Module 12 - Lecture 8 - Movement

Question 1

What does a motor unit represent?

Answer 1

A motor unit represents the single LMN and all of the muscle fibers innervated by it.

Question 2

Where are LMN located in the body?

Answer 2

• Their cell body is located in the CNS
• Most of the LMN are in the spinal cord “spinal nerves”, contained within the “ventral horn”
• Some of the LMN are in the brainstem “cranial nerves”, starting in nuclei (III-VII, IX-XII)

Question 3

How is the spinal cord organized?

• Dorsal horn = ?
• Middle = ?
• Ventral Horn = ?

Answer 3

• Neurons cluster in highly “regionalized” within the spinal cord
• Dorsal horn = sensory interneurons = recall afferent cell body in dorsal root ganglia this is now a second “inter” step
• Middle = Interneurons spanning SC sides or levels
• Ventral horn = motor neurons cell body = extra regions Rexed’s laminae VIII and IX

Question 4

What is “somatotopic” organization of the spinal cord and what areas of the ventral horn you would find specific groups of motor pools?

Answer 4

The most medial part of the ventral horn contains Lower Motor Neuron pools that innervate axial muscles or proximal muscles of the limbs, whereas the more lateral parts contain lower motor neurons that innervate the distal muscles of the limb

Question 5

Medial section of the ventral horn = more _______muscles
Lateral section of the ventral horn = more ________muscles

TO ADD
Ventral section of the ventral horn = more ________ muscles
Dorsal section of the ventral horn = more _________muscles

Answer 5

Medial section of the ventral horn = more proximal muscles
Lateral section of the ventral horn = more distal muscles

TO ADD
Ventral section of the ventral horn = more extensor muscles
Dorsal section of the ventral horn = more flexors muscles

Question 6

What is the definition of a motor pool, how it is different from a motor unit?

Answer 6

A motor pool is all of the motor units that innervate a “whole muscle group”. They’re grouped together on the longitudinal axis of the spinal cord, and can lie across one or more spinal cord segments.

A motor unit represents the single LMN and all of the muscle fibers innervated by it.

Question 7

Explain the innervation ratio

Answer 7

The number of muscle fibers connected to one motor unit = is always a bit different. BUT on average, specific muscle root, have specific innervation ration that are linked to their function.
- Example = gastrocnemius = on average 2000 muscle fibers connected to a single motor unit = very large range of how many muscle fibers are connected.
○ It is a high innervation ratio since it is a power/force muscle. If you activate it, you activate a big range of muscle fibers and generate more force.
Compare this to eye muscles = high precision = low innervation ratio

Question 8

Important distinction = a single muscle fiber, is only synapse by a __________.

Answer 8

Important distinction = a single muscle fiber, is only synapse by a single motor neuron.

Question 9

Know the definition of an innervation ratio. What properties of a muscle group with a high or low innervation ratio?

Answer 9

Innervation ratio: the number of muscle fibers that synapses by a single motor neuron (a muscle fiber is only synapsed by one motor neuron).

The properties of a muscles group with high or low innervation ratio
Differences in #s of motor units in a muscle (muscle specific)
Innervation ratio differs (i.e. # of muscle fibers/motor unity)
Innervation ratio differs within a muscle (there is a range) – power/force muscles = higher innervation ratio, precision muscles = smaller innervation ratio

Question 10

Know the three types of motor units. How do they vary in max force, rate of force production, and fatigue?

Answer 10

Three types of muscle fiber/motor unit types:

1. Slow (fatigue resistant) (SO), these are important for activities the require sustained contraction (like staying in an upright posture).
2. Fast, fatigue resistant (FR), intermediate size, not as fast as FF but produce twice the force as SO and are also fatigue resistant
3. Fast, fatigable (FF), important for brief activities that require a lot of force

Question 11

What is rate coding and the influence on contraction force?

Answer 11

Rate coding: the amount of force a muscle will produce depends on: how fast the motor units that are recruited discharge (rate or frequency coding), and the number of motor units that are recruited in a motor pool (recruitment). Motor units first have low firing rater and these rates increase as the force contraction increases. The higher the action potential rater = more calcium in muscle fibre moves more troponin/tropomyosin so more binding sites.

Question 12

Understand the principles of recruitment and Henneman’s size principle

Answer 12

Recruitment: produce more force= activate more motor units. New motor units start at the same firing rate but generate more force and reach higher firing rates. HSP: Motor units are recruited from smallest to largest (orderly fashion, SO → FF). Bigger change in maximum force with larger Motor units.

Question 13

How is rate coding and recruitment linked?

Answer 13

Rate coding and recruitment = LINKED TOGETHER!!!
Basic principle = our motor units, share a common drive…
- Imagine that there was one axon that was activated and it spoke to many LMN, and it produce the same excitatory post-synaptic potential in all LMN… BUT what differs is that the LMN (the post-synaptic structure), it varies in SIZE.
○ RED IN THE IMAGE = When our slow innervation ratio motor units, the cell bodies are quite small and you can imagine that the receptor potential that would occur from a LMN, that it could flow passively to this closer axon Hillock to any of the dendrites structure
○ YELLOW/DOTED BROWN IN THE IMAGE = if you compare the red to this = a higher innervation ratio of fast fatigable muscle
Because of this depolarization that is happening, it is MORE LIKELY TO REACH THRESHOLD AND DEPOLARIZE IN THESE SMALLER MOTOR UNITS FIRST.

Question 14

What are the 3 inputs to the LMN are they inhibitory/ excitatory or both?

Answer 14

• Upper motor neurons à descending control
• Spinal Cord Circuits à complex reflexes, CPG
• Sensory Inputs – Somatic sensation à simple reflexes

All can be EPSP or IPSP

Question 15

Why is the LMN considered the common final pathway?

Answer 15

LMN are considered the final common pathway because they are responsible for taking information from a variety of sources (including UMN which synapse on local neuron circuits in the brainstem and spinal cord) to skeletal muscles. LMN are the only way for the nervous system to cause muscle contraction. All neural control of movements requires the same LMN.

The LMN are the only way for the NS to cause muscle contraction and that means that all of the inputs to the LMN are competing for the exact same LMN. Whether it is a reflex coming from direct sensory input or a descending control from brain with goal orientated motion.
The movement is the result of excitation and inhibition from all of these sources to the same general lower motor neuron which we now know that is a pool of motor units and they have a common drive.

Question 16

What is the relevance of sensory information for motor function?

Answer 16

Sensory information provides information about the movement that allows for movement modification, information comes from proprioceptors, vision and vestibular. Sensory information is integrated to modulate the activity of LMN or UMN, (direct connections to LMN, indirect to LMn via interneuron network, most simple reflexes, informs UMN for their activity, leads to learning how to intelligently recruit UMN).

Question 17

How does the NS select which muscles to contract?

Answer 17

Question 18

What are the basic three properties of any senses and their function?

Answer 18

1. Stimulus: some form of energy in our internal/external environment
2. receptor: detects energy for “transduction”. Into an AP (how the Nervous system sends information)
3. Peripheral afferent: carries into CNS.

• Reflex circuits: rapid control of movement.
• Central pathway: higher level processing. Ascend in the CNS to Brainstem, cerebellum, cerebral cortex

• Peripheral afferents will carry information to the CNS in our peripheral nerve that will synapse in the dorsal root of the spinal cord.
• Once it gets to the CNS, it can participate into reflex circuits going to final cord interneurons or directly synapsing with LMN to activate them.
• BUT, these peripheral afferents can also ascend and go up to the higher brain centers –> brainstem, cerebellum, cerebral cortex
○ This happens through 2 main pathways for the proprioceptors
§ Dorsal column- medial lemniscus system (proprioception, discriminative touch)
Spinothalamic tract (pain, thermal sense, non-discriminative touch and pressure

Question 19

What is the design of the golgi-tendon organ. What stimuli is it most sensitive to? What energy is transduced and why?

Answer 19

GTO are designed to sense muscle tension from shortening. They are innervated by 1b axon nerve endings located in muscle tendon collagen fibrils. Collagen fibers and GTO are intertwined to create the response when tension is put on the muscle.
Tension propagates to tendon, fibrils “straighten”, axon “compressed”. GTO is most sensitive to tension produced by the muscle itself (active tension). Energy to increase firing is transduced to increase the muscle tension.

Question 20

What is the design of a muscle spindle (know the two parts). What stimuli is it most sensitive to? What energy is transduced and why?

Answer 20

Special sensor for muscle length. It is an intrafusal muscle fiber that is encapsulated in connective tissue. Sensory parts of the muscle spindle is the group 1a axon (which are around the nuclear bag and it senses dynamic “changes in length”) and group II axon (which are around the chair fibre and measure static length).
Most sensitive to changes in dynamic and static length, higher AP frequency when lengthened to prevent overstretch.

Question 21

Compare and contrast the GTO with the muscle spindle.

Answer 21

GTO senses force due to contraction, shortening MF. 1b axons

Muscle Spindle senses forces due to lengthening MF. 1a axons + group II axons

They complement one another to help regulate muscle performance through operations of distinct spinal cord reflexes.

Question 22

What is the function of the Gamma Motor Neurons? And contrast it with Alpha Motor Neurons.

Answer 22

Gamma motor neurons in the ventral horn, innervate intrafusal muscles and the muscle spindles. Help to control the sensitivity of muscle spindles.

Alpha motor neurons also in the ventral horn, innervate extrafusal muscles and produce contractions.

Question 23

How does a muscle spindle function with or without alpha-gamma co-activation?

Answer 23

Without co-activation there is a “gap” in afferent activity of the muscle contraction, which would be the case when there is only gamma neurons being activated. With co-activation there is no “gap” in afferent activity of the muscle. This gap is filled because gamma motor neurons are activated with alpha motor neurons, gamma motor neurons innervate intrafusal fiber, thus activation causes contraction, shortening the spindles and maintaining sensitivity.

Question 24

Alpha and gamma motor neurons are only coactivated when movement is coming from ?????
In the event of a reflex….

Answer 24

• Alpha and gamma motor neurons are only coactivated when movement is coming from a source of higher motor system = CNS

IF reflex = it is only activation of alpha motor neurons

Question 25

• Our extrafusal muscles, mainly producers of contraction which are innervated by ______ LMN.

On the other hand, and our muscles spindles and intrafusal muscles, they are innervated from their own LMN which are_________ = exist in the ventral horn.

Answer 25

Our extrafusal muscles, mainly producers of contraction which are innervated by alpha motor neurons LMN.

On the other hand, and our muscles spindles and intrafusal muscles, they are innervated from their own LMN which are gamma motor neurons = exist in the ventral horn.