Motor cortical control Flashcards
What is meant by the principle “Hierarchical organisation”
- High order areas of hierarchy are involved in more complex tasks (programme and decide on movements, coordinate muscle activity)
- Lower-level areas of hierarchy perform lower-level tasks (execution of movement)
What is meant by the principle “functional segregation”?
Motor system is organised in a number of different areas that control different aspects of movement
Describe the process by which information gets through the motor system hierarchy
- The motor cortex receives info (commands) from the other cortical areas
- The cerebellum and basal ganglia adjust the commands received from other parts of the motor control system
- The motor cortex sends the commands to the thalamus and brainstem
- The brainstem passes commands from the cortex to the spinal cord- which then sends it to the muscles of the body/ face and neck
What are the major descending tracts? describe their features
Voluntary movement=
1. Corticospinal tracts
2. Corticobulbar tracts
Involuntary/ automatic movement= EXTRAPYRAMIDAL TRACTS- do not pass pyramids in medulla
1. Vestibulospinal- head movement, position and posture adjustments
2. Tectospinal- orientation of the head and neck during eye movements
3. Reticulospinal- prepares the body to move limbs and posture control
4. Rubrospinal- innervates upper limbs
Describe the features of the voluntary tracts
Voluntary movement= PYRAMIDAL TRACTS- pass through the pyramids of the medulla
- Corticospinal tracts:
- Upper motor neurones (primary motor cortex)
- Pass through the cerebral perduncle,
- Then the midbrain
- Pass through pyramids of the medulla, some (lateral) fibres decussate some do not (anterior)
-Controls movements of torso, upper limbs, lower limbs - Corticobulbar tracts:
- Upper motor neurones in the precentral gyrus (primary motor cortex)
-Terminates on motor neurones WITHIN brainstem motor nuclei only
- Controls movement of face, head and neck
Describe the features of the involuntary tracts
Involuntary/ automatic movement= EXTRAPYRAMIDAL TRACTS- do not pass pyramids in medulla
- Vestibulospinal- Stabilises head during body movements, coordinates head movements with eye movements, medial postural adjustments
- Tectospinal- orientation of the head and neck during eye movements
(from superior colliculus of the midbrain) - Reticulospinal- preparatory (prepare body to move the limbs), changes in muscles tone, associated with voluntary movement, postural stability
(from medulla and pons) - Rubrospinal- innervates lower motor neurons of the upper limb (in humans, this activity is mainly taken over by the corticospinal tract, not much use in humans)
(from red nucleus of midbrain)
Where is the primary motor cortex located?
-In precentral gyrus, anterior to the central sulcus
What does the primary motor cortex control?
- Fine, discrete, precise voluntary movements
- Provides descending signals to execute movements
Where is the premotor area located?
Anterior to the primary motor cortex (in front of PMC and below the supplementary area)
What does the premotor area control?
- Involved in planning movements
- Regulates externally cued movements (interacting with objects)
Where is the supplementary area located?
Anterior and medial to the primary motor cortex (in front of PMC and above the premotor area)
What does the supplementary motor area control?
- Involved in planning complex movements (e.g. internally cued, speech)
- Becomes active prior to voluntary movement
What are the 2 types of corticospinal tracts?
- Lateral corticospinal tract:
- Fibres that cross at the medulla
- Make up 85-90% of the fibres
- Supply the limb muscles - Anterior corticospinal tract:
- Fibres do not cross at the medulla
- Make up 10-15% of the fibres
- Supply the trunk muscles
What is the motor homunculus?
Shows the proportion of brain that stimulates certain parts of the body (e.g. large proportions of the brain stimulate the hands, feet and tongue)
What is meant by somatotopic representation?
Which parts of the brain gets activated by stimulation of certain body parts
What motor nuclei make up the corticobulbar tract?
- Oculomotor nucleus= extraocular muscles (eye movements)
- Trochlear nucleus= extraocular muscles (eye movements)
- Trigeminal motor nucleus= muscles of mastication (jaw movements)
- Abducens nucleus= extraocular muscles (eye movements)
- Facial nucleus (muscles of facial expression)
- Hypoglossal nucleus (muscles of the tongue)
What is the difference between negative and positive signs/ symptoms?
Negative- things that have been “turned off”/ lost
Positive- things that have been “turned on/up”/ intensified
Name some negative signs of upper motor neuron lesions
Negative:
Loss of voluntary motor function
Paresis: graded weakness of movements
Paralysis (plegia): complete loss of voluntary muscle activity
Name some positive signs of upper motor neuron lesions
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 Apraxia?
A disorder of skilled movements - patients are not paretic (paralysed/weak) but have lost information about how to perform skilled movements
What causes apraxia?
Lesions of the inferior parietal lobe, frontal lobe (premotor cortex, supplementary motor area) and any diseases to these areas like stroke or dementia
Name some signs of lower motor neuron lesion
- Weakness
- Hypotonia (reduced muscle tone)/ floppy
- Hyporeflexia (reduced reflexes)
- Muscle atrophy
- Fasciculations: visible twitch (caused by damaged motor units causing sponataneous action potential, 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 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 is the basal ganglia?
a group of subcortical nuclei responsible primarily for motor control
What is the structure of the basal ganglia, what nuclei is it made up of?
C - caudate nucleus
P – putamen
G – (external) globus pallidus
T - thalamus
Acc – nucleus accumbens
Am – amygdala
AC – anterior commisure
What is the function of the basal ganglia?
“helps us to perform fluid movements”
- Decision to move
- Elaborating associated movements (e.g. swinging arms when walking; changing facial expression to match emotions)
- Moderating and coordinating movement (suppressing unwanted movements)
- Performing movements in order
What is Parkinson’s disease?
Degeneration of the dopaminergic neurons that originate in the substantia nigra and project to the striatum
What are some signs of Parkinson’s disease?
Bradykinesia - slowness of (small) movements (doing up buttons, handling a knife)
Hypomimic face - expressionless, mask-like (absence of movements that normally animate the face)
Akinesia - difficulty in the initiation of movements- cannot initiate movements internally
Rigidity - muscle tone increase, causing resistance to externally imposed joint movements
Tremor at rest - 4-7 Hz, starts in one hand (“pill-rolling tremor”); with time spreads to other parts of the body
What is Huntington’s disease?
“Genetic neurodegenerative disorder- Degeneration of GABAergic neurons in the striatum, caudate and then putamen”
- caused by CAG repeat in Chromosome 4
- autosomal dominant
What are some signs of Huntington’s disease?
- Choreic movements (chorea - dance)
- Rapid jerky involuntary movements of the body; hands and face affected first; then legs and rest of body
- Speech impairment
- Difficulty swallowing
- Unsteady gait
- Later stages, cognitive decline and dementia
What is ballism and when does it occur?
- Sudden uncontrolled flinging of the extremities
- Symptoms occur contralaterally
- Usually from stroke affecting the subthalamic nucleus
What is the role of the cerebellum?
Coordinator and predictor of movement
What is gait?
The manner or style of walking
What is the role of the vestibulocerebellum? What happens if it is damaged?
- Regulation of gait, posture and equilibrium
- Coordination of head movements with eye movements
Damage (tumour) causes syndrome similar to vestibular disease: - leading to gait ataxia (abnormal, uncoordinated movement)
- tendency to fall (even when patient sitting and eyes open)
What is the role of the spinocerebellum? What happens if it is damaged?
- Coordination of speech
- Adjustment of muscle tone
- Coordination of limb movements
Damage (degeneration and atrophy associated with chronic alcoholism): - affects mainly legs
- causes abnormal gait
- Abnormal stance (wide-based)
What is the role of the Cerebrocerebellum? What happens if it is damaged?
- Coordination of skilled movements
- Cognitive function
- attention
- Processing of language
- Emotional control
Damage affects: - mainly arms
- skilled coordinated movements (tremor)
- Speech
What are the main signs of cerebellar dysfuncion?
- 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 the alpha motor neurons?
- The lower motor neurons of the brainstem and spinal cord
- They innervate the extrafusal muscle fibres of the skeletal muscles
- Activation of these neurons causes muscle contraction
What is a motor neuron pool?
Motor neuron pool contains all alpha motor neurons innervating a single muscle
What is a motor unit?
“a single motor neuron together with all the muscle fibres that it innervates”
- It is the smallest functional unit with which to produce force.
- Stimulation of one motor unit causes contraction of all the muscle fibres in that unit
What are the different types of motor units?
- Slow (S, type I)
smallest diameter cell bodies
small dendritic trees
thinnest axons
slowest conduction velocity
(2 types of fast)
2. Fast, fatigue resistant (FR, type IIA)
3. Fast, fatiguable (FF, type IIB)
larger diameter cell bodies
larger dendritic trees
thicker axons
faster conduction velocity
How are the different motor unit types classified?
classified by the amount of tension generated, speed of contraction and fatiguability
What are the 2 mechanisms by which the brain regulates the force that a single muscle can produce?
- Recruitment
- Rate coding
What is meant by recruitment of motor units?
Motor units are not randomly recruited. There is an order to this.
Governed by the “size principle”. Smaller units are recruited first (these are generally the slow twitch units).
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 meant by rate coding mechanism?
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 (the additive effect of several electrical impulses) occurs when units fire at frequency too fast to allow the muscle to relax between arriving action potentials.
What are neurotrophic factors?
“A type of growth factor”
- promotes growth of neurons after injury
What determines the characteristics of motor units and muscle fibres?
- Motor unit and fibre characteristics are dependent on the nerve which innervates them (certain neurotrophic factors)
- If a fast twitch muscle and a slow muscle are cross innervated, the soleus becomes fast and the FDL becomes slow
- The motor neuron has some effect on the properties of the muscle fibres it innervates.
Fibre types can change properties under many different conditions, what are these conditions?
- Type IIB to IIA most common following training
- Type I to II possible in cases of severe deconditioning or spinal cord injury. Microgravity during spaceflight results in shift from slow to fast muscle fibre types
- Ageing associated with loss of type I and II fibres but also preferential loss of type II fibres. This results in a larger proportion of type I fibres in aged muscle (evidence from slower contraction times)
What is a reflex function?
“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.”
also..
“An involuntary coordinated pattern of muscle contraction and relaxation elicited by peripheral stimuli”
What determines the magnitude and timing of a reflex function?
- Magnitude is determined by the intensity of the stimulus
- Timing is determined by the onset of the stimulus
What is the difference between reflexes and voluntary movements?
Reflexes differ from voluntary movements in that once they are released, they can’t be stopped
What are the 2 types of reflexes?
- Automatic (e.g. knee jerk)
- Stereotyped behaviors (e.g. sneezing, coughing)
in response to stimulation of peripheral receptors
What is the Jendrassik manoeuvre?
Medical manoeuvre where the patient is clenching the teeth, making a fist, or pulling against locked fingers when having patellar tendon tapped
Aim= make the reflex response larger
What motor control is in charge of reflexes?
Higher centres of the CNS exert inhibitory and excitatory regulation upon the stretch reflex
-Inhibitory control dominates in normal conditions (N)
-Decerebration (removing the cerebrum) reveals the excitatory control from supraspinal areas (D)= causes muscles to tighten and increased rigidity
-Rigidity and spasticity can result from brain damage giving over-active or tonic stretch reflex.
Describe the supraspinal control of reflexes
- Activating alpha motor neurons
- Activating inhibitory interneurons
- Activating propriospinal neurons
- Activating gamma motor neurons
- Activating terminals of afferent fibres
What is meant by hyper- reflexia?
“Overactive reflexes”
-Loss of descending inhibition
-Associated with upper motor neuron lesions
What is Babinski sign?
When sole stimulated with blunt instrument the big toe:
- Curls downwards - normal
- Curls upwards – abnormal in adults. This is a positive Babinski sign.
- Associated with upper motor neuron lesions
Note: Toe curls upwards in infants – this is normal
What is meant by hypo-reflexia?
“Below normal or absent reflexes”
-Associated with lower motor neuron disease
