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
What is an overview of motor control?
But the Basal Ganglia and Cerebellum are doing quite different things, not just opposing one another (simplified schematics have their limits!)
What are the Descending Projections from Motor Cortex?
How are the dorsolateral and ventromedial tracts similar?
Both contain a direct corticospinal route
Both contain an indirect route via brainstem nuclei
What occurs in the dorsolateral tract?
Via red nucleus
Innervate contralateral side of one segment of spinal cord
Sometimes project directly to alpha motor neuron
Project to distal muscles, e.g. fingers
What occurs in the ventromedial tract?
Via tectum, vestibular nuclei, reticular formation & cranial nerve nuclei
Diffuse innervation projecting to both sides and multiple segments of spinal cord
Project to proximal muscles of trunk and limbs
How movement modulated by basal ganglia and cerebellum?
What is the basal ganglia?
A group of structures beneath the cortex that act as a ‘gate-keeper’ for control of the motor system (muscles)
What is the basal ganglia like?
The basal ganglia are a group of nuclei lying deep within cerebral hemispheres
•Widely studied (including here at Sheffield)
•Role in motor control not fully understood
•Basal ganglia dysfunction implicated in many disorders
What is the basal ganglia involved in?
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 (pars compacta & pars reticulata)
Caudate & Putamen (together=striatum)
Globus Pallidus (internal and external segments)
Subthalamic Nucleus
What is the selection problem?
Multiple command systems
•Spatially distributed
•Processing in parallel
•All act through final common motor path
•[Cannot do more then one thing (well) at a time]
•How do you resolve the competition???
How does disinhibitory gating of motor cortex output occur in the basal ganglia?
Excitatory Input from Cortex
Dopaminergic Input from Substantia Nigra
What are the loops?
Motor
Oculomotor
Prefrontal
Limbic
What is the cerebellum?
The cerebellum is a large brain structure that acts as a ‘parallel processor’, enabling smooth, co-ordinated movements. It may also be very important in a range of cognitive tasks.
What is the cerebellum like?
Like basal ganglia, no direct projection to the lower motor neurons – instead modulate activity of upper motor neurons
Contains approx half total number of CNS neurons
•Just 10% of total brain weight
•Projects to almost all upper motor neurons
What are the inputs and outputs of the cerebellum?
Spinal cord, cerebral cortex and vestibular system -> pons -> cerebellum -> thalamus -> motor cortex
Cortical
- Mostly from motor cortex (copies of motor commands)
- Also somatosensory and visual areas of parietal cortex
Spinal
- Proprioceptive information about limb position and movement (muscle spindles, other mechanoreceptors)
Vestibular
- Rotational and acceleratory head movement (semicircular canals / otoliths in inner ear)
What is the cerebellar function?
A) It knows what the current motor command is
A)It knows about actual body position and movement
B)It projects back to motor cortex
Computes motor error and adjusts cortical motor commands accordingly
- Not just motor control, but motor learning too, in collaboration with basal ganglia and cortical circuits.
2.Functional brain imaging studies have demonstrated that the cerebellum is involved in a wide variety of non-motor tasks
Could recordings of cortical electrical activity be used to directly control lower motor neurons, thus bypassing (possibly damaged) spinal pathways?
DISCUSS. The direct projection from primary motor cortex to lower motor neurons is very direct.
More resent answer yes
Where do motor commands originate?
Motor commands originate in upper motor neurons of primary motor cortex and descend along corticospinal tracts to lower motor neurons originating in spinal cord
Where does a great deal of motor control and muscle activity, happen?
outside of conscious awareness
What is the final common pathway of motor control?
•The motor unit innervated by a single alpha motor neuron, is the final common pathway of motor control
What is precise motor control governed by?
Precise motor control is governed by the size principle, different types of muscle fibre, and the antagonistic arrangement of muscles
What are motor commands like?
•Motor commands are ‘gated’ by the basal ganglia nuclei, by a system of ‘disinhibition’
What does the cerebellum provide?
The cerebellum provides precise control, fine adjustment and co-ordination of motor activity based on continual sensory feedback
What is the organisation of the motor pathway - peripheral?
How do we move?
We need constantly to process a vast amount of
afferent information from muscle spindles
What do basal ganglia do?
processing all that afferent information
What are peripheral nerves like?
Peripheral nerves are motor and sensory
• Some carry autonomic fibers
• Pathology could be axonal, demyelination or
both (Read)
• When impaired features are
– Numbness, tingling, burning, freezing pain
– Weakness and muscle wasting
– Poor balance as a result
– Deformities secondary to weakness
What are some causes of peripheral nerve damage?
Diabetes
Idiopathic
(Leprosy)
(HIV)
Deficiency states e.g. B12 Folate
Alcohol/Toxins/Drugs
Hereditary Neuropathies
Paraneoplastic Syndromes
Metabolic abnormalities Porphyria
What are the major event in neuromuscular transmission?
Motor neuron depolarization causes action potential to
travel down the nerve fiber to the neuromuscular junction
(1).
• Depolarization of the axon terminal causes an influx of
Ca2+ (2) which triggers fusion of the synaptic vesicles (3)
and release of neurotransmitter (Acetylcholine; ACh) (4).
• ACh diffuses across the synaptic cleft and binds to post-
synaptic ACh receptor (AChR) located on the muscle fiber
at the motor end-plate (5).
What does the binding of ACh to AChRs do?
Binding of ACh to AChRs opens the intrinsic ion
channel, resulting in a cation entry locally. This results
in a local depolarisation of the sarcolema.
• Local sarcolemmal depolarisation results in opening
of the sodium channels (VGSC) causing an influx of
Na (5), depolarization of the sarcolemma that travels
down the t-tubules (6) and ultimately causes the
release of Ca2+ from the sarcoplasmic reticulum -
CONTRACTION.
• Unbound ACh in synaptic cleft defuses away or is
hydrolyzed (inactivated) by acetylcholinesterase
(AChE) (7).
What is skeletal muscle like?
Muscle cell = muscle fiber (a single cell with one
nucleus)
– Muscle fibers are made of myofibrils (striated)
– Myofibrils are made of units called sarcomeres
– Sarcomeres are made of thick and thin filaments
– Z line is the end of the sarcomere
– Thick and thin filaments slide over one another to
shorten the muscle during contraction
What is the sliding filament theory?
Links the structure of a sarcomere to its function
• During contraction thin filaments slide over thick
filaments
• Thick filaments= myosin and have “heads”
• Thin filaments = actin, these slide
• Ca and ATP required for sliding and attachment
Describe the sliding filament theory
- ATP binds to myosin head, which is released from an actin filament
- Hydrolysis of ATP cocks the myosin head
- The myosin head attaches to an actin binding site with the help of calcium
- The power stroke slides the actin (thin) filament
What is the structure of muscle like?
100-1000‘s muscle fibres
make up a muscle
Muscle fibres long
structure
multinucleated
bound by sarcolemma
What is the structure of myofibril like?
Myofibril
The contractile element
within the myofibre
Myofibril
Thin filaments (I) - Actin
Thick filaments (A) -
Myosin
What is needed to coordinate the movement?
Coordinate the movement- extrapyramidal
system is needed
Coordination is also mediated by cerebellum
(read features of cerebellar dysfunction, ataxia)
• Sensory input about place of joints in relation to
the 3D space (read about joint position sense,
dorsal column structure and purpose, sensory
ataxia)
Why do we have 2 eyes?
Useful to have a spare
•Allows us to see in 3D (stereopsis)
•Widens our visual field
What are the layers of the eye?
Outer layer
Sclera and cornea
•Middle layer
Uvea
•Inner layer
Retina
What is the outer layer for the eye like?
Sclera – tough fibrous outer coat, made up of collagen
•Cornea – also made up of collagen!
Light transmission – must be transparent
Barrier to trauma and infection – must be tough
Responsible for ~2/3 refractive power of the eye (43D)
What is the outer layer for the eye like?
Sclera – tough fibrous outer coat, made up of collagen
•Cornea – also made up of collagen!
Light transmission – must be transparent
Barrier to trauma and infection – must be tough
Responsible for ~2/3 refractive power of the eye (43D)
What are the layers of the outer layer of the eye?
5 layered structure
- Epithelia
- Bowman’s layer
- Stroma
- Descemet’s layer
- Endothelium
What are the layers of the outer layer of the eye?
5 layered structure
- Epithelia
- Bowman’s layer
- Stroma
- Descemet’s layer
- Endothelium
What is the middle layer of the eye like?
Made up of iris, ciliary body and choroid
What is the middle layer of the eye like?
Made up of iris, ciliary body and choroid
What is the iris like?
Iris –
coloured part at front of the eye
contains dilator and sphincter pupillae muscles
pupillary reflexes
What is the ciliary body like?
Ciliary body –
glandular epithelium produces aqueous humour
ciliary (smooth) muscle controls accommodation
Choroid –
blood supply to outer third of retina
heat sink
What is the choroid like?
Choroid –
blood supply to outer third of retina
heat sink
What is the inner layer of the eye?
Retina
•Specialised organ of
phototransduction
•Many layers
What is the retina made up of?
Macula lutea
•Fovea centralis
•Cones
•Rods
What are the layers of the retina?
8 layers?
What does the retina contain?
Retinal photorecptors
•Bipolar cells
•Amacrine & horizontal cells
•Mullers glial cells
•Retinal ganglion cells
What is Phototransduction?
What is the anterior segment?
Aqueous humour
•Nutrition to lens and
cornea
•Maintains intraocular
pressure
What is the lens like?
Biconvex
•Responsible ~1/3 refractive
power of the eye (~20D)
•Accommodation
What is the lens like?
Biconvex
•Responsible ~1/3 refractive
power of the eye (~20D)
•Accommodation
What is Emmetropia?
What is Hypermetropia?
Underpowered to focus near objects on retina
•May be due to:
- corneal curvature too shallow
- lens not flexible enough
- axial length of eyeball too short
What is myopia?
Overpowered so can’t focus far objects on retina
•May be due to:
- corneal curvature too steep
- axial length of eyeball too long
What is in the posterior segment?
Vitreous humour
•Avascular viscoestalic gel
•Hyaluronic acid (GAG)
•Collagen
What is adnexae?
Lids
•Conjunctiva
•Tear film
What is the structure of the adnexae?
Levator palpebrae superior is muscle
Superior suspensory ligament of the globe
Aponeurosis
Superior tarsal (mullers muscle)
Superior suspensory ligament of the fornix
What does teh adnexae include?
Lids – protect the globe
•Anterior skin
•Eye lashes
•Meibomian glands
•Orbicularis oculi
•Tarsal plate
•Tarsal conjunctiva
•Levator palpebrae superioris & sympathetic muscle
What are …. Of the adnexae?
Conjunctiva
palpebral (tarsal) vs. bulbar (ocular)
•Conjunctival fornix
•Limbal stem cells
•Mucous membrane (Goblet cells)
•Lymphoid cells (protective)
What is the tear film like in the adnexae?
Tear film
•3 layers
anterior lipid
middle aqueous
posterior mucous
•Protective
•Nutrition for cornea
What is the tear film like in the adnexae?
Tear film
•3 layers
anterior lipid
middle aqueous
posterior mucous
•Protective
•Nutrition for cornea
What is the arterial supply of the eye?
Internal carotid a. → ophthalmic a.
•Branches of ophthalmic a. (ocular group)
central retinal a.
posterior ciliary a. → long and short posterior ciliary a.
muscular a. → anterior ciliary a.
•Branches of ophthalmic artery (orbital group)
lacrimal a.
What is the blood supply of the inner retina?
Inner 2/3 retina supplied
by central retinal a.
•Branches into
superior/inferior/temporal/nasal
branches
•Drained by branch retinal veins
→ central retinal v. →
ophthalmic v. → cavernous sinus
→ internal jugular v.
What is the blood supply of the outer retina?
Outer 1/3 supplied by choroid
•Posterior ciliary a. → choroidal a. → choriocapillaris
•Blood-retinal barrier at RPE
regulates movement of nutrition and solutes from choroid into subretinal space
What is the venous drainage for the eye?
Vortex veins drain the choroid
•Usually one for each quadrant
•Superior drain to SOV, inferior drain to IOV
Where do the veins drain into for the eye?
Superior ophthalmic veins drain directly into the cavernous sinus
•Inferior ophthalmic veins drain into pterygoid venous plexus
•Valveless system – orbital cellulitis/facial infection can precipitate cavernous sinus thrombosis
What is the lymphatics for the eye?
No lymphatic drainage from the globe
•Conjunctiva and lids do have lymphatic drainage to submandibular and pre-auricular nodes
How does light travel through the eye?
