In one breath
The cortex makes the plan: motor cortex settles it with the basal ganglia and cerebellum, tunes it against sensory and association cortex, hands part of the work to premotor cortex and the supplementary motor area, and then sends the signal down the descending pathways. Remove the cortex and the animal keeps every midbrain reflex, shows only a faint decorticate rigidity at rest, and loses hopping and placing reactions, conditioned reflexes and the ability to use past experience. The cortical motor areas are area 4 (primary motor, with its upside-down homunculus), lateral area 6 (premotor), medial area 6 (supplementary motor), areas 3, 1, 2 (somatosensory), areas 5 and 7 (posterior parietal) and area 8 (frontal eye field).
Builds on: Midbrain integration and righting reflexes · Corticospinal (pyramidal) tract · Somatosensory cortex · Leads to: Basal ganglia: circuits and functions · Cerebellar functions, lesions and tests
Cortical integration
Before a command can reach the spinal motor neurons, the plan has to be made, and it is made in the cortex. The sequence is:
- Motor cortex works with the basal ganglia and cerebellum to finalize the plan and the programme of the movement.
- It then checks the plan against the sensory cortex and the association cortex, tuning and altering the command so that what the body does matches what was planned.
- Part of the information goes to the premotor cortex and supplementary motor area, which generate further motor signals to get the right movement started.
- The finished signal goes to the spinal motor neurons down the descending motor pathways (see Corticospinal (pyramidal) tract).
Decortication
Removing the cerebral cortex in an experimental animal is decortication; the animal is a decorticate animal.
Features of the decorticate animal
- No features of shock.
- Every reflex activity of the midbrain animal is intact.
- Decorticate rigidity is very slight, and it is present at rest only.
- Hopping and placing reactions are badly impaired.
- The striking defect: the animal cannot use past experience to shape what it does.
- Conditioned reflexes are lost, though special training can build them up again.
- Temperature regulation works.
- Visceral homeostasis works.
The hypothalamus is left behind by decortication, so temperature regulation and visceral homeostasis carry on — which is exactly why a decorticate animal is comparatively easy to keep alive.
Decorticate rigidity
The cortex inhibits the medullary reticulospinal tract, so taking the cortex away facilitates γ motor neuron discharge — the same mechanism as in Medullary integration and decerebrate rigidity, but with only one of the three inhibitory drives removed instead of two. Hence the rigidity is minimal, is seen only at rest, and goes the moment a phasic postural reflex is brought into play.
Hopping and placing reactions
Push a normal animal — or a person — sideways, and two things happen in order:
- Hopping reaction: short steps sideways to catch the balance.
- Placing reaction: the feet are then planted firmly on the ground, so the new position is stable.
These are seriously impaired after decortication, which is the evidence that their integrating centre is the motor cortex.
What each preparation keeps
The four preparations together give the whole chapter’s answer — spinal reflexes and supporting reactions in the cord, antigravity (static postural) reflexes in the medulla, righting reflexes in the midbrain, hopping and placing in the cortex:
- Spinal reflexes — present in all four preparations, and strongest of all in the spinal animal.
- Antigravity reflexes — absent in the spinal animal; most marked in the decerebrate; present in the midbrain and decorticate animals.
- Righting reflexes — absent in the spinal and decerebrate animals; most marked in the midbrain animal; still present after decortication.
- Conditioned reflexes — absent in the first three; in the decorticate animal barely there, and only with special training.
- Hopping and placing reactions — absent in the first three; grossly impaired after decortication.
Read the table downwards, not across
A reflex is absent in every preparation cut below its own centre, and exaggerated in the preparation cut just above it — that is the release phenomenon at work. So righting reflexes are absent in the decerebrate animal (cut below the midbrain) but at their strongest in the midbrain animal. If you can reconstruct that rule you never need to memorize the grid.
Cortical motor areas
A large number of cortical areas are involved in movement or associated with it:
| Area | Brodmann | Main job |
|---|---|---|
| Primary motor cortex | 4 | Plans and initiates voluntary movement |
| Premotor cortex | Lateral 6 | Sets posture at the start of a planned movement |
| Supplementary motor area | Medial 6 | Plans and programmes motor sequences; bimanual tasks |
| Primary somatosensory cortex | 3, 1, 2 | Learned sequences of movement; modulates afferent input |
| Posterior parietal cortex | 5, 7 | Uses complex sensory information to produce movement |
| Frontal eye field | 8 | Eye movements and saccades |
Primary motor cortex
Area 4, in the precentral gyrus. The corticospinal and corticobulbar tracts arise mainly from here, and the area is responsible for the planning and initiation of voluntary movement.
Connections of area 4
- Projects to: premotor cortex, brainstem and spinal cord.
- Reciprocally connected with: thalamus, sensory cortex and supplementary motor area.
- The wiring is crossed, so each cortical motor area controls the contralateral muscles.
Motor homunculus
The parts of the body are mapped onto the precentral gyrus:
- Face is represented bilaterally; the rest of the body unilaterally.
- The map is upside down: the feet lie uppermost on the gyrus, the face lowest of all.
- The size of a part’s territory follows the skill with which it is used voluntarily, not its bulk. So the hand and digits take up a large area, because the skilled movements are theirs.
- Man being specialized for speech, the lips, jaw and tongue also get a disproportionately large territory — the vocalization of the cortex.
The homunculus is inverted, and the face is the exception
Feet at the top of the precentral gyrus, face at the bottom — students routinely draw it the right way up. The second half of the trap is that the face is represented bilaterally while everything else is unilateral, so one-sided cortical damage spares much of the face — a point that comes back in Upper vs lower motor neuron lesions.
Worth knowing, though not in your pages: because the leg sits at the top, over the medial edge of the hemisphere, a lesion near the vertex tends to weaken the leg, while one low on the lateral convexity picks out the face and hand.
Draw it: motor areas and the homunculus
Left half of the page — lateral view of the hemisphere. Draw the outline with the frontal pole to the left, then mark the central sulcus running down the middle. Shade the gyrus in front of it as area 4 (primary motor) and the gyrus behind it as areas 3, 1, 2 (somatosensory). In front of area 4, shade area 6 and split it: the part on the lateral surface is premotor, the part carried over the medial edge is the supplementary motor area. Put area 8 (frontal eye field) in front of area 6, and areas 5 and 7 behind the somatosensory strip in the parietal lobe. Right half — the homunculus. Draw a coronal strip of the precentral gyrus from the midline down to the lateral fissure and lay the body along it in this order from top to bottom: toes, foot, leg, hip, trunk, shoulder, arm, elbow, wrist, hand and fingers (huge), thumb, neck, brow, eyelid, face, lips, jaw, tongue (huge), pharynx. Draw the hand and the lips grossly out of proportion and label “area ∝ skill, not size”. Write “feet at the top, face at the bottom” and “face bilateral, rest unilateral” beside it.
Premotor cortex
Lateral area 6.
- Receives from the supplementary motor area and the primary motor cortex.
- Projects to the brainstem areas that look after postural control.
- Function: it fixes the posture a planned movement will start from, so it gets the body ready to carry the movement out.
Supplementary motor area
The medial part of area 6.
- Receives from the somatosensory cortex.
- Function: the higher motor jobs — planning and programming motor sequences — and it controls bimanual tasks.
- Lesion: difficulty with complex motor activities, and with anything needing the two hands to work together.
Primary somatosensory cortex
Areas 3, 1 and 2.
- Projects to the premotor cortex and the supplementary motor area.
- Lesion: motor performance fails where the movement is a learned sequence of events — eating with a knife and fork, for instance, is badly affected.
- Its own contribution to the corticospinal tract ends mostly in the dorsal horn, where those fibres modulate afferent input coming into the CNS rather than driving muscle.
Posterior parietal cortex
Areas 5 and 7. It contributes fibres to the corticospinal tract and is connected with the motor cortex. Its job is to take complex sensory information and use it to produce movement.
Frontal eye field
Area 8.
- Receives from the primary and premotor cortices.
- Projects to the pontine nuclei that control eye movements and to the superior colliculus, which integrates saccades.
Applied: reading the lesion off the area
Each area fails in its own way, and examiners like the pairing:
- Area 4 — loss of planning and initiation of voluntary movement on the opposite side.
- Medial area 6 (SMA) — complex and two-handed activities break down, though single movements are possible.
- Areas 3, 1, 2 — learned sequences go: the patient can move the hand but cannot manage a knife and fork.
- Areas 5, 7 — movement guided by complex sensory information suffers.
- Area 8 — voluntary conjugate eye movement and saccades are affected.
Exam-answer skeleton: "Describe the motor areas of the cerebral cortex" (long essay)
- Open with cortical integration: the cortex makes the plan, checks it with basal ganglia and cerebellum, tunes it against sensory cortex, then sends it down the corticospinal and corticobulbar tracts.
- Name all the areas with their Brodmann numbers, as a table — this is the marking skeleton.
- Primary motor cortex: area 4, precentral gyrus, origin of the corticospinal and corticobulbar tracts, planning and initiation of voluntary movement.
- Its connections: to premotor cortex, brainstem and cord; reciprocal with thalamus, sensory cortex and SMA; control of the opposite side.
- Motor homunculus: draw it. Inverted, face bilateral, territory proportional to skill, hand and the lips/jaw/tongue huge — the vocalization of the cortex.
- Premotor cortex (lateral 6) and supplementary motor area (medial 6): inputs, outputs, functions, and the lesion of each.
- Somatosensory cortex (3, 1, 2), posterior parietal cortex (5, 7) and frontal eye field (8): what each contributes to movement.
- Close with the evidence from decortication: midbrain reflexes intact, slight rigidity at rest, hopping and placing reactions lost, so the cortex integrates those.
Asked in exams
- Motor areas of the cerebral cortex: Sep/Oct 2019, 10 marks · Jan 2023, MCQ