6.1 Motor Control Flashcards
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
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- 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
Difference between complex: aspects of motor control vs motor tasks (2)
Spinal cord mediates complex aspects of motor control such as feedback control of muscle stretch and force, and integration of reflex and rhythmic activity.
More complex motor tasks - like maintaining body posture and formulating and executing voluntary movements - require the brain: the main sources of this input are the brainstem and the motor areas of the cerebral cortex..
What forms the basis of alternative movements such as breathing and locomotion? (2)
Pattern generating circuits in the spinal cord
Basis of these pattern generators are intrinsically active neurons firing bursts of action potentials, and their reciprocal interconnections such that alternating output to flexors and extensors, and to the left and right limbs is produced.
What are the ventral-medial tracts, specifically? What information do they carry?
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Ventral-medial: Brainstem to spinal cord
Ventral / medial tracts (tectospinal, vestibulospinal and reticulospinal) carry information related to maintenance of posture.
Their activity is influenced by visual, vestibular (ear,balance) and proprioceptive activity (proprioceptive = ““body position sense””: mainly muscle spindle activity).
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Common features of ventral-medial tracts (2)
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- Axons project ipsilaterally in medial white matter tracts
- Synapses with medially located interneurons on either side of the midline in cervical spinal cord
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Descending Pathways from motor cortex (2)
- Corticobulbar Pathways
- Corticospinal Pathways
Descending Pathways from motor cortex (2): Corticobulbar Pathways
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Synapses at brainstem
Reticular formation is often a target of the motor cortex neuron axons, which then descend down the reticulospinal tract
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Descending Pathways from motor cortex (2): Corticospinal Pathways
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Directly from motor cortex to spinal cord
- Internal capsule into brainstem
- ~90% will decussate in brainstem (medella), and continue in the lateral corticospinal tract’;
~10% will continue ipsilaterally and continue to the ventral corticospinal tract, decussating in spinal cord
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Organisation of motor cortex
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Regions of functionally related movements (rather than a topographic map of the body), corresponds with their degree of motor innervation
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What do motor maps represent?
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- Mapped on the M1 are relevant combinations of muscle action that produce movements
- Purposeful movements result from prolonged stimulation of M1
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Role of Motor association cortex
Motor association cortex encodes more complex movements and more abstract representations of motor goals
What are lower motoneuron signs
Without innervation from motoneurons, small regions within muscles (single fibre or groups of fibres - fascicles) may exhibit weak spontaneous contractile activity; prolonged denervation results in muscle atrophy.
What are upper motoneuron signs
Upper motoneuron signs are characterised by over-excitability of muscles, due to a loss of the net inhibition the brain exerts over the spinal cord.
Example of an upper motoneuron syndrome
“Babinski sign:
Brain unable to control spinal cord and tonically inhibit reflex, causing extensor plantar response
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Feedforward/Feedback Mechanisms
Feedforward system from central command anticipates postural adjustments based on previous experience
Once the movement is executed, if we still have postural instability, the feedback system from postural reflexes will correct this by feedback for unanticipated postural instability
