🧠Neurology🧠 - Motor & Neuromuscular Control Flashcards
What 2 main principles is the motor control system built upon?
Hierarchical organisation
Functional segregation
What is meant by hierarchical organisation?
Higher orders of hierarchy in nervous system are involved in more complex tasks (programme and decide on movements, co-ordinate muscle activity)
Lower level areas of hierarchy perform lower level/more basic tasks (e.g. execution of movement)
What is meant by functional segregation?
Motor system organised in a number of different areas that control different aspects of movement
Outline the motor system hierarchy
What 2 categories can the major descending tracts be divided into?
Pyramidal tracts - pass through the pyramids of the medulla
Extrapyramidal tracts - do not pass through the pyramids of the medulla
What is the overall function of the pyramidal tracts?
Voluntary movements of body and face
What is the overall function of the extrapyramidal tracts?
Involuntary (automatic) movements for balance, posture and locomotion
Where to the pyramidal tracts originate and terminate?
The motor cortex to the spinal cord/cranial nerve nuclei in brainstem
Where do the extrapyramidal tracts originate and terminate?
Brainstem nuclei to the spinal cord
Name the pyramidal tracts
Corticospinal
Corticobulbar
Name the extrapyramidal tracts
Vestibulospinal
Tectospinal
Reticulospinal
Rubrospinal
Where is the primary motor cortex located?
Pre-central gyrus, anterior to the central sulcus
What is the function of the primary motor cortex?
Controls fine, discrete, precise voluntary movements
Provides descending signals to execute movements
Where is the premotor area located?
Located anterior to primary motor cortex
What is the function of the premotor area?
Involved in planning movements
Regulates externally cued movements
What is meant by an externally cued movement?
Interacting with the environment
e.g. seeing an apple and picking it up
Where is the supplementary motor area?
Located anterior and medial to the primary motor cortex
What is the function of the supplementary motor area?
Involved in planning complex movements (e.g. internally cued movements, such as speech)
Becomes active prior to voluntary movement
Describe the first part of the path of the corticospinal tract?
Upper motor neurones originating in the cerebral cortex, travel downwards, to the cerebral peduncle and into the midbrain
Where does the corticospinal tract go after the midbrain?
Into the medulla, and through the pyramids
Decussates somewhere along the pyramids
Which fibres of the corticospinal tract decussate?
85-90% of the fibres decussate (cross) - Limb muscles - lateral corticospinal tract
10-15% of the fibres do not - trunk muscles - anterior corticospinal tract
Outline the pathway of the corticospinal tract
What is the corticobulbar tract?
Principal motor pathway for the voluntary movements of the face and neck - passes through the basal ganglia
Some fibres terminate on the cranial nerve nuclei in the brainstem
What are the cranial nerve nuclei found in the brainstem, on which some fibres of the corticospinal tract terminate?
Oculomotor+trochlear nucleus
Trigeminal motor nucleus
Abducens nucleus
Facial nucleus
Hypoglossal nucleus
What is the function of the oculomotor+trochlear nuclei?
Control eye movements
What is the function of the trigeminal motor nucleus?
Controls the muscles of the jaw
What is the function of the abducens nucleus?
Controls movements of the eye
What is the function of the facial nucleus?
Controls muscles of the face
What is the function of the hypoglossal nucleus?
Controls movements of the tongue
Outline the vestibulospinal tract
Stabilise head during body movements, or as head moves
Coordinate head movements with eye movements
Mediate postural adjustments
Outline the reticulospinal tract
Most primitive descending tract - from medulla and pons
Changes in muscles tone associated with voluntary movement
Postural stability
Outline the tectospinal tract
From superior colliculus of midbrain
Orientation of the head and neck during eye movements
Outline the rubrospinal tract
From red nucleus of midbrain
In humans mainly taken over by corticospinal tract
Innervate lower motor neurons of flexors of the upper limb
What are the two categories of signs that can be identified with an upper motor neuron lesion?
Positive and negative signs?
What are the negative signs that can be identified with an upper motor neuron lesion?
Loss of voluntary motor function
Paresis: graded weakness of movements
Paralysis (plegia): complete loss of voluntary muscle activity
What are the positive signs that can be identified with an upper motor neuron lesion?
Increased abnormal motor function due to loss of inhibitory descending inputs
Spasticity: increased muscle tone
Hyper-reflexia: exaggerated reflexes
Clonus: abnormal oscillatory muscle contraction
Babinski’s sign
What is Babinski’s sign?
the big toe moves upward and the other toes fan out or curl down in response to the sole of the foot being stroked with a blunt object
What is apraxia?
Disorder of skilled movement
Patients are not paretic but have lost information about how to perform skilled movements
What causes apraxia?
Lesion of inferior parietal lobe, the frontal lobe (premotor cortex, supplementary motor area - SMA)
How does a lesion of the inferior parietal lobe, or frontal lobe, causing apraxia arise?
Any disease of these areas can cause apraxia
Stroke and dementia are most common
What are the features of a lesion of the lower motor neurons?
Weakness
Hypotonia (reduced muscle tone)
Hyporeflexia (reduced reflexes)
Muscle atrophy
Fasciculations: damaged motor units produce spontaneous action potentials, resulting in a visible twitch
Fibrillations: spontaneous twitching of individual muscle fibres; recorded during needle electromyography examination
What is motor neuron disease (MND)?
Progressive neurodegenerative disorder of the motor system
Spectrum of disorders
Also known as Amyotrophic lateral sclerosis (ALS
What are the upper motor neuron signs of MND?
Spasticity (increased tone of limbs and tongue)
Brisk limbs and jaw reflexes
Babinski’s sign
Loss of dexterity
Dysarthria (difficulty speaking)
Dysphagia (difficulty swallowing)
What are the lower motor neuron signs of MND?
Weakness
Muscle wasting
Tongue fasciculations and wasting
Nasal speech
Dysphagia
What are the main components of the basal ganglia?
Caudate nucleus
Lentiform nucleus (putamen + external globus pallidus)
Caudate + putamen know collectively as the striatum
Nucleus accumbens
Substantia nigra
Ventral pallidum, claustrum, nucleus basalis
What is the function of the caudate nucleus?
Responsible for decisions to move
What is the function of the lentiform nucleus and the nucleus accumbens?
Elaborating associated movements (e.g. swinging arms when walking; changing facial expression to match emotions)
What is the function of the subthalamic nuclei and substantia nigra?
Moderating and coordinating movement (suppressing unwanted movements)
What is the function of the ventral pallidum, claustrum, nucleus basalis collectively?
Performing movements in order
Outline the components of the basal ganglia
Outline the basal ganglia circuitry
Don’t need to memorise, just for illustration
What part of the basal ganglia circuitry does Parkinson’s effect?
Degeneration of dopaminergic neurons that originate in the substantia nigra and project to the striatum
What are the features of Parkinson’s?
Bradykinesia
Hypomimic face - expressionless, lacks usual movements that animate the face
Akinesia - difficulty in the initiation of movements - can’t internally initiate
Rigidity - increased muscle tone, resistance in externally imposed joint movements
Tremor at rest - starts in hand, with time spreads to other parts of the body
What part of the basal ganglia circuitry does Huntington’s effect?
Degeneration of GABAergic neurons in the striatum, caudate and then putamen
Genetic disorder, chromosome 4, autosomal dominant, CAG repeat
What are the classic features of Huntongton’s?
Choreic movements (chorea = dance) - rapid, jerky, involuntary movements of the body - hands/face first - rest of the body follows
Speech impairment
Difficulty swallowing
Unsteady gait
Cognitive decline and dementia - later stages
What is ballism?
Sudden uncontrolled flinging of the extremities
Symptoms occur contralaterally
What is the usual cause of ballism?
Usually from stroke affecting the subthalamic nucleus
Where is the cerebellum located?
Located in posterior cranial fossa
Separated from cerebrum above by tentorium cerebelli
What is the function of the cerebellum?
Coordinator and predictor of movement
What sections can the cerebellum be divided into?
Vestibulocerebellum
Spinocerebellum
Cerebrocerebellum
What is the function of the vestibulocerebellum?
Regulation of gait, posture and equilibrium
Coordination of head movements with eye movements
What would damage to the vestibulocerebellum lead to?
Damage (tumour) causes syndrome similar to vestibular disease:
Gait ataxia - inability to coordinate the movements required for normal walking
Tendency to fall (even when patient sitting and eyes open)
What is the function of the spinocerebellum?
Coordination of speech
Adjustment of muscle tone
Coordination of limb movements
What would damage to the spinocerebellum lead to?
Damage (degeneration and atrophy associated with chronic alcoholism) affects mainly legs
Abnormal gait and stance (wide-based)
What is the function of the cerebrocerebellum?
Coordination of skilled movements
Cognitive function, attention
Processing of language
Emotional control
What would damage to the cerebrocerebellum lead to?
Damage affects mainly arms/skilled coordinated movements (tremor) Speech affected
What are the main signs of cerebellar dysfunction?
Apparent only on movement:
Ataxia
Dysmetria
Intention tremor
Dysdiadochokinesia
Scanning speech
What is ataxia?
General impairments in movement coordination and accuracy. Disturbances of posture or gait: wide-based, staggering (“drunken”) gait
What is dysmetria?
Inappropriate force and distance for target-directed movements (knocking over a cup rather than grabbing it)
What is intention tremor?
Increasingly oscillatory trajectory of a limb in a target-directed movement (nose-finger tracking)
What is dysdiadochokinesia?
Inability to perform rapidly alternating movements (rapidly pronating and supinating hands and forearms)
What is scanning speech?
Staccato, due to impaired coordination of speech muscles
What are alpha motor neurons?
Lower motor neurons of the brainstem and spinal cord
Innervate the extrafusal muscle fibres of the skeletal muscle
Activation causes muscle contraction
What is a motor neuron pool?
Motor neuron pool - all alpha motor neurons innervating a single muscle
What is a motor unit
Single motor neuron together with all the muscle fibres that it innervates
The smallest functional unit with which force can be produced
Stimulation of one motor unit causes contraction of all muscle fibres within the unit
How many muscle fibres does a motor neuron supply (on average)?
600 muscle fibres
Approximately how many motor neurons and muscle fibres do humans have?
420,000 motor neurons
250,000,000 skeletal muscle fibres
What are the three types of motor unit?
Slow (S, type I)
Fast, fatigue-resistant (FR, type IIA)
Fast, fatiguable (FF, type IIB)
What are the features of type I motor units?
smallest diameter cell bodies
small dendritic trees
thinnest axons
slowest conduction velocity
What are the features of type IIA motor units?
larger diameter cell bodies
larger dendritic trees
thicker axons
faster conduction velocity
What are the features of type IIB motor units?
larger diameter cell bodies
larger dendritic trees
thicker axons
faster conduction velocity
How are the three different types of motor unit classified?
The 3 different motor unit types are classified by the amount of tension generated, speed of contraction and fatiguability.
How does the amount of sustained force produced by the different types of motor unit compare to each other?
What are the two mechanism by which the brain regulates the amount of force produced?
Recruitment
Rate coding
What is muscle fibre recruitment?
Motor units are not randomly recruited
Governed by the “size principle”. Smaller units are recruited first - type I
As more force is required, more units are recruited
This allows fine control (e.g. when writing), under which low force levels are required
What is rate coding?
A motor unit can fire at a range of frequencies. Slow units fire at a lower frequency.
As the firing rate increases, the force produced by the unit increases
Summation occurs when units fire at frequency too fast to allow the muscle to relax between arriving action potentials
What are neurotrophic factors?
Are a type of growth factor
Prevent neuronal death
Promote growth of neurons after injury
How do neurons affect the muscle fibres which they innervate?
Motor unit and fibre characteristics are dependent on the nerve which innervates them.
If a fast twitch muscle and a slow muscle are cross innervated, the soleus(slow twitch) becomes fast and the FDL(fast twitch) becomes slow
What is meant by the “plasticity” of motor units/muscle fibres?
Fibre types can change properties under many different conditions
Type IIB to IIA most common following training
Type I to II possible in cases of severe deconditioning or spinal cord injury (e.g. microgravity from space flight)
Ageing associated with loss of type I and II fibres but also preferential loss of type II fibres - larger proportion of type I fibres in aged muscle
Define a reflex
An automatic response to a stimulus that involves a nerve impulse passing inward from a receptor to a nerve centre and then outward to an effector (as a muscle or gland) without reaching the level of consciousness
An involuntary coordinated pattern of muscle contraction and relaxation elicited by peripheral stimuli
How do reflexes differ from voluntary movement?
Do not reach the level of consciousness - are automatic
Reflexes differ from voluntary movements in that once they are released, they can’t be stopped
Outline the monosynaptic (stretch) reflex
Stretch of tendon caused by hammer sends signal up sensory neuron
Goes through a motor neuron, to the antagonist motor neuron
How can reflexes be controlled/influenced?
Higher centres of the CNS exert inhibitory and excitatory regulation upon the stretch reflex
Inhibitory control dominates in normal conditions (N)
Decerebration (i.e. head injury) reveals the excitatory control from supraspinal areas (D)
Rigidity and spasticity can result from brain damage giving over-active or tonic stretch reflex
What is the Jendrassik maneouvre?
Clenching the teeth, making a fist, or pulling against locked fingers when having patellar tendon tapped. The reflex becomes larger
What is hyper-reflexia?
Over-active reflexes
Loss of descending inhibition
Associated with upper motor neuron lesions
What is clonus (referring to hyper-reflexia)?
Involuntary and rhythmic muscle contractions
What is hypo-reflexia?
Below normal or absent reflexes
Associated with lower motor neuron diseases
