fatigue resistant then fast fatigable (stand, walk/run, jump/gallop)
help control muscle length (deep tendon reflex, a.k.a. stretch reflex, a.k.a. myotatic reflex)
muscle spindles
help control muscle tension
golgi tendon organs
moves you away from an unpleasant event in the periphery
flexion reflex
Activate contraction of 8 – 10 specialized intrafusal muscle fibers (muscle spindles) in proprioception
gamma motor neurons
Describe the role of 1a afferent fibers in keeping your beer from spilling when it’s heavy.
stretching of muscle causes 1a afferents to synapse on lower motor neurons which drive the activity in the homonymous muscle and inhibit the activity in the antagonist muscle
stretching of the muscle spindle causes 1a afferents to activate the (blank) muscle, but inhibit the (blank) muscle
homonymous; antagonist
Why must you activate gamma motor neurons as well as lower motor neurons in the muscle spindle reflex?
activation of lower motor neurons alone would cause the muscle to shorten and the 1a afferents would shut down *the cerebellum won’t know what is happening
Why is co-activation of lower and gamma motor neurons necessary?
An effective control center needs to know the activity in both the gamma efferents and the Ia afferent to know what is going on in the muscle
Sensitivity of muscle spindles is under constant adjustment. When you’re sitting or stretching is gamma activity high or low? When you’re standing on a moving bus or skiing, is gamma activity high or low?
low; high
Gamma motor neurons are wrapped in (blank) afferent neurons that sense muscle length
1a primary
Encapsulated afferent nerve endings between the muscle fibers and the tendon
golgi tendon organs
Golgi tendon organs utilize (blank) fibers
Ib
What are golgi tendon organs activated by?
tension
Passive stretch increases activity in afferents from both muscle spindles and golgi tendon organs. Spindles respond to (blank) change, while golgi organs respond only slightly to passive increase in (blank)
length; muscle tension
What does contraction do to activity in spindle afferents? What does it do to activity in Golgi afferents?
decreases; increases
Golgi afferents synapse on local circuit neurons that inhibit a response in the (blank) muscle and activate the (blank) muscle
homonymous; antagonis
If tension gets to big, what do golgi afferents cause the muscle to do?
relax to prevent damage
How do primary afferent neurons for pain and temp link to lower motor neurons?
directly and via interneurons
What happens when you step on something unpleasant?
a cutaneous afferent fiber from the a nociceptor in the foot will synapse in the dorsal horn on a lower motor neuron causing you to flex the ipsilateral leg and extend the contralateral leg
Can the spinal cord organize and generate sequences of movement without input from the cortex?
yes
**pattern generators in the spinal cord
What happens to a cat after sectioning the dorsal roots of the spinal cord?
it can still walk on a treadmill!
What do deficits due to damage or loss of lower motor neurons result in?
specific loss of neuro-muscular transmission
List some symptoms of lower motor syndrome
paralysis paresis areflexia loss of tone atrophy fibrillations fasiculations
