Neuroscience 7b - Cortical Motor Function
Anil Chopra
1. Explain the importance of integration of sensory and motor systems in the
voluntary motor control.
2. Know the organisation of the anterior and lateral corticospinal tracts from the
motor cortex to a given motoneurons pool.
3. Explain the role of the pre-motor cortex and supplementary motor area in the
planning of motor tasks
4. Explain how CNS damage can lead both to exaggerated motor activity (i.e.
spasticity and hyper-reflexia) and at the same time depressed motor activity
(i.e. weakness)
5. Explain how stroke affecting the motor cortex can produce symptoms and how
plasticity in the nervous system can aid recovery.
The different parts of the central nervous system are associated with different aspects
of cortical motor function.
Association Neocortex: uses information about where the body is in space (based on
vision, audition and somatic sensations). The frontal lobe and limbic system generate
the desire to move into a certain position by considering all the possibilities and then
filtering them through the basal ganglia.
Basal Ganglia: filter out the different options to decide the best plan of action.
Motor Areas and Cerebellum: make the tactical decision.
Brainstem and spinal Cord: cause the movement to be made accordingly by sending
signals down the different tracts.
Descending Pathways
Extrapyramidal Pathways: these are indirect
descending motor pathways that originate in:
- red nuclei: rubro-spinal tract
- reticular nuclei: reticulospinal tracts
- vestibular nuclei: vestibulo-spinal tracts
Corticospinal Tracts: most of the motor pathways
descend in the pyramidal tracts. These are direct
connections between the motoneurons in the primary
motor and pre-motor corticies and those in the spinal
cord.
They cross over at the pyramidal decussation in the lower medulla oblongata to
form the contra-lateral (lateral) corticospinal tracts. There are small number of fibres
that remain uncrossed influencing muscles of the back, thorax and abdomen. 
Path of the Corticospinal tract:
»
Axons leave pyramidal cells and enter the white matter just below Layer VI.
Every gyrus has this core of white matter entering and leaving the cortex
»
The slips of white matter form the corona radiata (radiating crown)
»
The corona radiata runs deeper into the hemispheres and splits the caudate and
putamen nuclei in two
»
The axons are now called the internal capsule
»
At around the level of the mid brain the internal capsule coalesces to form a tight
bundle – the cerebral peduncles or stalks of the cerebrum in order to exit the
brain. These stalks form the floor of the brainstem and contain the descending
motor fibres
»
After the mid brain it reaches the pons where the
fibres split. Several synapse into the pontine nuclei
to form corticopontine fibres. The remaining
corticospinal tract gets fragmented and is no longer
visible as a single tract, but as a number of smaller
ones
»
In the medulla the corticospinal fibres come
together again to form pyramids. Pyramids run the
entire length of the medulla in the ventral surface
»
At the most caudal (bottom) part of the medulla the
fibres cross the midline – decussation of the
pyramids
»
As each individual fibre decussates it takes up
position in lateral white matter of the spinal cord
forming the lateral corticospinal tract. From here
they dive into the ventral horn at the level of their
target and synapse with α-motorneurons or
interneurons
»
15-20% of the corticospinal fires do not dross the
decussation and so form the anterior corticospinal
tract.
The Internal Capsule:
 A major two-way highway
 Sensory information travels up to the cortex from the thalamus, motor information
travels down from the cortex to the spinal cord
 It has two limbs:
 Anterior Limb
 Posterior Limb – further divided into the anterior part which contains the
corticospinal pathway and the posterior part which contains sensory fibres
going from the thalamus to the cortex
The corticospinal tracts originate in three main areas of the cerebral cortex:
 Primary Motor Cortex (Brodmann’s area 4): 38%
 Premotor Cortex PMA and supplementary motor cortex SMA
(Brodmann’s area 6): 30%
 Somatosensory cortex (Brodmann’s areas 1,2 & 3): 32%
Corticospinal Tract
Corticospinal tract has 2 parts
The largest is the lateral corticospinal tract
with 70-90% of the fibers
It originates from primary motor cortex,
premotor cortex and somatosensory cortex
It crosses at the pyramidal
decussation
It projects to the lateral ventral horn and
makes monosynaptic connections on the
motoneurons of distal muscle.
This enables fine independent finger
movements
premotor cortex
Frontal lobe
Parietal lobe
Corticospinal Tract
premotor cortex
primary motor cortex
primary motor cortex
somatosensory cortex
Anterior (ventral) corticospinal tract
remains uncrossed until the spinal cord
Here bilateral and polysynaptic connections
are made on medial motoneurons of
proximal and axial muscle
These muscle are used primarily for posture
It is not fully developed at birth
Cortical Motor Areas
Primary motor cortex: Low stimulation from the primary
motor cortex is needed to produce a muscle movement. The
activities relating to motor acts all converge on M1 (the
primary motor cortex).
NB: M1 contributes to most of the fibres in the corticospinal
tract.
A small population of cortical neurones in M1 controls small
movements. These are movements that are specific, distal (i.e.
fingers) and delicate/precise.
Premotor Cortex: electrical stimuli from this area of the brain does not produce muscle
movement unless the stimuli is very intense (much more so than in M1). This are of the
brain prepares M1 for the motor act. It does this by facilitating multiple columns in M1.
These neurones are more easily stimulated by impulses from other parts of the brain and
are close to the threshold level needed to produce the movement. Lesions in the PMA do
not cause paralysis but only result in the slowing of complex movements.
Supplementary Motor Cortex: this area of the brain elicits complex movements
involving many muscle groups (entire arm, hands and postural movements). It is not
needed for simple repetitive acts that require little skill, and is particularly active when
carrying out learned tasks. Ablation of this area results in inability to perform complex
bimanual tasks + reduction in spontaneous motor activity.
Lesions to the motor systems
Upper motor neuron: all motor neurons from the cerebral cortex to the synapse in the spinal cord.
Lower motor neuron: motor neurons that directly cause movement; i.e. peripheral motor neurons.
Upper motor Neuron lesions
Symptoms and signs:
 Weakness (initially)
o Paresis = muscle weakness
o Plegia = total loss of muscle control
o Hemi = one side of the body
o Para = affecting both legs
o Quadra = affecting all 4 limbs.
 Spasticity (later)
o Increased muscle tone in the affected voluntary muscles.
o Hypotonia = dencreased muscle tone and weakness.
 Increase muscle stretch reflexes (hyperreflexia & clonus)
 Babinski sign: normally, when the lateral inferior surface of the of the foot is
scratched from heel to little toe, the toes flex i.e. curl downwards, however, in an
upper motor neuron lesion this is damages and the toes extend out and flare.
Consequences of Stroke
Stroke is the consequence of cerebrovascular disease that interrupts blood flow to part
of the brain causing ischaemia and infarction. The diagnosis depends on establishing
the time course and the symptoms:
» Time course – symptoms develop over a few seconds to a few hours. Few other
neurological conditions develop this quickly
» Symptoms – Because stroke is of vascular origin the symptoms are often limited to the
region in the brain and so functions dependant of the territory of a single vessel
There are three causes of stroke:
 Thrombosis – ischaemic stroke
 Embolism – ischaemic stroke
 Haemorrhage – usually from a ruptured aneurysm
An ischaemic stroke in the left middle cerebral artery would lead to:
 R-sided upper motor neuron facial weakness
 R-sided hemiplegia
 Severe speech disturbance - aphasia