Sensorimotor System Flashcards
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.
Types of motor control
Voluntary
Goal-directed
Habit
Involuntary
Voluntary motor control examples
Walking
Running
Talking
Goal-directed motor control
Conscious
Explicit
Controlled
Habit motor control
Unconscious
Implicit
Automatic
Examples of involuntary motor control
Eye movements
Facial expressions
Jaw
Tongue
Postural muscles
Hand and fingers
Diaphragm
Cardiac
Intercostals
Digestive tract
Lower motor neurones
Cell body in brainstem or spinal cord and projects to the muscle
Upper motor neurons
Originate in higher centres and project down to meet the lower motor neurones
Smallest muscle
Stapedius- found in the inner ear
Largest muscle
Gluteus maximus - found in the hip/buttock
Strongest muscle
Masseter- jaw
3 types of muscle
Cardiac
Smooth
Skeletal
Antagonistic arrangement
Combined co-ordinated action
How do we achieve a range of movements and forces
Antagonistic arrangement
Recruitment of muscle fibres
Muscle size and strength is dependent on
Cross-sectional area of individual fibres and different proportions of the different types of fibres
Number of muscle fibres
Varies across individuals
Changes little with either time or training
Genetically determined
What attaches muscle to bone
Tendon
Muscle fasciculus
Several muscle fibres
Rigor mortis
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
Motor unit
Single alpha motor neurone and all the extrafusal skeletal muscle fibres it innervates
Fewer fibres innervated by a motor neurone means
Greater movement resolution eg finger tips and tongue
Muscle fibres innervated by each motor unit
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
Average number of muscle fibres innervated by single motor neuron (a motor unit) varies according to two functional requirements for that muscle:
Level of control
Strength
Size principle
Units are recruited in order of size (smallest first)
Fine control typically required at lower forces
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.
Motor pool
All the lower motor neurones that innervate single muscle
Contains both alpha and gamma motor neurones
Often arranged in a rod like shape within the ventral horn of the spinal cord
Cell bodies in the ventral horn are activated by
Sensory information from muscle
Descending information from brain
Muscles can be contracted or relaxed to provide movement, but a good control system (the CNS) needs to know two things:
how much tension is on the muscle;
what is the length (stretch) of the muscle
What proprioceptor senses stretch
Muscle spindles
Which proprioceptors sense tension
Golgi tendon organs
Golgi tendon organs
Within the tendon
Sends ascending sensory information to the brain via the spinal cord about how much force there is in the muscle
Under conditions of extreme tension
Golgi tendon organs act to inhibit muscle fibres to prevent damage
Muscle spindles
Sense the length of muscles —>amount of stretch
Embedded within most muscles
Composed of intrafusal fibers
Detect stretch regardless of the current muscle length
Most simple reflex
Monosynaptic- eg patellar tendon reflex
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
Muscle spindle feedback
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
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
Alpha motor neurons located laterally
Control distal muscles
Alpha motor neurones located medially
Control proximal muscles
Muscle tone
The degree of contraction of a muscle or the proportion of motor units that are active at any one time
High muscle tone
Feels firm or rigid
Resists passive stretch
Low muscle tone
Feels soft or flaccid
Offers little resistance to passive stretch
Alpha motor neurones
Produce clinical signs of LMN syndrome when damaged
Cell bodies originate in laminae VIII and IX of the ventral horn - somatotopically organised
Function of alpha motor neurones
Can be voluntary via UMNs
Can also elicit the myotatic stretch reflex
Gamma motor neurones function
Regulation of muscle tone and maintaining nonconscious proprioception
Signal length and velocity of a muscle
Which motor neurones are activated during voluntary movement
Both Alpha and gamma simultaneously
Signs of LMN damage
Hypotonia - reduced or absent muscle tone
Hyporeflexia- decreased or absent reflexes
Flaccid muscle weakness or paralysis
Fasciculations - small involuntary muscle twitches
Muscle atrophy
Which neurotransmitter is commonly involved with UMN
Glutamate
Damage to a UMN
Causes weakness or paralysis of movement for the group of muscles it innervates
Signs of UMN damage
Hypertonia - abnormally high level of muscle tone due to loss of descending inhibition
Hyperreflexia - brisk reflexes
Spasticity - muscle is tight and stiff on passive movement
Positive babinski sign- large toe extends in response to a blunt object stroked on the plantar surface (instead of flexes)
Clonus- where a muscle is suddenly stretched and held there
Clasp knife reflex- rapid decrease in resistance when flexing a joint
Common cause of UMN signs
Stroke when it affects the cerebral cortex of internal capsule
Types of stretch receptors
Nuclear chain fibres
Nuclear bag fibres
Nuclear chain fibres
Respond to how much the muscle is stretched
Nuclear bag fibres
Respond to both magnitude of stretch and the speed it occurs at
What innervates the ends of the intrafusal fibres
Gamma motor neurons
Keep the fibres at a set length —> optimises muscle stretch detection
How is the muscle spindle composed
2 ends are contractile
Centre is non-contractile
Middle 1/3 is associated with fast type 1a Afferent sensory nerves
inferior and superior thirds are associated with type 2 afferent sensory nerves (slower conducting)
Upper motor neuron lesions
Everything is going up
What is the middle third of the spindle associated with
Fast type 1a Afferent sensory nerves
What are the inferior and superior thirds of the spindle associated with
Type 2 afferent sensory nerves
How are muscle spindles connected
Attached by connective tissue in parallel to extrafusal fibres
Alpha-gamma co-activation
Prevents loss of sensory information by preventing the central region of the muscle spindle from going slack during a shortening muscle contraction
Ensures information about muscle length will be continuously available
What slows the rate of firing in the stretch receptor
Contraction of extrafusal fibres —> shortening of the muscle removed tension on the spindle
Muscle spindle
Receptors have peripheral endings of afferent nerve fibres
Wrap around modified muscle fibres. = intrafusal fibres
What does tension depend on
Muscle length
Load on the muscle
Degree of muscle fatigue
Golgi tendon organs
Measure the force developed by the muscles and any resultant change in length
Golgi tendon organs structure
Endings of afferent fibres that wrap around collagen bundles in the tendons
1b fibres —> run to anterior horn
Posses slower afferent fibres than muscle spindles
Which afferent fibres lead from GTO to spinal cord
1b fibres
Run to anterior hirn
Output of Golgi tendon organ
Output is proportional to muscle tension
Do muscle spindles or Golgi tendon organs have slower afferent fibres
GTOs
What do Golgi tendon organs stimulate
Motor neurones of antagonistic muscle
Inverse stretch reflex
1b fibres inhibit muscle contraction via inhibiting alpha motor neurones
Synergy between this and interneurones regulates muscle tension and prevents overload
How do 1b fibres inhibit muscle contraction
Inhibit alpha motor neurones
Stretch reflex
Afferent fibres activate excitatory synapses directly on motor neurones which return to the muscle
Monosynaptic arc
Important in posture
Knee jerk reflex
Patellar tendon is tapped
Thigh muscles are stretched
Stretch receptors activated
Afferent nerve fibres activated—> activate excitatory synapses on the motor neurones that control this muscle
Stimulation of motor units
Contraction of muscle
Extension of lower leg
Polysynaptic reflex arc
At least one interneuron between the afferent and efferent neurones
Polysynaptic reflex example
Motor neurons of synergistic muscle are activated
Reciprocal innervation
Polysynaptic
Afferent nerve fibres end in inhibitory interneurons
When activated, inhibit motor neurones of the antagonistic muscle whose interaction would interfere with the reflex response
Withdrawal reflex
Activates flexor muscles
Inhibits extensor muscles
When legs affected= crossed-extensor reflex occurs simultaneously to allow shift of weight into other foot
Crossed-extensor reflex
Motor neurones to contralateral extensors activated and flexors inhibited to shift weight when pick up foot
Motor cortex
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
Upper motor neurones
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.
Pyramidal tract
axons of these upper motor (pyramidal) neurons form the pyramidal tract
Most cortical projections innervate …
Contralateral motor units
The homunculus
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
Descending projections from motor cortex
Dorsolateral tracts
Ventromedial tracts
Dorsolateral tracts
Contain a direct corticospinal route
Contain a indirect route via brainstem nuclei = red nuclei
Innervate contralateral side of one segment of spinal cord
Sometimes project directly to alpha motor neuron
Project to distal muscles, e.g. fingers
Ventromedial tracts
Contain a direct corticospinal route
Contain an indirect route via brainstem nuclei = tectum, vestibular nuclei, reticular formation and cranial nerve nuclei
Diffuse innervation projecting to both sides and multiple segments of spinal cord
Project to proximal muscles of trunk and limbs
What do the Dorsolateral tracts project to
Distal muscles eg fingers
What do the ventromedial tracts project to
Proximal muscles of trunk and limbs
Brainstem nuclei involved with Dorsolateral tracts
Red nucleus
Brainstem nuclei involved with ventromedial tracts
Tectum
Vestibular nuclei
Reticular formation
Cranial nerve nuclei
Basal ganglia
A group of structures beneath the cortex that act as a ‘gate-keeper’ for control of the motor system (muscles)
Basal ganglia receives input from
Many areas of cortex (glutamate)
Neurotransmitter involved with excitatory input to basal ganglia
Glutamate
Output of basal ganglia
Back to cortex via thalamus
Mainly inhibitory (GABA)
5 principle nuclei of basal ganglia
Substantia Nigra (pars compacta & pars reticulata)
Caudate & Putamen (together=striatum)
Globus Pallidus (internal and external segments)
Subthalamic Nucleus
Inhibitory neurotransmitter of basal ganglia
GABA
What forms the striatum
Caudate and putamen nuclei of basal ganglia
Function of basal ganglia
Disinhibitiry gating of motor cortex output
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]
Cerebellum
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.
Cerebellum structure
Contains approx half total number of CNS neurons
•Just 10% of total brain weight
•Projects to almost all upper motor neurons
Cerebellum function
Modulates activity of UMN
Types of Inputs to cerebellum
Cortical
Spinal
Vestibular
Cortical input to cerebellum
Mostly from motor cortex (copies of motor commands)
Also somatosensory and visual areas of parietal cortex
Spinal input to cerebellum
Proprioceptive information about limb position and movement = muscle spindles, Golgi tendon organs
Vestibular inputs to cerebellum
Rotational and acceleratory head movement (semicircular canals / otoliths in inner ear)
Output of cerebellum
Thalamus to motor cortex
Cerebellar function
It knows what the current motor command is
It knows about actual body position and movement —> 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.
Functional brain imaging studies have demonstrated that the cerebellum is involved in a wide variety of non-motor tasks
What is precise motor control governed by
Size principle
Different types of muscle fibres
Antagonistic arrangement of muscles
Final common pathway of motor control
Single alpha motor neurone
Pyramidal tracts originate
In cerebral cortex
Role of pyramidal tracts
Carry motor fibres to spinal cord and brainstem
Responsible for voluntary control of musculature
Where do extrapyramidal tracts originate
Brainstem
Extrapyramidal tracts function
Carry motor fibres to spinal cord
Responsible for involuntary and autonomic control of musculature
Which descending tracts are involved in voluntary motor control
Pyramidal teacts
Which descending tracts are involved in involuntary and autonomic control of musculature
Extrapyramidal
2 types of pyramidal tracts
Corticospinal
Corticobulbar
Inputs to corticospinal tracts
Primary motor cortex
Premotor cortex
Supplementary cortex
Lateral corticospinal tract
Decussate and then descends, terminating in ventral horn
Pathway of corticospinal tracts
Cortex —> descends through internal capsule —> crus cerebri —> pons —> medulla
Where does the corticospinal tract divide into 2
Caudal part of the medulla
Anterior corticospinal tract
Remains ipsilateral to the spinal cord, then decussates and terminates in the ventral horn of the upper thoracic levels
Where does the corticobulbar tract begin
Lateral aspect of primary motor cortex
Pathway of corticobulbar tract
Cortex —> descend through internal capsule —> crus cerebri —> brainstem —> terminate on motor nuclei of cranial nerves (acting on facial and neck muscles)
Hypoglossal nerve
Only provides contralateral innervation
Facial nerve
Has contralateral innervation
Only affects muscles in lower quadrant of the face (below eyes)
Where does the facial nerve affect
Lower quadrant of face (below eyes)
Which cranial nerves are exceptions to innervating motor neurones bilaterally
Facial
Hypoglossal
Corticobulbar fibres innervate
Innervate motor neurones bilaterally
Where do the corticobulbar tracts terminate
Motor nuclei of cranial nerves acting on facial and neck muscles
Number of extrapyramidal tracts
4
Where do extrapyramidal tracts originate
Brainstem
Which extrapyramidal tracts decussate
Rubrospinal
Tectospinal
What are the 4 extrapyramidal tracts
Vestibulospinal
Reticulospinal
Rubrospinal
Tectospinal
Vestibulospinal tracts arise from
Vestibular nuclei
Vestibulospinal tracts supply
Ipsilateral information
Vestibulospinal tracts control
Balance and posture
Vestibulospinal tracts - types
Medial and lateral tracts
Medial Reticulospinal tracts arise from
Pons
Lateral Reticulospinal tracts arise from
Medulla
Medial Reticulospinal tracts function
Facilitates voluntary movements
Increases muscle tone
Lateral Reticulospinal tracts function
Inhibits voluntary movement
Reduces movement tone
Rubrospinal tracts arise from
Red nucleus
Rubrospinal tracts function
Fine control of hand movement
Tectospinal tracts arise from
Superior colliculus
Tectospinal tracts function
Coordinates movement of the head in relation to vision stimuli
Where does the lateral corticospinal tract decussate
Caudal medulla
Where does the anterior corticospinal tract decussate
Uncrossed
Some cross in spinal cord
Where does the rubrospinal tract decussate
Level of origin in midbrain
Where does the medial vestibulospinal tract decussate
Bilateral from origin
Does not extend beyond cervical region
Where does the pontine Reticulospinal tract decussate
Uncrossed
Where does the medullary Reticulospinal tract decussate
Partially
Target of the lateral corticospinal tracts
Alpha motor neurones related to hand and digits or interneurones
Target of the anterior corticospinal tract
Motor neurones related to the trunk muscles
Input is bilateral
Target of the rubrospinal tract
Alpha motor neurones of proximal muscles especially flexors
Target of the medial vestibulospinal tracts
Alpha and gamma motor neurones for extensor muscles
Target of the Reticulospinal tracts
Cervical and lumbosacral pattern generatirs
Target of the MLF ascending
Links vestibular and nuclei related to moving eye
Function of lateral corticospinal tracts
Initiation of movement
Function of the anterior corticospinal tracts
Initiation of movement in trunk muscles
Function of the rubrospinal tract
Supraspinal control of flexor motor neurones and proximal limb muscles
Function of the medial vestibulospinal tracts
Supraspinal control of extensor muscles
Function of the Reticulospinal tracts
Locomotion
Posture
Function of the MLF - ascending
Coordinates head and eye movements
Muscles of the lower limbs are represented where in the motor cortex
Medially
Muscles of the face are represented where in the motor cortex
Laterally
Where are axons of upper motor neurons mainly located
Lateral white matter of the spinal cord
The anterior corticospinal tract mainly supplies the
Contralateral side of the body
How are fibres of the corticospinal tract organised
Somatotopically
Alternative name for primary motor cortex
Brodmann’s area 4
What produces an abnormal rhythmical output in Parkinson’s disease
Basal ganglia
At what level do the lateral corticospinal tracts decussate
Level of medullary pyramids
Where are the cell bodies of LMN located
Ventral horn of spinal cord
What is a motor unit
Motor neuron and all MMUs it innervates
Where do the LMN leave the spinal cord
Anteriorly (ventrally)
Where does the lateral corticospinal tract decussate
In medullary pyramids
What percentage of the corticospinal tract is anterior
15%
Where is the anterior corticospinal tract located in relation to the anterior horn of grey matter
Antero-medially
Which corticospinal tracts contains more fibres
Lateral (85% vs 15%)
Which motor neurones innervate extrafusal muscle fibres
Alpha motor neurons
Which motor neurons innervate intrafusal muscle fibres
Gamma motor neurones
Function of extrafusal muscle fibres
Muscle contraction
Function of intrafusal muscle fibres
Body position
Proprioception
Somatotopical organisation of corticospinal tract
Lower extremity fibres located laterally
Upper extremity and head fibres located medially
Direct pathway function
Increase movement
Direct pathway
Primary motor cortex —> striatum [excitatory = glutamate]
Striatum —> internal Globus pallidus and pars reticulata [inhibitory= GABA]
—> thalamus and pars compacta [inhibitory = GABA]
Excitatory signals further excite inhibitory pathway via dopamine [D1 receptors]
—> excitatory signals from thalamus to primary motor cortex
Function of indirect pathway
Decreases or stop movement
Indirect pathway
Primary motor cortex —> Putamen [excitatory= glutamate]
Striatum —> external Globus pallidus [inhibitory = GABA]
Inhibits Globus pallidus so cannot inhibit Subthalamic nucleus
—> excitatory to internal Globus pallidus and pars reticulata [glutamate]
—> inhibitory signals being sent to thalamus [GABA]
Knee jerk reflex
- The muscle spindle is stretched in the quadriceps following stretch of the patellar tendon- caused by the tap stimulus
- This causes action potentials to be fired by la afferent fibres which then synapse within the spinal cord with alpha-motor neurones which innervate extrafusal fibres
- The antagonistic muscle is inhibited by inhibitory interneurons and the agonist muscle contracts- quadriceps contract and hamstrings relax
Overview of movement pathway
Plan out the motor programme
Voluntarily execute the programme—>
Motor signal relays to the peripheral nervous system to activate the muscles
Signals cross to the muscles via NMJ
Muscles activated
Smoothen the execution- extra pyramidal
Brain receives feedback- muscle spindles/joint position
Co-ordinate the movement
Where are primary motor neurones located
Primary motor cortex
Where do the axons from the primary motor cortex travel through
Projections through internal capsule to brainstem and then spinal cord
Where do the UMN synapse
Anterior horn cells of spinal cord
Rhyme to remember nerve roots
S1,2: tie my shoe - ankle reflex.
L3,4: kick the door - knee reflex.
C5,6: pick up sticks - biceps reflex.
C7,8: lay them straight - triceps reflex.
A reflex is an automatic, involuntary reaction to a stimulus.
Which group of spinal nerves innervates the biceps reflex?
C5/C6
The spinal cord has a variety of tracts each with their own function.
Which descending motor tract originates in the cerebral cortex and synapses in the spinal cord.
Corticospinal
A reflex is an automatic, involuntary reaction to a stimulus.
Which group of spinal nerves innervates the ankle reflex?
S1/S2
