Motor system I

Lecture 6

How does the system work ?

Initiates motor movement: Brain - muscles - contraction of muscles.

How are the motor pathways affected ?

In stroke

Descending tracts of the CNS: What are they ? Where do they originate ? Divisions ?

• The descending tracts are the pathways by which motor signals are sent from the brain to lower motor neurones. The lower motor neurons then directly innervate muscles to produce movement.

• Originate in brain in cortex, go through spinal cord to muscles.

• Can be functionally divided into 2 major groups

1. Pyramidal tracts

2. Extrapyramidal tracts

Pyramidal tracts: originate ? What do they carry ? Function ? Subdivions ?

originate in the cerebral cortex, carrying motor fibres to the spinal cord and brain stem.

Function: responsible for the voluntary control of the musculature of the body and face. E.g. grabbing a cup of tea.

Functionally, these tracts can be subdivided:

corticospinal tracts: supplies the musculature of the body

Corticobulbar tracts: supplies the musculature of the head and neck

Corticospinal tract: Function ? Originate ? Pathway ? Division ?

Function: supplies the musculature of the body.

Originate in the cerebral cortex (primary motor cortex, premotor cortex, supplementary motor area), then descends through the internal capsule, then passes through the crus cerebri of the midbrain, the pons and into the medulla.

Tract divided into two:

• Lateral corticospinal tracts

• Ventral corticospinal tracts

Corticobulbar tract: Function ? Originate ? Pathway ?

Function: supplies the muscaluture of the head and neck.

Originate from lateral aspect of the primary motor cortex, then descend through the internal capsule to the brainstem to terminate on the motor nuclei of the cranial nerves (hypoglossal (CN XII)), then synapse with lower motor neurones, which carry the motor signals of the face and neck (help us with facial expressions, muscle movement of head + neck)

Clinical relevance ?

Internal capsule is particlarly susceptible to compression from haemorrhagic bleeds, known as a “capsular stroke” - bleeding goes into internal capsule.

» could cause a lesion of the corticospinal and corticobulbar tract

» loss of motor function

Extrapyrmidal tracts: Targeted by what ? Originate ? Function ? Divisions ? Human body ?

often targeted by medications and other neuro-degenerative disorders.

Originate in the brainstem, carrying motor fibres to the spinal cord.

Function: responsible for the involuntray and automatic control of all musculature, such as muscle tone, balance, posture and locomotion.

Can be functionally divided:

1. Vestibulospinal tract

2. Reticulospinal tract

3. Rubrospinal tract

4. Tectospinal tract

We have two vestibular spinal tracts in our human body: medial and lateral

Vestibulospinal tract: How many ? Arise from ? Function ?

Two vestibulospinal pathways (medial and lateral)

arise from the vestibular nuclei, which receive input from organs of balance (inner ear)

The tracts convey this balance information to the spinal cord, where it remains ipsilateral.

Function: Fibres in this pathway control balance and posture by innervating the “anti-gravity” muscles (flexores of the arm, and extensors of the leg), via lower motor neurones.

Reticulospinal tract: Functions ? Arises from what ?

The two reticulospinal tracts have different functions:

Medial reticulospinal tract: arises from the pons, It facilitates voluntray movements, and increases muscle tone.

Lateral reticulospinal tract: arises from the medulla. It inhibits voluntary movements and reduces muscle tone.

Rubrospinal tract: Originate ? Pathway ? Function ?

Originates from the red nucleus in the midbrain.

Decussate (cross over to the other side of the CNS) and descend into the spinal cord.

Function: play a role in the fine control of hand movements.

Tectospinal tract: Originates ? Function ?

Originates at the superior colliculus of the midbrain (receives inout from the optic (visual) nerve.

Terminate at the cervical levels of the spinal cord.

Function: controls head and eye movement (becomes often damaged in stroke or TBI)

Classes of motor neurons: What does it help ? What are motor neurons ? Two main subtypes ? What are they named based on ?

Helps explain a lot of diseases.

Motor neurons: cells of the CNS transmitting signals to muscle cells or glands (innervate muscles, lead to contraction of muscles)

Two main subtypes (named based on location):

Upper Motor Neuron: (Big neuron, 1 of the largest neurons in the human body)

• originate in the cerebral cortex and travel down to the brainstem or spinal cord.

• Use glutamate as neurotransmitter (Excitatory, leads to excitation of muscles)

Lower motor neuron: very short, straight to muscles

• Begin in the spinal cord and their terminals extend all the way to the muscle fibers and tendons

• Use acetylcholine as neurotransmitter (excitatory)

Division of the lower motor neurons: Based on what ?

Lower motor neurons can be further classified based on the type of muscle fiber they innervate.

• Alpha Motor Neurons: Innervate extrafusal muscle fibers of skeletal muscle. Cause muscle contractions that generate movements

• Gamma motor neurons: innervate the intrafusal muscle fibers of a specialised sensory organ (muscle spindle) respond to stretch of the muscle spindle.

Input to lower motor neurones: sources of input to lower motor neurones ?

Sources of input to lower motor neuron: The alpha motor neurons that directly cause muscle contraction receive inputes from three different sources.

Sensory input from muscle spindles: provide sebsory information from the muscles

Upper motor neurons from the motor cortex: reponsible for initiating voluntary movement

Interneurons in the spinal cord: represents the largest input to the alpha motor neurons and can either provide excitation or inhibition to the alpha motor neuron

Sources of input to lower motor neuron:

Upper motor neurons send input to alpha neurons via several pathways:

Coricobulbar

Corticospinal

Rubrospinal

Reflex arc: Need input from brain ? Reflex ? Eg. ?

Do not need input from brain.

Reflex: involuntary and nearly instantaneous movement in reposnse to a stimulus. The reflex is an automatic repsonse to a stimulus that does not receive or need conscious thoughts as it occurs through a reflex arc. Reflex arcs act on an impulse before that impulse reaches the brain.

Primitive reflex arc (Knee-Jerk Reflex) when a muscle is unexpectantly stretched it contracts in an attempt to keep its length constant. Muscle spindle sends a sensory signal into the spinal cord, which synapses directly on a motor neuron, without signalling to the brain first.

Monosynaptic stretch reflex: very quick corrective action.

Reflexes examples ?

not all transduction of sensory nerve fiber signals into musculature action via brain:

• Visual light reflex (midbrain)

• Corneal reflex (pons)

• gag reflex (medulla)

• Biceps reflex (cervical spinal cord)

• Triceps reflex (cervical spinal cord)

• Quadriceps reflex (lumbar spinal cord)

Motor cortex: what happens here ? What is it ? Divisions ?

• Motor commands are generated

• Region of the frontal lobes (directly in front of sensory cortex) involved in the planning, control, and excecution of voluntray movements.

The motor cortex can be divided into 3 areas:

• primary motor cortex (Brodmann’s area 4) often affected in stroke

• Premotor cortex

• Supplementary motor area

Primary motor reflex: What is it critical for ? How are neurons arranged ?

• critical for initiating motor movements

• Neurons are arranged topographically (means each area of the primary motor cortex contains neurons with a specific function for movements)

» Functional map of motor cortex can be created (Motor Homunculus)

• Motor cortex for the foot and toes is located at the top of the primary motor cortex.

• Representation for the knee, hip, shoulder, elbow, wrist, hand, neck and eye progresses laterally downward (if you hit lower part of patients would have troubles in movement of tongue, problems in speech production)

Primary motor cortex (Brodmann area 4): Contains what ? pathway ?

• The primary motor cortex contains large neurons with triangular-shaped cell bodies that are called pyramidal neurons (Betz cells) these are the primary output cells of the motor cortex.

• Axons of pyramidal cells leave the motor cortex carrying info about a desired movement and enter one of the tracts of the pyramidal systems (cortciospinal, cortciobulbar tract)

• Pyramidal neurons of the motor cortex can also form direct connections with alpha motor neurons which innervate the skeltal muscles to cause movement

Premotor cortex: Affected by what ? Found where ? Involved in what ? contains what ?

• often affected in TBI + stroke patients

• Found directly in front of primary motor cortex

• Thought to be involved in planning and executing motor movements.

• Contains mirror neurons (motor learning - observation of motor skills in development of children). For eg. sequencing of movement is affected if damaged. Mirror neurons are often affected in ASD.

Supplementary motor area: Critical for what ?

Critical to the execution of sequences of movement, the execution of motor skills and the control of moevements.

Disorders of the motor system & lesions: Damage at different levels of motor system ?

Spinal Cord (e.g. SCI)

• Paraplegic: complete paralysis of lower limbs (can affect trunk and pelvic organs)incontinent

• Quadraplegic: complete paralysis that affects both arms and legs, trunk, pelvic organs.

Motor Cortex (e.g. Stroke)

• Hemiplegia: Complete paralysis to 1 side of the body

• Hemiparesis: Partial weakness of one side of the body.

Secondary / Association Motor Areas (e.g Stroke)

Apraxia: is the loss of ability to execute or carry out skilled movement and gestures, despite having the physical ability and desire to perfrom them.

Cause: stroke in premotor cortex or parietal lobe

Aphasia vs Apraxia ?

Patients with apraxia and aphasia might both have difficulty expressing themself with words.

Aphasia: problem in a person’s ability to understand or use words in and of themself. This may make it hard for someone with the condition to speak, read or write.

Apraxia:

• does not describe a problem with language comprehsnion

• Difficulty someone has initiating and performing the movements needed to make speech. This difficulty arises despite the fact that there is no weakness in the necessary muscles. Problems in the motor cortex which initaites and which gives the commands to the muscles to generate muscle moevement

Upper Motor Neuron Syndrome (UMNS): symptoms, causes

damage to the descending upper motor neuron pathways.

Symptoms:

Muscle altered performance:

• Weakness (decreased ability for the muscle to generate force)

• Decreased motor control (decreased speed, accuracy)

• Altered muscle tone (hypotonia or hypertonia)

• decreased endurance

• spasticitity and clonus (a series of involuntary rapid muscle contractions)

Causes: Motor areas in the brain and or spinal cord are damaged or fail to develop normally.

• Spinal Cord Injury

• Cerebral palsy (neurodevelopmental disorder - cortex is not formed properly)

• Multiple Sclerosis(motor neurons attached)

• Stroke

• TBI

Lower Motor Neuron Syndrome (LMNS): symptoms ? Causes ?

Symptoms that arise from damage to the lower motor neurons of the brainstem and spinal cord.

Symptoms: Damage to lower motor neuron cell bodies or their peripheral axons result:

• Paralysis (loss of movement)

• Paresis (weakness) of the affected muscles

• Areflexia (loss of reflexes)

• Loss of muscle tone

• Atrophy of the affected muscles

• Muscles involved may also exhibit fibrillations+fasciculations

Causes:

• Trauma or compression of the peripheral nerves.

• Carpal tunnel syndrome

• Viruses selectively attack ventral horn cells.

Example of UMN and LMN damage ?

Facial Nerve Weakness:

Distribution of facial weakness provides important localsing clues:

• Damage to primary motor cortex (UMN lesion; A)

• Damage to corticobulbar projections (lesion; B)

• Facial motor neuron or nerve damage (LMN lesion; C)

Extrapyramidal symptoms (EPS):

Symptoms that are associated with the extrapyramidal system.

movement dyfunction:

Dystonia: involuntary muscle spasms + contractions

Akathisia: motor restlessness

Parkinsonism charachteristic symptoms:

• Rigity (inface)

• Bradykinesia (slowness of movement)

• Tremor (most frequent symptom)

• Tardive dyskinesia (irregular, jerky movements)

Causs:

• Medication - Antipsychotic drugs (dopamine blockade) cause extrapyramidal symptomes

• Non medication: Brain damage, Meningitis - inflammation of the brain (due to viris / bacteria)