The Motor System Flashcards
What is motor control?
Motor control involves a dynamically changing mix of conscious and unconscious regulation of muscle force, informed by a continuous and complex sensory feedback, operating in a framework sculpted by evolutionary pressures
What different types of motor control are there?
- Voluntary = runnning, walking, talking etc
- Goal-directed = conscious, explicit, controlled
- Habit = unconscious, implicit, automatic
- Involuntary = eye movements, facial expressions, diaphragm, cardiac etc
What is the hierarchical control architecture of movement from an evolutionary perspective?
- Pain = Spinal Cord
- Loom = Sensorimotor
- Learned threat = Cortex + Limbic System
All use motor, autonomic and endocrine systems to create a defence related output
Describe an overview of the sensorimotor system
Association cortex
Secondary motor cortex
Primary motor cortex
Brainstem motor nuclei
Spinal motor units/motor units
Muscle
Descending circuits and continuous sensory feedback
What modulates movement in this sensorimotor system?
Basal ganglia = WHAT to do
Cerebellum = HOW to do it
What are the 4 main systems involved in the control of movement?
- Descending systems (upper motor neurons) = Motor cortex (planning, initiating voluntary movement) and brainstem centres (rhythmic, stereotyped movements, postural control)
- Basal ganglia = initiation of intended movement
- Cerebellum = coordination of ongoing movement
- Spinal cord and brainstem circuits = local circuit neurons (sensorimotor integration), motor neuron pools (lower motor neurons)
What governs motor control?
Lower and upper motor neurons
Lower motor neurons begin in the brainstem or spinal cord and project to the muscles
Upper motor neurons originate in higher centres and project down to meet lower motor neurons
What are the three types of muscle?
Skeletal muscle
Smooth muscle
Cardiac muscle
Describe the structure of a muscle
Contains several muscle fasciculi
Muscle fasciculi comprises several muscle fibres/sarcolemmas
Muscle fibre is constituted of several myofibrils
Myofibrils made up of short units called sarcomeres
Sarcomeres contain 2 protein filaments = actin and myosin myofilaments
How do muscles contract?
- Nerve impulse reaches the neuromuscular junction
- Acetylcholine released by the motor neuron
- Ach binds to the muscle receptor site
- Sodium channels in the muscle open causing sodium ions to rush into the cell initiating an action potential
- Action potential impulse causes the sarcoplasmic reticulum to release Ca+ into the cell and around the sarcomere contraction units
- Ca+ binds to troponin releasing tropomyosin out of the actin-myosin binding site = causes myosin heads to fit into actin filaments
- Myosin heads pull actin filaments toward the centre allowing filaments to slide past each other and shorten the sarcomere
ATP also binds
What causes rigor mortis?
ATP is required to break the bond between the myosin head and actin filament
So the muscle becomes contracted and remain that way until enzymes begin to disrupt the actin/myosin
Therefore, if there is no ATP i.e., if the person is dead, the bond between the myosin head and actin filament cannot be broken so the muscles stay contracted
What is the motor unit?
Single alpha motor neuron (lower motor neuron) + all the muscle fibres it innervates
The less fibres means the greater movement resolution
Activation of an alpha motor neuron depolarises and causes contraction of all muscle fibres in that unit (all or none)
What determines the average number of muscle fibres innervated by a single motor neuron?
- Level of control
- Strength
Size principle = units are recruited in order of size (smallest first)
Fine control typically required at lower forces
What are the different types of muscle fibre?
Fast and slow muscle fibres
- Slow (sustained muscular contraction e.g., maintaining posture)
- Fast fatigue resistant (generate twice the force of slow motor unit)
- Fast fatiguable (brief exertions that require large forces e.g., runnning, jumping)
Can the thickness of muscle fibres and the proportion of different muscle fibre types change?
Yes with training and exercise
What is the motor pool?
All the lower motor neurons that innervate a single muscle
The motor pool contains both the alpha and gamma motor neurons
Motor pools are often arranged in a rod like shape within the ventral horn of the spinal column
What 2 types of lower motor neurons are found in the motor neuron pools of the ventral horn?
Alpha motor neurons = control muscle contraction involved in voluntary movement
Gamma motor neurons = control muscle contraction in response to external forces acting on the muscle - in response to external forces the gamma motor neurons produce the involuntary reflexive movement
Where does the sensory input and motor output occur in the spinal cord?
Sensory input = dorsal root
Motor output = ventral root
What information is needed by the control system allowing muscles to contract and relax - a key part of proprioception?
- How much tension is on the muscle = Golgi tendon organs sense tension
- What is the length (stretch) of the muscle = muscle spindles sense stretch
What is the role of the Golgi tendon organs?
Found within the tendons
Sends ascending sensory information to the brain via the spinal cord about how much force there is in the muscle
Critical for proprioception
Under conditions of extreme tension, it is possible that GTOs act to inhibit muscle fibres to prevent damage
What are muscle spindles important for?
Sense the length of muscles i.e., the amount of stretch - this information forms a key part of reflex circuits
What is a monosynaptic reflex?
Most simple reflex e.g., patellar tendon reflex
A reflex arc that provides direct communication between sensory and motor neurons innervating the muscle
Describe the reflex circuit in movement
Motor pool in the ventral root of the spinal cord connected to the muscle fibres via alpha motor neurons
Within the muscle there are 2 types of muscle fibres
- Extrafusal muscle fibres (innervated by gamma motor neurons)
- Intrafusal muscle fibres which contain the muscle spindles (innervated by alpha motor neurons)
Explain the stretch reflex
- Muscle spindle is stretched
- Once detected it causes action potentials to be fired by sensory afferent fibres
- These synapse within the spinal cord with alpha motor neurons which innervate extrafusal fibres
- The antagonistic muscle is inhibited and the agonist muscle contracts
Why are intrafusal fibres innervated separately to extrafusal fibres and what innervates both types?
Intrafusal fibres = gamma motor neurons
Extrafusal fibres = alpha motor neurons
Need a system to detect stretch regardless of the current muscle length
If both muscle fibres are controlled by the same motor neurons, when a muscle is slack the system wont be sensitive to slight changes
Gamma motor neurons keep intrafusal fibres set at a length that optimises muscle stretch detection
What is the vestibular righting reflex or the righting reflex?
Vestibular system detects that the body is not upright as well as any acceleration due to gravity
Information from the vestibular system is combined with visual, somatosensory and proprioceptive sensory input in order to specify a pattern of motor activity that will restore uprightedness and a safe landing
The cerebellum which compares the intended motor plan with the actual situation is critical for computing the desired motor activity
Why is it so hard to build robots based off of these simple circuits?
Control of gross movement patterns can be devolved to simple spinal circuitry, but constant modulation based on sensory feedback is required to account for the unexpected
Higher CNS centres constantly adjust ongoing activity to resolve conflicting demands on the motor system and direct it towards goals
What is the organisation of descending motor control?
The most medial part of the ventral horn contains lower motor neuron pools that innervate axial muscles or proximal muscles of the limbs
The most lateral parts contain lower motor neurons that innervate the distal muscles of the limbs
Motor command originate in the motor cortex pyramidal cells = upper motor neurons
Pyramidal cell axons project directly or indirectly to spinal cord where they synapse with lower motor neurons
What is the main descending motor pathway?
The pyramidal tracts
= most cortical projections innervate contralateral motor units
What are the 2 tracts within the pyramidal tracts?
Dorsolateral corticospinal tract/ corticuorubrospinal tract
Ventromedial corticospinal tract/ ventral cortico-brainstem tract
Explain the corticospinal and corticobulbar tracts
Neurons in the motor cortex give rise to axons that travel through the internal capsule and come together on the ventral surface of the midbrain within the cerebral peduncle
Axons continue through the pons and come to lie on the ventral surface of the medulla giving rise to the medullary pyramids
The components of the corticobulbar tract innervating cranial nerve nuclei, the reticular formation and the red nucleus leave the pathway at the appropriate levels of the brainstem
Lateral corticospinal tract decussates at the base of the pyramids = 85% (controls movement of the limbs)
Anterior corticospinal tract 15% do not decussate and enter the spinal cord ipsilateral (control of axial muscles e.g., trunk)
What are the main differences in the dorsolateral and ventromedial (anterior) tracts?
Dorsolateral tracts
- Innervate contralateral side of one segment of spinal cord
- Sometimes project directly to alpha motor neurons
- Project to distal muscles
Ventromedial tracts
- Diffuse innervation projecting to both sides and multiple segments of spinal cord
- Project to proximal muscles of trunk and limbs
How does the basal ganglia modulate movement?
Receives excitatory input from many areas of cortex (glutamate)
Output goes back to cortex via the thalamus
Output is mainly inhibitory (GABA)
Complex internal connectivity involving 5 principle nuclei
- Substantia nigra
- Caudate & Putamen
- Globus pallidus
- Subthalamic nucleus
Explain disinhibitory gating of motor cortex output
At rest the caudate/putamen are quiet which means the globus pallidus is tonically active as the caudate and putamen is not inhibiting it
Tonic activation of the globus pallidus inhibits the thalamus causing reduced excitation of the motor cortex
When excited, a dopamine input from the substantia nigra causes the caudate and putamen are transiently excited, transiently inhibiting the globus pallius. This disinhibits (excites) the thalamus causing increased excitation in the motor cortex
What is the cerebellum’s role in modulating movement?
Plays a major role in adapting and fine-tuning motor programs to make accurate movements through a trial-and-error process
The cerebellum is a large brain structure that acts as a parallel processor, enabling smooth, controlled movement
No direct projection to the lower motor neurons, instead modulate activity of the upper motor neurons
What are the inputs and outputs of the cerebellum?
CORTICAL
- Mostly from motor cortex
- Also somatosensory and visual areas of the parietal cortex
SPINAL
- Proprioceptive information about limb position and movement (muscle spindles, other mechanoreceptors)
VESTIBULAR
- Rotational and acceleratory head movement (semicircular canals)
How can this cerebellar function be used in robots and brain machine interfaces?
Record and decode brain activity
Specify desired movement sequence
Bypass the motor cortex and lower motor neurons or damaged spinal pathways
Move limbs
What are BMI systems and explain their design?
Brain machine interface systems
1. The acquisition of brain-generated signals that reflect information processing and the encoding of action goals
2. The processing and decoding of brain signals using artificial neural networks to extract salient features and translate them into pragmatic control signals
3. The implementation of control signals for the operation of digital and mechanical systems
4. The generation of sensory-based feedback signals to promote adaptive plasticity and improved brain control of BMI technology
- Acquisition of brain-derived signals/BOLD signals e.g., fMRI, EEG, NIRS
- BOLD signal processing - feature extraction, machine learning, pattern classification, translation algorithm
- Device commands - simple = yes or no communication, intermediate = control of external devices, complex = limbs controlled by electrodes
