Lecture 29: Spinal Motor Neurons Interneurons

motor neuron pool

all motor neurons innervating a single muscle

(which neurons the muscle talks to)

Axial muscles

(give an example)

* motor neurons located medially on the spinal cord
* interneurons connect across distant segments and across midpoint

(shoulders)

distal muscles

* motor neurons located laterally on the spinal cord
* interneurons connect only near segments unilaterally

(fingers, feet, hands)

Are motor neuron pools located in just one segment of the spinal cord or spread across many?

spread across many

What type of neurons coordinate muscle contraction?

local circuit neurons (interneurons)

What are the three main characteristics of axial muscles?

* responsible for posture
* work in concert over a large portion of the body
* both sides of the body work together

What are the two main characteristics of limb muscles?

* more fine-tuned movement
* each side of the works seperately

Long distance local circuit neurons travel through (blank) segments whereas short distance local circuit neurons only travel through (blank) segment

several; one

What are the two main types of motor neurons and what do they innervate?

1. Alpha motor neurons

Innervate the muscles that generate forces (extrafusal)


2. Gamma motor neurons

Innervate muscle spindles (intrafusal)

(more of a sensory function)

Motor Unit

One alpha motor neuron (including all of the muscle fibers it talks to)

Most extrafusal skeletal muscle fibers are innervated by…

a single alpha motor neuron

What do alpha motor neuron axons do and what are the advantages?

branch and synapse on different fibers that work together

* force is spread evenly
* all muscle fibers that work together are stimulated together

more muscle fibers activated =

more force used

What are the three types of motor Units?

* slow
* fast fatigable
* fast fatigue resistant

Slow Motor unit

* sustained muscle contraction, small force
* think posture

fast fatigable motor unit

* brief exertions, large forces
* running, jumping

fast fatigue-resistant motor units

* intermediate in size and force
* think walking

Motor Units:

Weak synaptic stimulation =

Stronger synaptic stimulation =

Strongest synaptic stimulation =

* smaller, s motor units
* mid sized, fr motor units
* biggest, ff motor units

How does the firing rate of a neuron relate to muscle tension?

The faster the rate of fire, the greater the tension