6.1 Motor Control Flashcards
(38 cards)
Define: Lower Motor Control
Lower Motor Control: If this is active, skeletal muscles innervates are active
Define: Upper Motor Control
Upper Motor Control: Anything that controls the excitability of a lower motor neuron**
What kind of neurons are motoneurons, Where are their cell bodies, What do they do?
Large cholinergic neurons, with cell bodies in ventral spinal cord and large fast-conducting myelinated axons, innervates muscles and cause contractions
How are motor neurons organised?
- Motoneurons are organised into distinct, rod-shaped clusters called motor neuron pools
Motor Neuron Pools
- Motor neuron pools are clusters of motoneurons that innervate muscle fibres within a single muscle
- Motor neurons pools have little overlap and follow a broad topography: There is a relationship between where muscles are and where motor neurons are located in the spinal cord (i.e., neurons that are clustered together tend to innervate muscles which are adjacent to each other)
Relationship between motor neurons and muscles - Organisational structure
Somatotropic Organisation
Proximal muscles are innervated by lower motor neurons whose cell bodies are located medially in the ventral spinal cord
Distal muscles are innervated by lower motor neurons whose cell bodies are located laterally in the ventral spinal cord
Relationship between motor neurons and fibres.
Also, how are muscle fibres distributed and why?
- Each single motor neuron innervates a number of muscle fibres. This is called a motor unit
- Each muscle fibre is innervated by a single lower motor neuron
- Muscle fibres that are innervated by a single motor neuron are distributed widely (not located adjacently). This is to prevent muscle damage.
How do motor units differ in number they innervate?
Some motor units innervate a small number of muscle fibres
Some motor units innervate a large number of muscle fibres
How do motor units differ in type
- Slow: Small motor neurons produce the least amount of force, and are recruited first
- Always recruited, can contract constantly, aerobic capacity
- Allows us to do fine, delicate things
- Fast fatriguable-resistant
- Fast faiguable: Large motor neurons produce the most amount of force, and are recruited last
* Recruited less, anaerobic capacity
Maximum force = All motor neurons recruited
What is the Size Principle
The size principle states that motor units will be recruited in order of size from smallest to largest depending upon the intensity
Synaptic inputs to lower motoneurons includes
2 ++
- Sensory input from muscle spindles
- Key sensory input from muscles and comes through to dorsal spinal cord
- Either: (a) directly to alpha motor neurons or (b) indirectly through spinal interneurons to alpha motor neurons
2. Inputs from upper motor neurons originating in the brain - Either: (a) directly or; (b) more commonly indirectly through circuit neurons
What do muscle spindles detect and what is their main role?
Do all muscles have muscle spindles?
- Stretch sensory receptors (Mechanoreceptors) embedded in the intrafusal muscle detects changes in muscle length or stretch
- Maintain joint and muscle position by detecting muscle length
What are the properties of Group 1 and 2 afferents?
Group 1 and 2 afferents are the biggest, fastest conducting afferent axons
Muscle spindle reflex: Activation
When muscle spindles are activated beyond set point, they generate a set of APs which travels to the spinal cord via sensory afferent
Sensory afferent of muscle spindles (2). What does this act maintain
Disynapse:
- Directly excites (monosynapse) the (alpha) lower motor neurons that innervate that same muscle
- Indirectly inhibits antagonistic muscles via an inhibitory interneuron
- This acts to maintain joint position by correcting changes made to joint position : Negative feedback
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Muscle Spindles: Reactions to muscle contracting or relaxing
Overview and Main Role of Golgi Tendon Organs (GTO)
Detect the force of the muscle which is being transferred through the tendon
Maintain force generated by detecting and regulating tension coming through tendons
GTO reflex: Activation
If force surpasses a set point, GTOs are activated and creates inhibitory reflexive circuits.
Sensory afferent of GTO (2)
Disynapse
- Synapse with an inhibitory interneuron (1b) in the spinal cord, which synapses with the alpha motor neuron innervating the same muscle
- Synapse with an excitatory interneuron, which synapses with the alpha motor neuron innervating the antagonistic muscles
Example: Stretch Reflex. When muscles contract, how do spindles and GTO respond and what is the implication?
- Reduced length decreases muscle spindle activity
- The increased force increases GTO activity
- Hence, they provide 2 independent information about muscles
What are Gamma motor neurons (vs alpha motor neurons)
Gamma motor neurons innervates intrafusal fibres at 2 ends of the spindle while alpha motor neurons innervates extrafusal fibres. This is often done in synchrony (Tuning function)
Flexion-crossed extension reflex (2)
Noxious stimulus activates nociceptor, and its sensory afferents called cutaneous afferent fibres (via. interneurons) will:
- Activate flexor muscles and inhibit extensor muscles on the same leg, allowing us to withdraw from the noxious stimulus
- Activate extensor muscles and inhibit flexor muscles on the opposite leg, allowing us to weight-bear
- Some of them also crossover
Ipsi = withdraw, Contral = Extend
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Pattern Generation: Walking. Phases in humans and animals
Sequentially:
- Flexion phase: Activating flexors and inhibiting extensors (Swing in animals)
- Extension phase: Activating extensors and inhibiting flexors (Stance in animals)
What controls locomotor pattern?
In cats, what are the findings?
Spinal motor pattern generator creates locomotor patterns
The transition between gait (i.e. walk to run) and pattern of gait is controlled by the spinal cord. When treadmill is sped up, the cat moves to match the treadmill speed
Supra-spinal inputs to motoneurons: Types (2)
Lateral white matter tracts: Axons from motor cortex
Medial white matter tracts: Axons from brain stem
