week 4 Motor Control concepts and motor cortex Flashcards
Primary Motor Cortex A.k.a. Area 4, M1
Located in the pre central gyrus (frontal lobe)
- Houses cell bodies of Upper motor neurons
- Executes commands to motor neurons
- Stimulation elicits simple movements of single joints
Pre-motor cortex
- Receives input from sensory areas
- Role in planning movement ( “P” stands for)
- Related to sensory input / sensory guidance of movement
- Spatial guidance of movement
Supplementary Motor Cortex
Sequencing movement
- Feeds correct motor instructions in correct sequence to the
primary motor cortex
- Active during mental rehearsal of coordinated movements
what are the 3 classes of movement
reflexes, Rhythmic motor patterns, Voluntary
Reflexes
- Involuntary, rapid, stereotyped movements: Eye-blink, coughing, knee jerk reflex
- Initiated by an eliciting stimulus
Rhythmic motor patterns
- Combines voluntary & reflexive acts: Chewing, walking, running
- Initiation & termination voluntary
- Once initiated, the movement is repetitive & reflexive
Voluntary
- Complex actions: writing, speaking, playing piano, preparing food (many activities of daily life)
- Purposeful, goal-oriented
- Learnt and can be improved with practice
where is voluntary movement initiated
cerebral cortex level
Voluntary movements must be
planned, programmed and executed
Central pattern generators (CPGs)
- CPGs are neuronal circuits that produce rhythmic motor patterns in the absence of sensory or
descending inputs that carry specific timing information. - E.g. Walking
where are Central pattern generators (CPGs) initiated
brainstem) and modified by sensory input from PNS
what is Stepping pattern generators (SPG)
Adaptable networks of spinal interneurons that activate the lower motor neurons (to be discussed) that innervate your hip flexors/extensors and your knee flexors/extensors to give you the pattern of alternate flexion and extension required for walking.
- Activated when you consciously send a signal from the brain to initiate walking.
define motor control
- Motor control is defined as the ability to regulate or direct the mechanisms essential to movement
sensory information during motor control
- Update & modify motor activity during movement
- Alter motor patterns to deal with environmental perturbations
Proprioceptive information during motor control
Provides information about weight bearing & about limb position before movement onset
comes from receptors in PNS
visual system during motor control
- Provides information about visual cues for movement and guidance during movement
- e.g. Reaching for object
Vestibular system during motor control
- Input from inner ear receptors tells us about head position relative to gravity and during movement
- Hierarchical model is
- Organizational control that is top down.
- Each successively higher level exerts control over the level below it, never bottom-up control.
- For example, higher centres inhibit these lower reflex centres and reflexes controlled by lower levels of
the neural hierarchy are present only when cortical centres are damaged.
- Hierarchical model limitations
- Cannot explain the dominance of reflex behaviour in certain situations in normal adults.
- E.g. Withdrawal reflex after stepping on something sharp
Dynamical systems theory (DST) is
- Whole body is a mechanical system, with mass, and subject to both external forces such as gravity and
internal forces such as both inertial and movement-dependent forces - Degrees of freedom: Human beings have many degrees of freedom that need to be controlled (E.g.
Joints) and therefore human movement has inherent variability that is critical to optimal function - DST sees variability not to be the result of error but necessary for optimal function
- Optimal variability provides for flexible, adaptive strategies, allowing adjustments to environment
- Too little variability can lead to injury
- Too much variability leads to impaired movement performance
- A small amount of variability indicates a highly stable behaviour.
Dynamical systems theory (DST) limitation
- Can presume the nervous system has a less important role, giving mathematical formulas and principles of body mechanics a more dominant role in describing motor control.
- Understanding the application and relevance of this type of analysis to clinical practice can be very difficult.
Ecological
- Suggests motor control evolved to cope with the environment
- Suggests actions require perceptual information specific to a desired goal-directed action performed within a specific environment.
- Theory has broadened our understanding of nervous system function from that of a sensory / motor system, reacting to environmental variables, to that of a perception/action system that actively explores the environment to satisfy its own goals.
- Expanded our knowledge significantly with regard to the interaction of the us and the environment
Ecological disadvantages
- Gives less acknowledgement to the structure and function of the nervous system.
The medial upper motor neuron tracts are involved in
unconsious control of muscle tension
Upper Motor Neurons (UMN)cell bodies found in
UMN cell bodies are found either in the Primary motor cortex or the brainstem
Pathways originating from the cortex include
- Corticospinal tract
- Corticobrainstem (a.k.a. corticobulbar) tract
- Pathways originating from the Brainstem
Vestibulospinal
- Reticulospinal
- Rubrospinal
- Tectospinal
Corticospinal tract origin
Primary motor cortex
Corticospinal tract Course
- Axons of UMN descend via internal capsule (in posterior limb)
- Descend in the in cerebral peduncle of mid-brain, pons and
pyramids of the medulla- ~85% of axons decussate [X] descend through centrally as lateral corticospinal tract (Axons to legs are laterally and arms are medially located)
- ~10% of axons remain & descend as anterior corticospinal tract and decussate in the spinal cord at levels close to exit (Contains fibres to trunk & proximal muscles)
Corticospinal tract termination
On lower motor neurons (alpha motor neurons) in anterior (a.k.a ventral) horn of spinal cord.
Corticospinal tract Function
- Voluntary control of precise movements involving distal muscles o limbs (lateral CST)
- Control of less precise movements of proximal muscles of limbs and trunk (medial CST)
- Small percentage of CST projects to dorsal horn to modify sensory information, allowing brain to suppress or filter certain incoming
stimuli and pay attention to others.
Corticobrainstem origin
Lateral aspect of primary motor cortex (homunculus
area representing face and head)
Corticobrainstem course
- Descend via internal capsule (medial to Corticospinal tract)
- Most cranial nerve nuclei receive bilateral UMN innervation except VII (only lower half of face) and XII
- Contralateral fibres decussate at the level of
brainstem where Cranial cell bodies are
Corticobrainstem termination (crainial nerve 5,7,9,10,11,12)
- V – Trigeminal: Muscles of mastication
- VII – Facial: Muscles of the face
- IX – Glossopharyngeal: Stylopharyngeal
muscle - X – Vagus: Soft palate, larynx, oesophagus
- XI – Accessory: Sternomastoid and trapezius
- XII – Hypoglossal: Tongue
Corticobrainstem function
- Serves as UMNs to all motor cranial nerves
- Facilitates voluntary control of all the aforementioned cranial nerves (LMNs)
Tectospinal tract: function
Reflexive head movement respond to visual or auditory input
Vestibulospinal tract function
: Arises from vestibular nucleus to help controlling neck and upper back muscles. Aids
in balance.
Rubrospinal tract functions
: Arises from red nucleus in the midbrain but has minimal contribution to upper limb
extensor muscles
Lower Motor Neuron (LMN)
- LMNs transmit signals directly to skeletal muscles, eliciting the contraction of muscle fibers that move
the upper limbs and fingers - Are the only neurons that convey signals to skeletal muscle fibers.
- Cell body lies in the CNS
- Anterior horn of the spinal cord – Axons travel within peripheral nerves
- Brainstem (Cranial nerves with motor output) - Axons travel within cranial nerves
- Two types of lower motor neurons
Alpha LMN. Gamma LMN
Alpha LMN
Large cell bodies, large myelinated axons and project to extrafusal muscles fibers
Gamma LMN
medium sized myelinated axons and project to intrafusal muscles fibers in the
muscle spindles
LMN Motor units
- One alpha LMN and all the muscle fibres it innervates
- When one neuron fires ALL of the muscle cells which are stimulated by that neuron will contract
- Alpha motor neurons releases Ach (acetylcholine) so that all of the muscle fibers it innervates contract.
- The strength of a muscle contractions is determined by the size and number of motor units being stimulated.
Large Motor unit =
↑ muscle fibres for gross control
Small motor unit
↓ muscle fibres for precise control
LMN motor units have an inverse relationship with what. eg large and small cortical tissue have what sizwe motor units
motor homunculus
Larger Cortical tissue (i.e. more UMN cells bodies) = Small motor unit
Smaller Cortical tissue (i.e. less UMN cells bodies) = Large motor unit
Dysfunction causes of upper motor neurons
- Lesions: tissue that show damage from injury or disease
- Spinal cord injury
- Stroke (UMN) – Note: Depending of where stroke is will have different symptom presentation
- A traumatic brain injury (UMN)
- Guillain-Barré syndrome (LMN)
- Polio (LMS)
- multiple sclerosis
- Myasthenia Gravis
UMN Dysfunction
- Paralysis or paresis of affected muscles
Spasticity
Hyperreflexia - Loss of fractionation of movement (with CST involvement)
- Paralysis or paresis of affected muscles
- Hypertonia: Increase in muscle tone – often following a short period of hypotonia in the acute stage)
- Spasticity: Velocity-dependent; resistance to passive movement varies depending on the velocity of movement
- Hyperreflexia: Loss of inhibitory corticospinal input plus enhanced excitability of LMN & interneuron results in excessive LMN response to afferent input
- Muscle atrophy: Wastage (with disuse)
- Impaired postural control
- Involuntary muscle contractions eg spasms, cramps, myoclonus
Decerebrate Rigidity (UMN dysfunction)
- Caused by severe midbrain lesions
- Rigid extension of the limbs & trunk, internal rotation of upper limbs & plantar flexion
Decorticate Rigidity (UMN dysfunction)
- Caused by severe lesions above the midbrain
- Rigid flexed upper limbs, extended neck and lower limbs & plantar flexion
LMN dysfunction
- Loss of fractionation of movement (with CST involvement)
- Paralysis or paresis of affected muscles
- Hypotonia (decrease in muscle tone)
- Flaccidity
- Hyporeflexia-
- Muscle atrophy – wastage (with disuse)
Reflexes
- Reflex is an involuntary motor response to an external stimulus
- Can be protective
- Can integrate motor movements so they function in a coordinated manner such as postural adjustments to external stimuli while walking
- Can also be polysynaptic circuits involving interneurons & several levels of spinal cord e.g. Withdrawal
reflex - Although spinal reflexes can operate without Cerebral input, they are facilitated by descending pathways from cortex & brainstem and damage to them will result in absence
Phasic stretch reflex
Muscle contraction is response to quick stretch eg quad tendon reflex
Cutaneous reflex
Afferent information from skin, muscles, and/or joints can elicit a variety of withdrawal movements modulated in the Spinal cord E.g. A person steps on something sharp and the withdrawal reflex automatically
Gag reflex:
A protective mechanism to prevent unwanted entry of foreign body to respiratory passage which could lead to choking.
- Sensory: sensory from IX (from soft palate, pharynx)
- Response: muscular from X to close the glottis, elevate palate and gagging
Babinski’s sign (abnormal reflex)
Babinski’s sign is the extension of the great toe, often accompanied by fanning of the other toes
Areflexia
Absence of reflexes
Hyperreflexia
Increased or overactive reflexes
- Loss of inhibitory corticospinal input combined with LMN and interneuron development of enhanced excitability results in excessive LMN response to afferent input from stretch receptors.
- Excessive muscle contraction occurs when spindles are stretched as a result of excessive firing of the LMNs.
Hyporeflexia
Decreased reflexes
