Motor Control Flashcards
What is motor control?
Motor control involves a dynamically changing mix of conscious and unconscious regulation of muscle force, informed by continuous and complex sensory feedback, operating in a framework sculpted by evolutionary pressures.
What are the types of motor control?
Voluntary: running, walking, talking playing guitar etc.,…….
Involuntary: eye movements, facial expressions, jaw, tongue, postural muscles throughout trunk, hand and fingers, diaphragm, cardiac, intercostals (around lungs), digestive tract……
What are the key concepts of motor control?
Motor control governed by lower and upper motor neurons.
•The lower motor neuron begins (has its cell body) in brainstem or spinal cord and projects to the muscle
•The upper motor neurons originate in higher centres and project down to meet the lower motor neurons
How do basal ganglia and cerebellum differ?
In the sensorimotor system
Basal ganglia - what to do
Cerebellum - how to do it
What is the overview of the sensorimotor system?
A descending controls system but with lots of ascending feedback
Association cortex -> motor cortex -> brainstem circuits -> spinal circuits -> motor unit -> effect on world -> sensory systems
How do muscle fibres act?
Individual muscle fibres act in an ‘all-or-none’ manner, and so control of muscle force depends on the way in which lower motor neurons activate different types of muscle fibre.
What are muscles like?
Make up about 40% of body weight
•Three types: cardiac, smooth, skeletal
•Smallest is the stapedius, found in the inner ear
•Largest is the gluteus maximus, found in the hip/buttock
•Strongest (based on weight) is the masseter, found in the jaw
•Hardest working?
•Heart (3 billion beats over average lifetime [no rest!])
•Eye muscles (10,000 precisely controlled movements in 1 hour of reading)
•Neck (keeps your 5Kg head in position)
How do muscles act?
Muscles can only contract or relax (i.e. stop contracting)
•The activation of muscle fibres is all or none
•So how do we achieve such a range of movements and forces ??
•Antagonistic arrangement – combined co-ordinated action
•Recruitment of muscle fibres – fast/slow twitch, small and large motor units (see later)
What are muscle fibres like across individuals?
The number of muscle fibres varies across individuals, but changes little with either time or training – appears to be genetically determined
How do muscles contract?
A skeletal muscle is attached to the bone by the tendon
•A skeletal muscle comprises several muscle fasciculi (group of muscle fibres)
•A muscle fasciculus comprises several muscle fibres (= muscle cells)
•A muscle fibre is constituted of several myofibrils
•Myofibrils contain protein filaments: Actin and Myosin myofilaments
•When the muscle fibre is depolarised actin and myosin slide against each other which produce muscle contraction
What is the myosin cross-bridge cycle?
Release of the neurotransmitter acetylcholine (ACh) triggers a biochemical cascade in muscle cells.
calcium ions, magnesium ions and ATP (energy)
Actin involved
What is the rigor mortis?
How does the myosin cross-bridge cycle occur?
The release of acetylcholine causes a cascade of events resulting in the release of packets of calcium from inside the muscle cell (fibre)
This causes the myosin head to change shape, enabling it to bind with the actin filament
ATP (provides energy for cells) is required to break the bond between the myosin head and the actin filament
ATP is produced by oxidative metabolism, which stops upon death
So the muscle become contracted and remain that way until enzymes begin to disrupt the actin/myosin
What is a motor unit?
Motor unit = single alpha () motor neuron + all the muscle fibres it innervates – Different motor neurones innervate different numbers of muscle fibres – fewer fibres means greater movement resolution - those innervating finger tips and tongue
What are motor units like?
The motor unit is the final common pathway for motor control
• Activation of an alpha motor neuron depolarises and causes contraction of all muscle fibres in that unit (all or none)
• Muscle fibres innervated by each unit are the same type of fibre and often distributed through the muscle to provide evenly distributed force (and may help reduce effect of damage)
•More motor units fire – more fibres contract – more power
What is the control of muscle force like?
Average number of muscle fibres innervated by single motor neuron (a motor unit) varies according to two functional requirements for that muscle:
1. Level of control
2. Strength
Typically a range of motor units in a muscle, some with few, some with many fibres.
What is the size principle for control of muscle force?
Size Principle
Control of Muscle Force
Units are recruited in order of size (smallest first)
Fine control typically required at lower forces
Try playing the violin with weights attached to your arms!!
What are the speed of muscle fibres?
Slow
Fast fatigue resistant
Fast fatigable
What is the difference between between fast and slow muscle fibres?
What are lower (alpha) motor neurons?
Originating in the grey matter of the spinal cord, or in the brainstem, an alpha motor neuron and the muscle fibres it connects to represent the ‘unit of control’ of muscle force.
What is the motor pool?
All the lower motor neurons that innervate single muscle
•The motor pool contains both the alpha and gamma motor neurons (see later)
•Motor pools are often arranged in a rod like shape within the ventral horn of the spinal column
Where do alpha motor neurones originate from?
Alpha Motor Neurons originate in the Spinal Cord
What are the arrangements for alpha motor neurons?
What are alpha motor neurone origination like?
Cell bodies in the ventral horn: activated by:
•Sensory information from muscle
•Descending information from brain
Note the closeness and prominence of sensory input to the dorsal horn indicated in this diagram
What is sensing in muscles?
Muscles can be contracted or relaxed to provide movement, but a good control system (the CNS) needs to know two things:
(1)how much tension is on the muscle;
(2)what is the length (stretch) of the muscle
Key part in proprioception
Golgi tendon organs sense tension
Muscle spindles sense stretch
What are Golgi tendon organs like in muscle tension (force)?
The GTO is within the tendon (where the muscle joins to bone)
•Mostly, it 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 (via a circuit in the spinal cord) to prevent damage
What are muscle spindles like In muscle length (stretch)?
Muscle spindles sense the length of muscles, i.e. the amount of stretch
•This information forms a key part of reflex circuits…….
What are the reflex circuits?
C5 – Biceps
C6 – Biceps, Brachioradialis
C7 – Triceps
L4 – Patellar (knee jerk)
S1 – Achilles (ankle jerk)
What are reflexes like?
Reflexes can be quite simple or quite complex. They can operate without engaging with the brain, and are critical for the avoidance of injury and effective motor control
What is the reflex circuit?
Most simple reflex – MONOSYNAPTIC – e.g. the patellar tendon reflex
Extradural and intramural muscle fibres etc
What is the reflex circuit?
Most simple reflex – MONOSYNAPTIC – e.g. the patellar tendon reflex
Extradural and intramural muscle fibres etc
What system detects stretch?
We need a system to detect stretch regardless the current muscle length
If intrafusal muscle fibre is controlled by same motor neurons as extrafusals, when muscle is slack (or taught), the system won’t be sensitive to slight changes
So, intrafusal fibres are innervated separately, by gamma () motor neurons
They keep the intrafusal fibres set at a length that optimises muscle stretch detection
What is the imp of muscle spindle feedback?
An efficient motor control system needs to know how much each muscle is stretching – information provided by muscle spindles
What occurs in muscle spindle feedback?
Embedded within most muscles
Composed of intrafusal fibers
Detect stretch regardless of the current muscle length
Sensory fibres are coiled around the intrafusal fibers
Intrafusal fibers are innervated separately, by gamma () motor neurons
They keep the intrafusal fibers set at a length that optimizes muscle stretch detection
Muscle sensory receptors = muscle spindles
What is the simplest reflex?
Stretch reflex
What occurs in the quadruped reflexes?
In quadrupeds, sequence and precise control of limb movements appears complex and is substantially altered by speed changes (e.g. walk, trot, gallop)
Under control of higher brain centre…?
NO!
Quadrupeds will walk on treadmill if weight supported if spinal cord damaged at thoracic level
Will change to appropriate patterns of limb movement as treadmill speed is altered
Complex reflex system responding to nothing more than stretch of muscle spindles!!
What occurs in the withdrawal reflex?
Reciprocal Innervation
Principle described by Sherrington (also called Sherrington’s Law of reciprocal innervation)
Reciprocal innervation of antagonistic muscles explains why the contraction of one muscle induces the relaxation of the other
Permits the execution of smooth movements
What is the buttered cat paradox?
Why do cats always land on their feet?
More complex reflexes (4)!
The righting reflex, or vestibular righting reflex
Vestibular system detects that the body is not upright (orientation), as well as any acceleration due to gravity (i.e. falling)
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 ‘uprightness’ and a safe landing
The cerebellum, which compares the intended motor plan with the actual situation is critical for computing the desired motor activity (see next in lecture)
What is variable muscle force provided by?
Variable muscle force is provided by gradual recruitment of motor units
why is it so hard to build robots that can walk, run, track moving objects with robotic eye etc….?
Control of gross movement patterns (which can be quite complex) 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 are the brainstem structures like?
Pathways and nuclei within the brainstem (and midbrain) connect sensory input to motor output in quite direct ways, providing an evolutionarily ancient but still very important control system.
What is the ancient brainstem motor control?
without cortical involvement
e.g. balance and postural control
…also orienting, gross limb movement/positioning
Vestibular organ - effects on oculomotor control - head movement - vertical positioning of eyes
e.g. branches of vagus nerve project to larynx to control speech – this circuit richly interconnected with cerebellum and other brainstem sensorimotor systems
Also control of respiration!!
What is a combination of ancient and modern cooperation?
Speech : primitive sounds sculpted by cortex
In many motor activities, ancient and modern (cortical) control systems work co-operatively…
What is the primary motor cortex like?
Primary motor cortex exerts quite direct, top down control over muscular activity, with as few as one synapse (in the spine) between a cortical neuron and innervation of muscle cells
What are descending projections from cortical motor areas?
Motor command originates in motor cortex pyramidal cells (in layer 5-6, grey matter).
•These are the upper motor neurons.
•Pyramidal cell axons project directly or indirectly (e.g. via brainstem) to spinal cord, where they synapse with lower motor neurons.
•The axons of these upper motor (pyramidal) neurons form the pyramidal tract
•Most cortical projections innervate contralateral motor units
What is the hommunculus?
Homunculus is a reasonable representation, but an oversimplification: damage to a single finger area doesn’t mean loss of voluntary control of that finger.
•Representations are more complex and overlapping
•After all, few motor commands require isolated activation of a single motor unit
Involved In motor control
