L5: Neuromuscular Considerations

What is the organization of the nervous system?

• divisions

• functional unit

  • types

Central Nervous System (CNS)

• Brain and spinal cord

• Where human movement is initiated, controlled, and monitored

Peripheral Nervous System (PNS)

• Branching nerves outside of the spinal cord

• 12 cranial

• 31 spinal

The functional unit is neurons

• Motor/efferent neurons: carry motor actions from the CNS to effectors

• Sensory/afferent neurons: carry sensory signals from receptors to the CNS

What is the anatomy of a motor neuron?

Soma/Cell body

• In the ventral gray horns of the spinal cord (thoracolumbar region for somatic motor neurons)

• Processes information received by the dendrites

Dendrites

• Projections on the cell body that receive information, generally, from sensory neurons

Axon

• Is a long fiber that projects out from the spinal cord to the target cell.

Terminal branch (axon terminal)

• End of the nerve axon, where they synapse to target cell.

Where are the somas of somatic motor neurons?

Originate in the ventral gray horn of the thoracolumbar region of the spinal cord. The axon then projects from the anterior root, all the way to the skeletal muscle.

What are alpha motor neurons?

Large neurons that are capable of sending information rapidly.

These neurons are involved with gross motor skills.

What is myelination? Schwann cells? Nodes of Ranvier?

When an axon is covered by myelin.

Schwann cells myelinate axons in the PNS. Insulating, reducing friction, and allowing the propagation of AP.

Nodes of Ranvier are gaps between the myelin, here the AP jumps from “node to node”

• saltatory conduction

What is multiple sclerosis (MS)?

Clinical condition where propagation of AP are delayed because schwann cells have harden.

What is the neuromuscular junction?

NMJ is the place where the terminal branches of a somatic motor neuron articulate (synapse) and transmit information to muscle fibers.

What is a synapse?

Synapse: the gap between the terminal branch of the motor neuron and the motor end plate

Synapse (verb): information is being exchanged

Where does the terminal branch end?

Ends at the motor end plate

Where is the body of sensory neurons located?

How are the fibers named?

The body of a sensory neuron is between the terminal branches and dendrites, in the dorsal root ganglion.

Peripheral fiber: from dendrites to the cell body

Central fiber: from the cell body into the spinal cord

What are connector/interneurons?

Exist completely within the CNS, serving as connecting links between sensory and motor neurons when they are too far apart to communicate through a mono-synapse.

How many interneurons link sensory and motor neurons?

May be one interneuron or several, depends on the distance the information is being delivered.

What are nerves? What do spinal nerves consist of?

Nerves are a bundle of fibers (axons) enclosed by a CT sheath which transmit impulses.

There are 31 spinal nerves, they typically consist of: Motor outgoing fibers through the ventral roots and sensory incoming fibers through the dorsal root. When the two come together they form the spinal nerve.

What are the pairs of spinal nerves? Plexuses?

8 cervical

12 thoracic

5 lumbar

5 sacral

1 coccygeal

Cervical - C1-C4

Brachial C5 - T1

Lumbar L1-L4

Sacral L4-S4

How many synapses are there between neurons and target cells?

One or several.

What happens to information processing rate when there are several synapses? How can this be faster?

Processing time increases with the increase of synapses. However, the more often the sensory-motor neuron pathway is used the faster the information is processed (efficient).

How is information exchanged at synapses?

Chemical messenger (ACh) is released from the presynaptic membrane, diffuses across the synapse, and binds to receptors on the post synaptic membrane to propagate ACh.

What is an AP? How is it generated?

An AP is an electrical impulse that has a minimum threshold of stimulus (neurotransmitter) required to initiate or propagate it.

What type of stimuli can neurotransmitters cause?

An excitatory stimulus, lowering the threshold.

• decreasing the amount of excitatory stimulus required to continue propagation

Or a inhibitory stimulus, increasing the threshold

• increasing the amount of excitatory stimulus required to continue propagation.

Where does the stimuli come from? What does it summation determine?

Stimuli may come from more than one neuron. The summation of all EPSPs and IPSPs determine if the minimum threshold of the postsynaptic neuron is reached and if an AP will be fired.

What is a motor unit?

Is a single motor neuron (MN) and all the muscle fibers it innervates

What is the general division of MU?

Generally all muscle fibers in MU are the same fiber type.

Type I - Slow twitch fibers characteristics

Are the “endurance units.” Slow contractions (>70ms) are sustained but generate less force. However, they are highly fatigue-resistant, makes them good for maintaining posture and activities such as distance running.

Type IIa - Fast Twitch

Fast contraction times (30-50ms) and more fatigue resistant the Type IIb

Good for swimming and bicycling

Type IIb - Fast twitch

Rapid contraction times (30-40ms)

These fibers tend to be innervated by alpha motor neurons, so information is received quickly and the muscle contract fast in response.

Also tend to have large neuron-to-fiber-ratios. Meaning one motor neuron goes to thousand of cells

Good for sprinting, jumping, and weightlifting

What are the sizes of MU? What are they correlated with?

The size is directly correlated with what the muscle action is

Bigger muscles tend to have larger MU: Gross movement

• gastrocnemius with 2000 + muscle fibers

Smaller muscles tend to have smaller MU : Fine motor skills

• eye muscle have fewer than 10 fibers.

What are the gradations in the strength of muscle contractions?

1) How many MU are being recruited?

2) What is the frequency of stimulation to the MU?

What is the all-or-none principle?

All muscle fibers of MU contract when threshold is reached by stimulus.

• applies to MU NOT the whole muscle

How are MU recruited typically?

Slow twitch MU tend to be recruited first due to their lower thresholds, however they have a low force output. When more force is then fast twitch MU are recruited, which have higher thresholds, but greater force output.

When does MU recruitment change?

Order of recruitment changes when individual is familiar with the movement.

• “Knows” the force required to do said movement

What is the frequency of stimulation?

The frequency of an AP can also gradiate force output of muscle.

Low frequency of AP → muscle fibers..

High frequency of AP → muscle fibers...

At a low frequency of AP firing allows for the muscle fibers to relax between contractions.

At a high frequency of AP firing, muscle fibers do not have time to relax resulting in summation of stimulus or max contraction, increasing force output.

Combining the number of MUs recruited and frequency of stimulus does what?

A combo of max number of fibers stimulated at a high frequency results in the maximal strength of contraction