Motor system pathways for students

Motor System

• Starts at the motor cortex

• Motor cortex is located at the frontal lobe

– precentral cortex

Motor homunculus

First discovered

by

Penfield

Brodmann areas Primary motor cortex

Area 4

Motor cortex

• different areas of the body are

represented in different cortical areas in

the motor cortex

• Motor homunculus

– somatotopic representation

– not proportionate to structures but

proportionate to function

– distorted map

– upside down map

Motor cortical areas

• primary motor cortex (MI)

– precentral gyrus

• Movements are executed

• secondary motor cortex (MII)

– premotor cortex

– supplementary motor area (SMA)

• Movements are planned together with cerebellum, basal

ganglia and other cortical areas

Primary motor cortex

• Corticospinal tract (pyramidal tract) originates

from the primary motor cortex

• Corticobulbar tract also originates from the

motor cortex and supplies brainstem and the

cranial nerves

• Cell bodies of the corticospinal tracts are

called Betz cells (large pyramidal shaped

cells)

• Corticospinal tract descends down the

internal capsule

Course of the corticospinal tract

• Descends through

– internal capsule

– at the medulla • cross over to the other side

• uncrossed tracts

– descends down as the corticospinal tract

– ends in each anterior horn cell

– synapse at the anterior horn cell (directly or through interneurons)

Medulla

internal capsule

Upper

motor

neuron

Lower

motor

neuron

anterior horn cell

Primary and secondary cortical

areas• Primary areas are primarily connected with the

peripheral organs/structures

– Primary motor cortex (area 4)

• Secondary areas are inter-connected to each

other by cortico-cortical pathways and perform

complex processing

– Premotor cortex (area 6)

– Supplementary motor area (superomedial part of

area 6)

Functional role of primary and

secondary motor areas• SMA (Supplementary motor area)

assembles global instructions for movements

• It issues these instructions to the Premotor cortex (PMC)

• Premotor cortex works out the details of smaller components

• And then activates specific Primary motor cortex (MI)

• Primary motor cortex through Corticospinal tracts (CST) activate specific motor units

SMA

PMC

MI

CST

Motor units

Complex nature of Cortical

Control of Movement

8.15

idea•premotor area

•supplementary

motor area

(SMA)

•Prefrontal

cortex (PFC)

Primary

motor cortexmovement

basal ganglia

cerebellum cerebellum

plan execute

memory, emotions

Motor system

• Consists of

– Upper motor neuron

– Lower motor neuron

Lower motor neuron

• consists of mainly

• alpha motor neuron

– and also gamma motor neuron

alpha motor neuron

gamma motor neuron

alpha motor neuron

gamma motor neuron

corticospinal tract

Arrangement at the

anterior horn cell

alpha motor neuron

• this is also called the final common pathway

• Contraction of the muscle occurs through this

whether

– voluntary contraction through corticospinal tract

or

– involuntary contraction through gamma motor

neuron - stretch reflex - Ia afferent

motor unit• muscle contraction occurs in terms of motor units

rather than by single muscle fibres

• a motor unit is defined as

– anterior horn cell

– motor neuron

– muscle fibres supplied by the neuron

• Muscle power/strength is obtained by the principle of

“Recruitment of motor units”

motor unit

• Innervation ratio

– motor neuron:number of muscle

fibres

• in eye muscles

– 1:23 offers a fine degree of

control

• in calf muscles

– 1:1000 more strength

Upper motor neuron

• Consists of – Corticospinal tract (pyramidal tract)

– Extrapyramidal tracts• Start from the brainstem

• Ipsilateral/contralateral

• Cortical pathways can excite/inhibit these tracts

• Modify the movement that is initiated by the CST

• Influence (+/-) gamma motor neuron, stretch reflex, muscle tone

• Important for postural control

• Cerebellar and basal ganglia influence on the lower motor neuron will be through extrapyramidal tracts

Extrapyramidal tracts

• starts at the brain stem

• descends down either ipsilaterally or

contralaterally

• ends at the anterior horn cell

• modifies the motor functions

Extrapyramidal tracts

• there are 4 tracts

– reticulospinal tracts

– vestibulospinal tracts

– rubrospinal tracts

– tectospinal tracts

reticulospinal tract

• relay station for descending motor impulses

except pyramidal tracts

• receives & modifies motor commands to the

proximal & axial muscles

• maintain normal postural tone

• excitatory to alpha & gamma motorneurons

• end on interneurons too

• this effect is inhibited by cerebral influence

• mainly ipsilateral

midbrain

pons

medulla

spinal cord

reticulospinal tract

• pontine reticular formation

– medial reticulospinal tracts

• controls proximal muscles (axial), excitatory to flexor

• medullary reticular formation

– lateral reticulospinal tracts (also medial)

• excitatory or inhibitory to axial muscles

Reticular formation

• A set of network of interconnected

neurons located in the central

core of the brainstem

• It is made up of ascending and

descending fibers

• It plays a big role in filtering

incoming stimuli to discriminate

irrelevant background stimuli

• There are a large number of

neurons with great degree of

convergence and divergence

Functions

• Maintain consciousness, sleep and arousal

• Motor functions (postural and muscle tone

control)

– Reticulospinal pathways are part of the

extrapyramidal tracts

• Pain modulation (inhibition)

– Several nuclei (PAG, NRM) are part of the

descending pain modulatory (inhibitory) pathway

vestibular nuclei & tracts

• responsible for maintaining tone in antigravity

muscles & for coordinating the postural

adjustments in limbs & eyes

• connections with vestibular receptors (otolith

organs) & cerebellum

• mainly ipsilateral

• supplies extensors

midbrain

pons

medulla

spinal cord

vestibulospinal tract

mainly extensors

• vestibulospinal tracts

– lateral vestibulospinal tract

– medial vestibulospinal tract

– excitatory to antigravity alpha motor neurons &

supplies interneurons too

– lateral tract

• excitation of extensor muscles & relaxation of flexor

muscles

– medial tract

• inhibition of neck & axial muscles

red nucleus

• present in the midbrain

• rubrospinal tract originates from the red nucleus

• ends on interneurons

• control the distal muscles of limbs

• excite limb flexors & inhibit extensors

• higher centre influence (cerebral cortex)

• mainly contralateral

• supplies flexors

• Functionally this tract is not important in human motor

system

midbrain

pons

medulla

spinal cord

rubrospinal tract

mainly flexors

tectospinal tract

• tectospinal tract originates from the tectum of

the midbrain

• ends on interneurons

• mainly contralateral

• supplies cervical segments only

• Functionally this tract is not important in human

motor system

midbrain

pons

medulla

spinal cord

tectospinal tract

cervical segments

inferior olivary nucleus

• present in the medulla

• function:

– motor coordination

• via projections to the cerebellum

• sole source of climbing fibres to the cerebellum

– motor learning

– Functionally this nucleus is not important in human

motor system

Upper

motor

neuron

Lower

motor

neuron

extrapyramidal tracts

pyramidal tracts

alpha motor neurone

gamma motor neurone

Clinical Importance of the motor system

examination

• Tests of motor function:

– Muscle power

• Ability to contract a group of muscles in order to make an

active movement

– Muscle tone

• Resistance against passive movement

Basis of tests

• Muscle power

– Test the integrity of motor cortex, corticospinal tract

and lower motor neuron

• Muscle tone

– Test the integrity of stretch reflex, gamma motor

neuron and the descending control of the stretch

reflex

Muscle tone

• Resistance against passive movement

– Gamma motor neuron activate the spindles

– Stretching the muscle will activate the stretch reflex

– Muscle will contract involuntarily

– Gamma activity is under higher centre inhibition

• There is a complex effect of corticospinal and extrapyramidal tracts on the alpha and

gamma motor neurons (in addition to the effect by muscle spindle)

• There are both excitatory and inhibitory effects

• Sum effect

– excitatory on alpha motor neuron

– Inhibitory on gamma motor neuron

Corticospinal

tractExtrapyramidal

tracts

Alpha motor

neuron

Gamma

motor

neuron•Voluntary movement

•Muscle tone

Muscle spindle

Clinical situations

• Muscle power– Normal

– Reduced (muscle weakness)

• Paralysis, paresis, plegia

• MRC grades 0 - no movement1 - flicker is perceptible in the muscle2 - movement only if gravity eliminated3 - can move limb against gravity4 - can move against gravity & some resistance exerted by examiner5 - normal power

• Muscle tone – Normal

– Reduced

• Hypotonia (Flaccidity)

– Increased

• Hypertonia (Spasticity)

Main abnormalities

• Muscle Weakness / paralysis

– Reduced muscle power

• Flaccidity

– Reduced muscle tone

• Spasticity

– Increased muscle tone

• Lower motor neuron lesion causes

– flaccid paralysis (flaccid weakness)

• Upper motor neuron lesion causes

– spastic paralysis (spastic weakness)

Lower motor neuron lesion

• muscle weakness

• flaccid paralysis

• muscle wasting (disuse atrophy)

• reduced muscle tone (hypotonia)

• reflexes: reduced or absent (hyporeflexia or areflexia)

• spontaneous muscle contractions (fasciculations)

• plantar reflex: flexor

• superficial abdominal reflexes: present

• eg. Brachial plexus damage

Upper motor neuron lesion

• muscle weakness

• spastic paralysis

• increased muscle tone (hypertonia)

• reflexes: exaggerated (hyperreflexia)

• Babinski sign: positive

• superficial abdominal reflexes: absent

• muscle wasting is very rare

• clonus can be seen:

– rhythmical series of contractions in response to sudden stretch

• clasp knife effect can be seen

– passive stretch causing initial increased resistance which is released

later

• eg. Stroke

Babinski sign• when outer border of the sole of the foot is

scratched

• upward movement of big toe

• fanning out of other toes

• feature of upper motor neuron lesion

• extensor plantar reflex

• seen in infants during 1st year of life (because

of immature corticospinal tract)

positive Babinski sign

• Observation

• When the spinal cord is suddenly transected, essentially all

cord functions, including spinal cord reflexes, immediately

become depressed

• This is called “spinal shock”

• Period of spinal shock is about 2 weeks in humans

• It may vary depending on the level spinal cord injury

• Higher the animal in evolution greater is the spinal shock

period

Spinal cord transection and spinal shock

During spinal shock period

• complete loss of all reflexes

• no tone

• paralysis

• complete anaesthesia

• no peristalsis

• bladder and rectal reflexes absent

• no sweating

• arterial blood pressure decreases

Possible mechanism of spinal shock

• Normal activity of the spinal cord reflexes depends to a great

extent on continual tonic excitation from higher centers

(pyramidal and extrapyramidal tracts)

• Spinal shock may be due to the sudden cessation of tonic

bombardment of spinal cord interneuron pool by descending

influences

• During recovery from spinal shock, the excitability of spinal cord

reflexes increase due to the lack of descending inhibition and

possible denervation hypersensitivity

• After the spinal shock period typical upper motor neuron

features appear

after the spinal shock

• reflexes will reappear, mostly exaggerated

• bladder become reflex

• mass reflex will appear

– afferent stimuli irradiate to several reflex centres

– noxious stimulus causes: withdrawal response,

evacuation of bladder, rectum, sweating, pallor

Site of lesions

Cortex

Internal capsule

Brain stem

Spinal cord

Anterior horn cell

Motor nerve

Neuromuscular junction

Muscle

Neurological diseases

Disease Involvement

• Stroke UMN

• Peripheral neuropathy LMN

– Mononeuropathy

– Polyneuropathy

• Plexopathy LMN

• Radiculopathy LMN

• Myelopathy LMN, UMN

• Motor neuron disease LMN, UMN

• Monoplegia (monoparesis)

• Hemiplegia (hemiparesis)

• Paraplegia (paraparesis)

• Quadriplegia (quadriparesis)

Site of lesions

monoplegia

only 1 limb is affected either UL or LL,

lower motor neuron lesion

hemiplegia

one half of the body including

UL and LL

lesion in the Internal capsuleparaplegia

both lower limbs

thoracic cord lesion

quadriplegia (tetraplegia)

all 4 limbs are affected

cervical cord or brain stem lesion

Some common neurological

diseases

Stroke

• Cerebrovascular accident (CVA)

• A serious neurological disease

• Large number of deaths per year

• Cerebrovascular ischaemia causing

infarction or haemorrhage

• Sudden onset hemiplegia

• Hypertension, diabetes, obesity are

risk factors

Peripheral neuropathies

• Mononeuropathies

– Carpal tunnel syndrome (CTS)

– Ulnar neuropathy - claw hand

– Saturday night palsy (radial nerve lesion) – wrist drop

– Common peroneal nerve lesion – foot drop

– Posterior tibial nerve lesion – tarsal tunnel syndrome

– Sciatic nerve lesion

– Facial nerve lesion – Bell’s palsy

• Polyneuropathies

– Diabetic, vitamin deficiency, toxic

Median nerve compression

(Carpal tunnel syndrome)

Ulnar nerve lesion

(Ulnar tunnel syndrome)

Clawing of the hand

Radial nerve lesion

(Saturday night palsy)

Wrist drop Wrist guard

Common peroneal nerve

lesion

Foot drop Ankle guard

Posterior tibial nerve lesion

(Tarsal tunnel syndrome)

Sciatic nerve lesion

Facial nerve lesion

(Facial palsy or Bell’s

palsy)

Brachial plexopathy

(Erb’s palsy)

Cervical spondylosis

Sciatica

Cervical or thoracic

myelopathy

Paraplegia

Quadriplegia

MND or Motor neuron

disease

• Anterior horn cell disease

• MND: motor neuron disease

• ALS: Amyotrophic lateral sclerosis

• Weakness of lower limbs, upper limbs

• Speech defect: dysarthria

• Difficulty in swallowing: dysphagia

MND or Motor neuron

disease