8.0 Motor Systems Flashcards
What is a consequence of a lesion in:<br></br><br></br>a) Association cortex<br></br>b) Cerebellum<br></br>c) Brainstem<br></br>d) lower motor neuron<br></br>e) Primary motor cortex/upper motor neuron<br></br>f) Basal ganglia
a) Association cortex → apraxia<br></br>b) Cerebellum → Ataxia/poor coordination<br></br>c) Brainstem → Postural deficits<br></br>d) lower motor neuron → Flaccid paralysis<br></br>e) Primary motor cortex/upper motor neuron → Spastic paralysis <br></br>f) Basal ganglia → Hyper/hypokinesia
Upper motor neuron vs lower motor neuron lesions:
Upper motor neuron = exaggerated reflexes + spastic paralysis<br></br><br></br>Lower motor neuron = Loss of reflexes + flaccid paralysis
Define ataxia:
Neurological disorder of voluntary coordination of muscle movements
What is noise (with regards to neural signals)?
Random variation in neural signals
What is the motor equivalence problem?
Describes redundancy in the motor system<br></br>Goal directed movement can be achieved in different ways
Define non-linearity
Mixing individual motor commands does not produce predictable results
Define non-stationarity
Behaviour of motor systems can change over time<br></br><br></br>Muscle contraction depends on history (thixotropy)
Define thixotropy
Muscle contraction depends on history
Define negative feedback systems:
A sensed parameter is compared to a desired ‘set point’<br></br>If they match → no output<br></br>If they differ → system will generate a corrective action
Examples of negative feedback systems:
1) Blood glucose<br></br>2) Temperature<br></br>3) Respiration rate<br></br>4) Blood pressure
Advantages of negative feedback systems:
Automatically compensate for unpredicted events that cause deviation from set point (e.g. noise)
Disadvantages of negative feedback systems:
1) Time delays (error signal can be out of date by the time it reaches brain)<br></br>2) Instability and oscillation
Define feed-forward systems:
Motor commands are prepared (by estimation) in advance based on sensory information available<br></br><br></br>Fast movements need feed-forward predictive control
What is an internal model system?
Brain contains an internal model system - it is a representation of the mechanics of the body and the behaviour of the external world
What are the two types of the internal model system?
1) Inverse model<br></br>- Starts with desired movement<br></br>- Needs to be learnt<br></br><br></br>2) Forward model<br></br>- Predicts the consequences of motor commands (before and during movement)<br></br>- Needs internal feedback or efference copy
Define efference copy
An internal copy created of the efferent motor signal, which is input into a forward model
What brain structures are regarded as centres for feedforward control?
1) Cerebellum<br></br>2) Motor cortex
Where are the alpha-motoneurons located in the spinal cord?
Ventral horn
Define a motor unit:
All the muscle fibres innervated by 1 motoneuron
What are the different motor unit categories?<br></br><br></br>Comment on anatomy, biochemistry and physiology
“<div><img></img></div>”
What is rate coding?
Mechanism of controlling motoneurons<br></br><br></br>Varying the motorneuron firing rate
What is motorneuron recruitment?
Mechanism of controlling motoneurons (more important than rate coding)<br></br><br></br>Varying the number of motoneurons recruited (↑ force needed → ↑ motoneurons recruited)
Define the size principle (motoneuron recruitment):
Motor units are recruited to action in an orderly sequence of increasing force
What are the 3 sources of input into motoneurons?
1) Spinal interneurons (most numerous)<br></br>2) Afferent fibres (only from muscle spindles)<br></br>3) Descending fibres (rare)
What is a proprioceptor?
A receptor that provides information about the state of the body (position/movement of joints/muscle force etc)
Define proprioception:
Perception of position and movement of the body (a.k.a. kinaestheia)
3 types of proprioceptors in motor system:
1) Muscle spindles afferents (signal stretch)<br></br>2) Golgi tendon organ afferents (signal tension)<br></br>3) Joint receptors (signal position and movement)
What are the different types of fibre in a muscle spindle?
1) Intrafusal<br></br>2) Extrafusal
What are the two types of fibre in an intrafusual fibre?
1) Bag fibre (dynamic response to changes in muscle length)<br></br>2) Chain fibre (Static response. Linear response)
What are the two types of sensory fibres in each intrafusal fibre?
1) Primary (Ia) spindle afferent (Aα)<br></br>2) Secondary (II) spindle afferent (Aβ)
What are the motorneurons that supply the muscle spindles? What is their role?
γ-motorneurons<br></br><br></br>Play a role in sensitisation and adaptation of receptors
Draw a stretch reflex:
“<div><img></img></div>”
Define reciprocal inhibition:
Stimulation of one agonist pathway also excites spinal interneurons that inhibit antagonist muscles
Define muscle clonus:
Brief involuntary muscle contraction (often repeated and rhythmic)
Define recurrent inhibition:
Motorneurons have recurrent collateral branches that innervate an inhibitory interneuron (Renshaw cell) to provide recurrent inhibition of the motorneuron, thus regulating the timing of motor neuron firing.
What are some normal neonatal reflexes?
1) Grasp reflex<br></br>2) Babinski’s sign<br></br>3) Reflex stepping
Define spasticity:
Exaggerated stretch reflexes (muscles are tense and stiff)
Define Babinski’s sign:
Toes turn up to plantar stimulation (normal for neonates, pathological in adults [due to cortical damage])
Define clasp knife reflex:
Limbs snap into extension or flexion
Define central pattern generators:
Central pattern generators (CPGs) are biological neural networks that produce rhythmic patterned outputs without sensory feedback
Muscle spindles comprise a fibrous capsule that contains a number of ____________ muscle fibres. The large primary afferent arising from the muscle spindle arise from annulospiral terminals on these fibres and respond to __________, however taken alone the signals are ambiguous as they can be altered by _______________. The central connections of the muscle spindle afferents include monosynaptic connections with motorneurones of the agonist muscle and with interneurones that mediate _____________inhibition of antagonist motorneurons. In addition to roles in reflex action and in proprioception, a very important role for the signals from muscle spindles is ______________.
Muscle spindles comprise a fibrous capsule that contains a number of <b>intrafusal</b> muscle fibres. The large primary afferent arising from the muscle spindle arise from annulospiral terminals on these fibres and respond to <b>change in length</b>, however taken alone the signals are ambiguous as they can be altered by <b>contraction of the intrafusal fibres</b>. The central connections of the muscle spindle afferents include monosynaptic connections with motorneurones of the agonist muscle and with interneurones that mediate <b>reciprocal</b> inhibition of antagonist motorneurons. In addition to roles in reflex action and in proprioception, a very important role for the signals from muscle spindles is <b>predictive feedforward control</b>.
What are the different descending motor pathways?
<b>Ventromedial pathways</b><br></br>1) Reticulospinal<br></br>2) Vestibulospinal<br></br>3) Tectospinal<br></br><br></br><b>Dorsolateral pathways</b><br></br>1) Corticospinal<br></br>2) Rubrospinal
What are the different functions of ventromedial and dorsolateral pathways?
<b>Ventromedial pathways</b><br></br>Controls axial + proximal limb muscles<br></br>Invovled in whole body movements like locomotion and posture<br></br><br></br><b>Dorsolateral pathways</b><br></br>Goal directed movements of the limbs (hand and feet) and face
List 3 systems that contribute to postural stability:
1) Visual system<br></br>2) Vestibular system<br></br>3) Somatosensory and proprioceptive systems
What are the receptors in the vestibular system?
1) Sensory hair cells (in semi-circular canals of labyrinth)<br></br><br></br>2) Otolith organs (saccule and utricle)
Define vestibular reflexes:
Vestibular signals generated from head movement to generate correcting responses (usually via vestibulospinal pathways acting on extensor muscles)
Define neck reflexes:
Equal and opposite reflexes to vestibular reflexes, to discriminate between head tilting and body swaying
Peripheral disease that can affect vestibular reflexes:
Labyrinthitis
Define gaze fixing:
Mechanisms used to keep eyes fixed relative to outside world<br></br><br></br>1) Vestibulo-ocular reflex<br></br>2) Optokinetic reflex
Define Vestibulo-ocular reflex:
A gaze-fixing mechanism where vestibular system detects head movement and drives compensatory (equal and opposite) eye movements
Define Optokinetic reflex:
A gaze-fixing mechanism where eyes move to follow the slow movement of the visual field
Define Nystagmus:
Sawtooth movement of eye caused by drift and saccade sequence
Causes of nystamus:
<b>Physiological</b><br></br>- Response to optokinetic or vestibular stimuli<br></br><br></br><b>Pathological</b><br></br>- Cerebellar damage<br></br>- Vestibular damage
Define gaze shifting:
Mechanisms used to move the eyes as quickly as possible to minimise the blurring period that occurs when eyes are moving
Define saccade:
Gaze shifting mechanism that quickly moves the eye to foveate visual stimuli
Neuronal structures involved in saccade production:
1) Retina<br></br>2) Superior colliculus<br></br>3) Brainstem reticular formation<br></br>4) Cervical spinal cord<br></br>5) Basal ganglia<br></br>6) Oculomotor nuclei
Define smooth pursuit:
Gaze-shifting mechanism where slower eye movements are used to ‘follow’ moving objects.<br></br><br></br>Feedforward predictive movement
Neuronal structures involved in smooth pursuit:
Cerebral cortex<br></br>Cerebellum
Location of motor cortex?
Frontal lobe (immediately rostral to central sulcus)
Role of motor association areas?
Involved with planning and preparation for action
Define hemiparesis:
Weakness/partial paralysis to one side of the body
Define hemiplegia:
Paralysis to one side of the body
What is unique to the corticospinal tracts in primates?
Corticomotoneuronal connections (some of the fibres in the corticospinal tract make direct conenctions with α-motorneurons)
What does an upper motor neuron cause?
Spastic paralysis
Signs of spastic paralysis?
1) Hypertonia<br></br>2) Hyperreflexia<br></br>3) Clonus
What does an lower motor neuron cause?
Flaccid paralysis
Signs of flaccid paralysis?
1) Hypotonia<br></br>2) Loss of reflexes
What goal directed movement does not occur via the motor cortex (corticospinal tract)?
Eye movements - because these need to be consensual to avoid diplopia<br></br><br></br>They have separate area of cortex which control movement via the brain stem
Why is the motor homunculus misleading?
Suggests an orderly representation of individual muscles in different regions of the cortex. This is not the case as a lesion in one location does not only affect one specific muscle/body part. It is more likely that muscles are represented in synergistic muscle groups.
Give an example of a movement that is controlled by feedback control:
Finger grip force
Give an example of a movement that cannot be controlled by feedback control:
Eye movement
Define long latency stretch reflex:
As well as spinal reflex, there is also a stretch reflex involving the cortex. This is slower but allows the brain to set its gain
What are the regions of motor association areas?
<b>1) Lateral premotor cortex</b><br></br>- Connected with sensory areas and cerebellum<br></br>- Important for sensory guided movement (esp. vision guided movements)<br></br><br></br><b>2) Supplementary motor area (SMA)</b><br></br>- Connected to many cortical areas + basal ganglia<br></br>- Important for internally generated movements<br></br><br></br><b>3) Cingulate motor area</b><br></br>- Expression of emotion via motor system
What are the connections to the motor association areas?
<b>1) Primary motor area</b><br></br>- Direct<br></br>- To control current command for movement<br></br><b>2) Spinal cord</b><br></br>- Direct<br></br><b>3) Cerebellum + basal ganglia</b><br></br>- Indirect<br></br>- To plan and prepare for future movements
What are mirror neurons?
Neurons in lateral premotor cortex<br></br>Neurons that fire in relation to making a movement but also in relation to seeing other make the movement (imitation)
What evidence is available to show that supplementary motor area is used in internally generated movements?
PET scanning shows that this area is active in mental rehearsal
Role of cerebellum:
Major role in feedforward control and Coordination of movement<br></br><br></br>Also learning
What do cerebellar lesions cause?
<b>D</b>ysdiadochokinesis/Dysmetria<br></br><b>A</b>taxia<br></br><b>N</b>ystagmus<br></br><b>I</b>ntention tremor<br></br><b>S</b>lurred speech<br></br><b>H</b>ypotonia
Define hypotonia:
State of low muscle tone with reduced muscle strength
Define dysmetria:
Inappropriate displacement (e.g. overreaching)
Define dysdiadochokinesis::
Inability to make rapid alternating movements
Define decomposition of movement:
Lack of coordination of different joint movements
Define dyskinesia:
Abnormality or impairment of voluntary movement. Unpredicted movements
What are the output cells of the cerebellum?
Purkinje cells<br></br>- Tree-like in sagittal plane<br></br>- Narrow in coronal plane<br></br>- Project to and inhibit the cerebellar nuclei
What neurotransmitter do Purkinje cells release?
GABA<br></br><br></br>They are inhibitory
How many layers are there in the cerebellar cortex?
3<br></br><b>1) Molecular layer</b><br></br>- Granule cell axons<br></br>- Dendrites of Purkinje cells<br></br><b>2) Purkinje cell layer</b><br></br>- Single cell thick<br></br><b>3) Granule layer</b><br></br>- Vast number of small granule cells
Function of granule cells in cerebellum:
“Give rise to parallel fibres that EXCITE purkinje cells<br></br><br></br>1 Purkinje can ““listen”” to 200,000 granule cells”
What are the two inputs to the cerebellum?
<b>1) Mossy fibres</b><br></br>- Most numerous<br></br>- From pons<br></br>- Excite granule cells<br></br><br></br><b>2) Climbing fibres</b><br></br>- From inferior olive<br></br>- Each Purkinje fibre receives single climbing fibre<br></br>- Mediate learning (synaptic plasticity)
What region in the cerebellum is responsible for the vestibulo-ocular reflex?
Flocculus
What is an inhibitory interneuron of the cerebellum?
Stellate cell
What are the main nuclei of the basal ganglia?
Caudate<br></br>Putamen<br></br>Globus pallidus
What are the two mid-brain nuclei that are functionally connected to the basal ganglia?
Subthalamic nuclei<br></br>Substantia nigra
What are the inputs to the basal ganglia?
From all lobes of cerebral cortex
What are the outputs of basal ganglia?
Inhibitory output to thalamus (GABA connections)
Examples of hyperkinetic movements:
1) Chorea (unexpected dancing movements)<br></br>2) Athetosis (writhing movements of hands/face)<br></br>3) Ballismus (ballistic movements)<br></br>4) Dyskinesia
Define Chorea:
Unexpected dancing movement
Define athetosis:
Writhing movements of the hands/face
Define ballismus:
Ballistic movements
Examples of hypokinesia:
1) Rigidity<br></br>2) Bradykinesia
Pyramidal vs Extra-pyramidal lesions:
Pyramidal = lesions of motor cortex/Corticospinal tract (eg caused by stroke)<br></br><br></br>Extra-pyramidal = Lesions of basal ganglia
Which part of the globus pallidus does direct pathway go to?
Internal globus pallidus
Which part of the globus pallidus does indirect pathway go to?
External globus pallidus
Effect of dopamine on direct and indirect pathways of basal ganglia:
Stimulates direct (D1 receptor)<br></br>Inhibits indirect (D2 receptor)
What is the role of the basal ganglia:
Decision making
What is the role of the ventral striatum?
Part of the limbic system. Plays a role in emotion and motivation
