Motor System

• Starts at the motor cortex

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

Motor homunculus

First discoveredbyPenfield

Brodmann areas

Motor cortex

• different areas of the body are represented in different cortical areas in the motor cortex

• Motor homunculus– somatotopic representation – not proportionate – distorted map– upside down map

Motor cortical areas

• primary motor cortex (MI)– precentral gyrus

• secondary motor cortex (MII)– premotor cortex– supplementary motor area

primary motor cortex

Functions• Corticospinal tracts (pyramidal tracts)

starts here• Cell bodies are located in the cortical area

(large cell bodies are known as Betz cells)• Corticospinal tract descends down

Course of the corticospinal tract

• Descends through– internal capsule– at the medulla

• cross over to the other side

– descends down as the corticospinal tract– ends in each anterior horn cell– synapse at the anterior horn cell

Medulla

motor cortex

internal capsule

Uppermotorneuron

Lowermotorneuron

anterior horn cell

Functional role of primary and secondary motor areas

• SMA (Supplementary Motor Area) assembles global instructions for movements

• It issues these instructions to the PreMotor Area.

• PreMotor Cortex (PMC) 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

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 neurone– muscle fibres supplied by the neuron

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

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

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

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

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

Renshaw cells• Renshaw cells are inhibitory interneurons

found in the spinal cord

• They receive excitatory collateral from the alpha motor neuron’s axon as they emerge from the motor root– (they are "kept informed" of how vigorously

that neuron is firing)

• They send their own inhibitory axon to synapse with the cell body of the initial alpha neuron

• In this way, Renshaw cell inhibition represents a negative feedback mechanism

• A Renshaw cell may be supplied by more than one alpha motor neuron collaterals and it may synapse on multiple motor neurons

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 tract

Extrapyramidal tracts

Alpha motor neuron

Gamma motor

neuron•Voluntary movement•Muscle tone

Muscle spindle

Clinical situations

• Muscle power– Normal– Reduced (muscle weakness)

• muscle paralysis• muscle paresis

• 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

• Upper motor neuron lesion causes– spastic paralysis

Lower motor neuron lesion

• muscle weakness• flaccid paralysis• muscle wasting (disuse atrophy)• reduced muscle tone (hypotonia)• reflexes: reduced or absent• spontaneous muscle contractions (fasciculations)• plantar reflex: flexor• superficial abdominal reflexes: present

Upper motor neuron lesion

• muscle weakness• spastic paralysis• increased muscle tone (hypertonia)• reflexes: exaggerated• 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 bee seen– Passive stretch causing initial incerased resistance

which is released later

tendon jerks (reflexes)

• reflex level

• biceps jerk C 5 6

• triceps jerk C 7 8

• knee jerk L 3 4

• ankle jerk S 1 2

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 (becuase of immature corticospinal tract)

positive Babinski sign

Site of lesions

Cortex

Internal capsule

Brain stem

Spinal cord

Anterior horn cell

Motor nerve

Neuromuscular junctionMuscle

Site of lesions

monoplegiaonly 1 limb is affected either UL or LL,lower motor neuron lesion

hemiplegiaon half of the body includingUL and LLlesion in the Internal capsule

paraplegiaboth lower limbsthoracic cord lesion

quadriplegia (tetraplegia)all 4 limbs are affected

cervical cord or brain stem lesion