Lower Motor Neuron

Lower Motor Neuron Circuits and Control

Cell bodies

Ventral horn , grey matter of spinal cord

Motor nuclei of cranial nerves, brainstem

Lower motor neuron

Local circuit neurons (interneurons)

Site of lower motor neuron integration

Sensory input

Skeletal muscles

Upper motor neurons

Motor cortex

Brainstem

Basal ganglia

Cerebellum

Organization of neurons in spinal cord

Proximal muscles, medial

Distal muscles, lateral

Postural control & balance

Skilled behaviour, fine control

Local circuit neurons, or interneurons display the same organization.

Medial LCN tend to travel longer distances through many spinal cord segments & cross midline as they ensure muscle groups on both sides maintain posture.

Lateral LCN tend to travel shorter spinal segments and are ipsilateral.

Types

Gamma

Alpha

Innervates muscle spindles, intrafusal

Regulates sensory input by changing length of muscle fibres

Innervates extrafusal muscle fibres

Striated muscles for posture, movement

Motor unit

1 alpha motor neuron, many muscle fibres

Size

Small

Large

Generate smaller force

Lower threshold for activation

Contract slowly

Sustained contraction; resistant to fatigue

Less muscle fibres per motor neuron

Recruited first

Generate large force

Higher threshold activation

Contract quickly

Easily fatigued

More muscle fibres per motor neuron

Recruited last

Increased motor unit size leads to:

Increased

- Cell body size

- More dendritic branches

- Larger axon diameter

- More axonal branches (muscle fibres innervated)

Decreased

- Input resistance

- Excitability

Synapse characteristics

Cholinergic , releases NT ACh only

Excitatory

*But how does inhibition occur then?

Muscle spindle

1. Intrafusal

- Innervated by gamma efferent motor neurons

- Innervated by group I/II afferent neurons

2. Extrafusal

- Innervated by alpha efferent motor neuron

Stretch reflex

Synergist muscle

Load --> Change in length of muscle fibre --> Detection by spindle receptors --> Afferent neuron output --> Alpha motor neuron --> Muscle contracts (excitation) --> Loop

Antagonist muscle

Load --> Change in length of muscle fibre --> Detection by spindle receptors --> Afferent neuron output --> Interneuron --> Muscle relaxes (inhibition) --> Loop

A negative feedback loop (negative because it counters it's initial movement of stretch) is formed.

Where does the gamma efferent neuron come into play?

They ensure muscle spindles maintain their tension as extrafusal muscles (innervated by alpha motor neuron) contract/become shorter.

This way the afferent neurons are able to constantly feedback in response to stretch (since they only respond to stretch).

Reflexes

Stretch, muscle fibres

Extrafusal

Intrafusal, muscle spindles

Force, golgi tendons

Alpha motor neuron

Sensory receptors, group Ia/II afferent neurons

Gamma motor neuron

Maintains tension of IMF

Negative feedback

Striated; sensitive to stretch

Meshed; sensitive to tension

Reflex maintains muscle force

Reflex maintains muscle length

Ib Afferent neuron

Negative feedback

Both muscle spindle and golgi tendons respond to muscle stretch. Muscle spindle responds more actively than golgi.

Only golgi tendons respond actively to muscle contraction. Muscle spindle still fires but markedly decreased. That's why gamma efferent neuron is necessary in maintaining spindle length.

Golgi tendon Negative Feedback

Force increase --> Sensory receptors --> Ib afferent --> Interneuron --> Inhibitory signal to alpha motor neuron in same muscle (flexor) --> Loop

Excitatory signal to alpha motor neurons in extensor muscles

Flexion reflex

Slowly conducting afferent axon that synapses via interneurons with several alpha motor neurons

Excitation of ipsilateral flexor, inhibition of ipsilateral extensor; opposite reaction in other limb

Local circuitry; muscle self regulates without brain input

Monosynpatic circuit for the synergist