Lower motor neuron circuit

motor systems

plan, coordinate, and execute programs responsible for movement

functions of motor system

• move through environment

• manipulate external world

• maintain posture/balance

• autonomic - respiration

• speech/gestures/writing

• sensations translation

4 sections of motor system

1. spinal chord + brainstem circuits

2. descending systems (above spinal chord)

3. cerebellum

4. basal ganglia

motor sections in lower motor neuron circuits (2)

1. spinal chord and brainstem

2. descending and modulatory pathways

sections of motor system in UPM (2)

1. cerebellum

2. basal ganglia

type of motor neuron in the brainstem

upper and lower motor neurons

lower motor neurons

a neuron in brainstem or spinal chord that projects to skeletal muscle

• the “final common path”

damage to LMN

hypoactive reflexes and decreased tone

hypoactive reflexes

loss of detemention to movements

where of LMNs located in the spinal chord?

ventral horn (of grey matter)

what type of LMNs in spinal chord?

\alpha, \gamma

motor neuron pools

groups of motor neuron in a column along the ventral horn of the spinal chord project

• form spatial map of neurons onto muscles

what was shown from the feline study?

motor neuron-muscle relationships

what happened in the feline study?

• they found out that each motor neuron pool has a muscle that those neurons go to

topical organization of spinal chord LMNs

medial of dorsal horn → proximal muscles (extremities) , mostly axial muscles

lateral of dorsal horn → distal muscles (farther like fingers and hand), mostly limb

long distance local circuit neuron

• communicate and connect with other motor neurons and have commiserations axons (cross midline)

• run medially

short distance local circuit neurons

• run near motor pools they control

• only go a few segments at a time

• connect motor neurons so muscles are able to work together

motor unit

alpha motor neuron, its axon, and the muscle the cell interfaces

• smallest motor functional unit

how does a motor neuron intervate a muscle?

a neuron goes to one muscle, but to different fibers, spread out

“S” slow motor unit (size, force, color, and function)

• smallest motor neuron

• intervate fewest fibers

• smallest forces, fatigue resistant

• high mitochondria # (red fibers)

• sustained muscular activity (ex. posture)

“FF” fast fatigable motor unit: (size, force, color, and function)

• large → intervate many fibers

• greater forces → easily fatigues

• pale (few mito and vessels)

• brief exertions (es. run and jump)

“FR” fast fatigue-resistant” motor unit: (size, force, color, and function)

• median size

• not as fatigueable

• 2x force of S unit

stretch reflex

• muscle spindle stretches→ afferent sensory neuron sends to SC→ synapses with alpha motor neuron→ which contracts the muscle

monosynaptic reflex

sensory neuron communicating directly with a motor neuron, no interneurons needed

gamma motor neurons

control how sensitive the muscle spindle is

• tight intramural fibers inside spindles so it keeps reporting the stretch accurately

gamma gain

how much force/stretch it takes for spindle to fire

high vs. low gamma gain

• High: spindle is cranked up → slight stretch makes a lot of afferent activity→ more alpha motor neurons, incr. firing rate, stronger muscle tension

• Low: spindle is lazy → need more of a stretch to get the same type of response

how does the NS adjust gamma gain?

depending on the task

Ex. walking vs balancing on ice

Golgi tendon organ (GTO)

A force detector located where the muscle meets the tendon.

what does GTO sense?

muscle tension from contraction

what does the GTO do?

inhibits a-motor neurons of the same muscle

• keeps muscle force stable and prevents damage

GTO pathway

Ib afferent → inhibitory interneuron → inhibits the same muscle’s motor neuron → reduces force

muscle spindle vs. GTO

muscle spindle: senses length

GTO: senses force

intrafusal vs. extrafusal fiber

inside muscle/ outside muscle

flexion reflex

• painful stimuli → activates ipsilateral flexors (inhibits ipsilateral extensors)

• pulls affected limb away fast

• multiple synapses

crossed extension reflex

• painful stimuli→ contralateral limb extensor muscles stimulated and flexor muscle inhibited→ support

• polysynaptic

central pattern generators (CPGs)

neuronal circuits that generate repeating motor patterns w/out input or brain commands

• ex. walking

like autopilot

CPG locations

SC, brainstem, ganglia

lobster example of CPG

• lobsters have a CPG system in their stomatogastric ganglion

  • Job: grinds food and moves food to hindgut

  • set of multiple neurons form network

  • different NTs can modulate rhythm, frequency, and pattern

examples of CPGs

1. swimming, scratching, locomotion

what creates gait patterns in locomotion?

spinal CPGs coordinate flexor-extensor timing

how many/limb activity during: walking, trot, gallop, incr speed)

walking: 1 limb off ground at a time

trot: 2 legs in swing

incr speed: limb is in contact with ground for less time

lower motor neuron syndrome

• paralysis or paresis (weakness)

• areflexia (loss of reflex)

• loss of muscle tone

• atrophy (muscles shrink)

areflexia

damage to LMN circuit

• loss of reflex