The Motor System 23.02.23 Flashcards
What is goal-directed and habit motor control?
Goal direct: conscious, explicit, controlled
Habit: unconscious, implicit, automatic
What is voluntary and involuntary motor control?
Voluntary: running, walking, talking
Involuntary: eye movement, facial expression, jaw etc…
What places in the body evoke motor responses?
Spinal cord
Sensorimotor midbrain
Cortex and limbic system
What does activation of muscle fibres create ?
Muscle fibre activated means it contracts and they act in an all or one manner. Depends on LMNs and how they activate different muscle fibres
How do we achieve such a wide range of movement and forces?
- Muscles arranged in antagonistic arrangement (tricep and bicep, eye muscles) so combined co-ordination
- recruitment of muscle fibres (fast/slow twitch)
How do muscle types vary in individuals?
Varies but changes with time and changing. Appears to be genetically determined
How do muscles contract?
Different filaments of muscles interlock and move in and out. Actin (thin) and myosin (thick) they slide against each other producing contraction. Head of myosin filament walking along actin.
What neurotransmitter is released for the binding of actin and myosin for muscle contraction?
Acetylcholine triggers biochemical cascade in muscle cells releasing packets of calcium from inside muscle cell.
Causes myosin head to change shape so it can bind with actin filament
Why is ATP involved in muscle contraction?
ATP require to break the bond between myosin head and actin filament
Muscle contracted and remains that way until enzyme begins to disrupt actin and myosin
What is the motor unit?
Single alpha motor neuron and all the muscle fibres it innervates
How many muscle fibres do motor neurons innervates?
Different for each one.
Fewer fibres means greater movement resolution (finger tips and toes)
What does an activation of alpha motor neuron cause?
Depolarisation of this neuron and so contraction of all muscle fibres in that unit
How do we determine how many muscle fibres are innervated by a single neuron?
- The level of control the muscle needs (eye needs a lot)
- Strength of the muscle
Typically there is a range of motor units size
Where do lower alpha motor neurons come from and travel to?
Originate in the gray matter of spinal cord, or in the brainstem.
An alpha motor neuron and muscle fibres it connects represent unit of control of muscle force
What is the motor pool?
All the LMN that innervate a single muscle
Contains alpha and gamma motor neurons
Arranged in rod like shape within ventral horn of spinal cord
What are cell bodies in the ventral horn activated by?
- Sensory information from muscle (which is sent to the brain)
- Descending information from the brain
What senses and controls tension of muscles?
Golgi tendon organ sense tension
These are located within the tendon and sends ascending information to the brain via spinal cord about how much force there is in muscle
Critical for proprioception
How do the Golgi Tendon organs sense tension?
When a muscle is stretched, tension is exerted on the tendon, the bundles straighten and activate the GTO receptor endings
What is the inverse stretch reflex?
- 1b fibres inhibit muscle contraction (via inhibiting alpha motor neurons).
- The synergy between this and interneurons regulates muscle tension and prevents overload
What senses stretch and controls stretch of muscles?
Muscle spindles sense the length of muscles and this information forms key part of reflex circuits
What is the most simple reflex?
Monosynaptic
Within muscle fibres:
1. Intrafusal muscle (sensory with muscle spindle)
2. Extrafusal muscle (function more to do with being a muscle)
What innervates intrafusal muscle fibre to detect stretch?
Gamma motor neurons innervate intrafusal. This is because if intra and extra were both controlled by alpha, when the muscle is slack the system won’t be sensitive enough to these slight changes
So this gamma innervation keeps fibres set at length that optimises muscle stretch detection
- Sensory fibres are coiled around intrafusal fibres
Why are reflexes important?
- They can operate without engaging with the brain
- Critical for the avoidance of injury and effective motor control
- They can be simple or complex
What is the withdrawal reflex?
- Reciprocal innervation of antagonistic muscles (muscle that opposes the movement of another) explains why the contraction of one muscle induces the relaxation of the other
- This permits the execution of smooth movements
Describe the pathway of a monosynaptic stretch reflex (e.g. knee jerk)?
- Patellar tendon tapped
- Thigh muscles are stretched
- Stretch receptors activated
- Burst of action potentials in the afferent nerve fibres
- Activate excitatory synapses on the motor neurons that control this muscle
- Stimulations of motor units
- Contraction of the muscle
- Extension of the lower leg
How is the sensitivity of the monosynaptic stretch reflex regulated?
Regulated by gamma motoneurons
- These lead to tightening or relaxing of muscle fibres within the muscle spindle
Are the ends and centre of a muscle spindle contractile?
- 2 ends of muscle spindle are contractile
- Centre is non-contractile
