Motor System• Starts at the motor cortex

• Motor cortex is located at the frontal lobe– precentral cortex

Motor homunculus

First discoveredbyPenfield

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

motor cortex

internal capsule

Uppermotorneuron

Lowermotorneuron

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• supplementar

y motor area (SMA)

• Prefrontal cortex (PFC)

Primary motor cortex

movement

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 tractor– 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 fil ter ing incoming stimuli to discriminate irrel e vant back ground stim uli

• 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

Uppermotorneuron

Lowermotorneuron

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 movement 1 - flicker is perceptible in the muscle 2 - movement only if gravity eliminated 3 - can move limb against gravity 4 - can move against gravity & some resistance exerted by examiner 5 - 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)

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