science

Primary motor cortex

Primary motor cortex is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Primary motor cortex rather than just read about it. In short: The primary motor cortex (Brodmann area 4) is a brain region that in humans is located in the dorsal portion of the frontal lobe. It is the primary region of the motor system and works in association with other motor areas including premotor cortex, the supplementary motor area, posterior parietal cortex, and several subcortical brain regions, to plan and execute voluntary movements.

Key takeaways

Primary motor cortex belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Primary motor cortex to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Primary motor cortex from memory before moving on to harder problems.

Reference excerpt

The primary motor cortex (Brodmann area 4) is a brain region that in humans is located in the dorsal portion of the frontal lobe. It is the primary region of the motor system and works in association with other motor areas including premotor cortex, the supplementary motor area, posterior parietal cortex, and several subcortical brain regions, to plan and execute voluntary movements. Primary motor cortex is defined anatomically as the region of cortex that contains large neurons known as Betz cells, which, along with other cortical neurons, send long axons down the spinal cord to synapse onto the interneuron circuitry of the spinal cord and also directly onto the alpha motor neurons in the spinal cord which connect to the muscles. At the primary motor cortex, motor representation is orderly arranged (in an inverted fashion) from the toe (at the top of the cerebral hemisphere) to mouth (at the bottom) along a fold in the cortex called the central sulcus. However, some body parts may be controlled by partially overlapping regions of cortex. Each cerebral hemisphere of the primary motor cortex only contains a motor representation of the opposite (contralateral) side of the body. The amount of primary motor cortex devoted to a body part is not proportional to the absolute size of the body surface, but, instead, to the relative density of cutaneous motor receptors on said body part. The density of cutaneous motor receptors on the body part is generally indicative of the necessary degree of precision of movement required at that body part. For this reason, the human hands and face have a much larger representation than the legs. For the discovery of the primary motor cortex and its relationship to other motor cortical areas, see the main article on the motor cortex.

Structure The human primary motor cortex is located on the anterior wall of the central sulcus. It also extends anteriorly out of the sulcus partly onto the precentral gyrus. Anteriorly, the primary motor cortex is bordered by a set of areas that lie on the precentral gyrus and that are generally considered to compose the lateral premotor cortex. Posteriorly, the primary motor cortex is bordered by the primary somatosensory cortex, which lies on the posterior wall of the central sulcus. Ventrally the primary motor cortex is bordered by the insular cortex in the lateral sulcus. The primary motor cortex extends dorsally to the top of the hemisphere and then continues onto the medial wall of the hemisphere. The location of the primary motor cortex is most obvious on histological examination due to the presence of the distinctive Betz cells. Layer V of the primary motor cortex contains giant (70-100 μm) pyramidal neurons which are the Betz cells. These neurons send long axons to the contralateral motor nuclei of the cranial nerves and to the lower motor neurons in the ventral horn of the spinal cord. These axons form a part of the corticospinal tract. The Betz cells account for only a small percentage of the corticospinal tract. By some measures, they account for about 10% of the primary motor cortex neurons projecting to the spinal cord or about 2-3% of the total cortical projection to the spinal cord. Though the Betz cells do not compose the entire motor output of the cortex, they nonetheless provide a clear marker for the primary motor cortex. This region of cortex, characterized by the presence of Betz cells, was termed area 4 by Brodmann.

Cellular components The primary motor cortex alone has been shown to have as many as 116 different types of cells differentiated in their morphology, electrophysiological properties (including firing patterns) and gene expression profile (for example, by type of neurotransmitter released (GABA, glutamate etc.).

Pathway As the primary motor axons travel down through the cerebral white matter, they move closer together and form part of the posterior limb of the internal capsule. They continue down into the brainstem, where some of them, after crossing over to the contralateral side, distribute to the cranial nerve motor nuclei. (Note: a few motor fibers synapse with lower motor neurons on the same side of the brainstem). After crossing over to the contralateral side in the medulla oblongata (pyramidal decussation), the axons travel down the spinal cord as the lateral corticospinal tract. Fibers that do not cross over in the brainstem travel down the separate ventral corticospinal tract, and most of them cross over to the contralateral side in the spinal cord, shortly before reaching the lower motor neurons. In addition to the main corticospinal tract, Motor cortex projects to other cortical and subcortical areas, including the striatum, hypothalamus, midbrain and hindbrain, as well as the thalamus, basal ganglia, midbrain and medulla

Corticomotorneurons Corticomotorneurons are neurons in the primary cortex which project directly to motor neurons in the ventral horn of the spinal cord. Axons of corticomotorneurons terminate on the spinal motor neurons of multiple muscles as well as on spinal interneurons. They are unique to primates and it has been suggested that their function is the adaptive control of the distal extremities (e.g. the hands) including the relatively independent control of individual fingers. Corticomotorneurons have so far only been found in the primary motor cortex and not in secondary motor areas.

Blood supply Branches of the middle cerebral artery provide most of the arterial blood supply for the primary motor cortex. The medial aspect (leg areas) is supplied by branches of the anterior cerebral artery.

Function

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with Primary motor cortex

Start with the simplest possible case. Write down what Primary motor cortex claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Primary motor cortex before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Primary motor cortex ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Primary motor cortex

In research: Primary motor cortex appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Primary motor cortex in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Primary motor cortex is common in secondary-school and first-year university syllabi. It links to neighbouring topics Cerebral cortex, Frontal lobe, Motor system, so understanding it makes those chapters shorter.
In everyday life: Look for Primary motor cortex outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Primary motor cortex in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Primary motor cortex means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Primary motor cortex out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Primary motor cortex in simple terms?

Why does Primary motor cortex matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Primary motor cortex?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Primary motor cortex.