Brain Anatomy: The Major Regions and What They Do

You are helping with a neurological assessment when a patient is asked to smile, lift both arms, follow a finger with their eyes and answer a few simple questions. None of those checks is random. Each gives you information about how different parts of the nervous system are working together.

That becomes much easier to understand once you can picture the brain beneath the assessment. A change in speech is not the same neurological finding as an unsteady gait. Weakness is different again from poor coordination. The anatomy helps explain why.

The brain is often taught as a list of structures and functions, but in a real patient those structures do not work in isolation. We are going to use the major regions as our map, then keep coming back to what they can look like at the bedside.

Start with the cerebrum

The cerebrum is the largest part of the brain. It has right and left cerebral hemispheres, joined principally by the corpus callosum so that information can pass between them. Its folded outer layer is the cerebral cortex. The raised folds are called gyri and the grooves between them sulci. Folding allows a large cortical surface area to fit inside the skull.

The cortex is grey matter, containing neuronal cell bodies and their connections. Beneath it is white matter, made largely from myelinated axons carrying signals between cortical areas and between the cortex and deeper parts of the nervous system.

We divide each cerebral hemisphere into lobes because it gives us a useful anatomical map. Just be careful with the familiar shortcut of learning one function for each lobe. Speech, movement, memory, attention and behaviour depend on connected networks, so damage in one area can affect several abilities at once.

At the front is the frontal lobe. The primary motor cortex lies in the precentral gyrus, immediately in front of the central sulcus, and is involved in voluntary movement. Many motor fibres descend through deeper structures and the brainstem before crossing to influence the opposite side of the body. This is why damage affecting motor pathways in the left cerebral hemisphere can present as right-sided weakness.

But the frontal lobe is not simply the brain's movement area. Frontal networks are also involved in planning, attention, judgement, behaviour and language production. In most people, language is predominantly represented in the left hemisphere. A left-sided cerebral lesion can therefore produce a pattern that includes both right-sided weakness and a disturbance of language.

Behind the central sulcus is the parietal lobe. Its postcentral gyrus contains the primary somatosensory cortex. Sensory pathways bring information about touch, pain, temperature and body position towards the brain, while parietal networks help integrate that information into an awareness of the body and surrounding space. A patient with parietal dysfunction may therefore have more than altered sensation. They can have difficulty attending to or making sense of part of their environment.

The temporal lobe sits lower on each side of the cerebrum. It is involved in auditory processing and contains structures important for memory, including the hippocampus on its medial aspect. Language comprehension also depends on networks involving the dominant temporal and neighbouring parietal cortex. A patient can therefore be awake and able to produce fluent speech while struggling to understand spoken language.

At the back of the cerebrum is the occipital lobe, where the primary visual cortex receives visual information. This is an important reminder that vision is not only about the eyes. Light has to be detected by the retina, carried through the visual pathways and then processed by the brain. Damage to the pathways or visual cortex can cause visual field loss even when the eye itself is intact.

There is more beneath the cortex

If you only learn the lobes, you miss a large part of how the brain works. Beneath the cerebral cortex sit structures that connect, filter and regulate the information moving through the nervous system.

The thalamus lies within the diencephalon. It contains groups of nuclei with extensive connections to the cerebral cortex. Most sensory information travelling towards the cortex is processed through thalamic nuclei, with olfaction being the notable exception. The thalamus also participates in motor, cognitive and arousal networks. Calling it a relay station is helpful at first, but it does more than simply pass information onwards.

Just below it is the hypothalamus. It links nervous-system activity with autonomic and endocrine regulation and is involved in temperature control, thirst, appetite, sleep-wake regulation and other aspects of homeostasis. Its relationship with the pituitary gland also gives it an important role in hormonal control. It is a relatively small structure doing an enormous amount of physiological work.

The basal ganglia are groups of deep nuclei connected with the cortex and thalamus. Their motor circuits help select, initiate and regulate movement rather than directly making muscles contract. This becomes clinically easier to appreciate in disorders affecting basal ganglia circuits, where movement may become slow, difficult to initiate, excessive or involuntary even though the muscles themselves are capable of contracting.

These deeper structures also show why neurological signs do not always fit neatly into the four-lobe map. A problem can lie in the cortex, in a deep nucleus, or in the white-matter pathways connecting one region to another.

The brainstem connects the brain with the rest of the nervous system

Follow the brain downwards and the anatomy narrows into the brainstem: the midbrain, pons and medulla oblongata. It connects higher brain regions with the spinal cord, but describing it only as a connection seriously understates what is packed into this small area.

Ascending sensory pathways and descending motor pathways pass through it. Most cranial nerve nuclei are found here. Brainstem networks also contribute to arousal, breathing, cardiovascular regulation, swallowing, eye movements and other functions needed for everyday neurological and physiological stability.

The three parts have different anatomy. The midbrain is involved in pathways controlling eye movement, motor function, reflex responses and arousal. The pons has extensive connections with the cerebellum and contains nuclei and pathways involved in functions including facial movement and sensation, hearing and balance. Lower down, the medulla contains important autonomic networks involved in respiratory and cardiovascular control, as well as pathways and nuclei involved in swallowing and protective reflexes.

Because so much passes through such a compact region, brainstem dysfunction can produce combinations of findings that initially seem unrelated: altered eye movements, facial changes, swallowing difficulty, limb weakness or sensory change, problems with balance, or altered consciousness. Severe injury can also compromise vital functions.

At the bedside, you are not expected to identify the exact brainstem nucleus responsible for a new sign. You do need to notice the change, describe what you have found and recognise when the pattern requires urgent escalation.

The cerebellum is about the quality of movement

The cerebellum sits behind the brainstem and communicates extensively with the cerebral cortex, brainstem, spinal cord and vestibular system. It helps coordinate the timing, accuracy and smoothness of movement and contributes to posture, equilibrium, eye movements and motor learning.

Think about reaching for a cup. The cerebral motor system contributes to initiating the movement, but the movement also has to be continuously adjusted. Your arm needs the right direction, distance, speed and force. Cerebellar circuits use incoming information to help refine that movement as it happens.

When those circuits are disrupted, the problem may not be straightforward weakness. The patient may have ataxia, an unsteady gait, poorly coordinated limb movements, dysarthria or abnormal eye movements. During a finger-to-nose assessment, for example, a patient may have enough power to lift the arm but repeatedly miss or overshoot the target. That is a different finding from an arm that cannot remain raised because it is weak.

This is why the word “balance” is too small a description of the cerebellum. Coordination tells you about how movement is being controlled, not simply whether the patient can generate movement at all.

Put the assessment back together

Return to the patient from the beginning. They now have a new right-sided arm weakness and their speech has changed. Those two findings immediately deserve attention, but knowing the anatomy gives you a clearer idea of why they may appear together.

Motor pathways arising from one cerebral hemisphere predominantly influence the opposite side of the body, and language is predominantly represented in the left hemisphere in most people. A new language disturbance alongside right-sided weakness can therefore be consistent with dysfunction involving the left cerebral hemisphere or connected pathways. It does not tell you the cause, and it does not replace urgent assessment or imaging. It tells you that the findings form a meaningful neurological pattern.

Now picture another patient. Their limb power appears relatively preserved, but they have suddenly become markedly unsteady. Their finger-to-nose movement is inaccurate and their eye movements are abnormal. That pattern asks different anatomical questions, with the cerebellum and brainstem becoming particularly relevant.

This is what neurological observations are doing. Speech and language, pupils and eye movements, limb power, sensation, coordination, behaviour and consciousness are not a random collection of bedside tests. Together they give us information about an interconnected nervous system.

The pattern will not always point cleanly to one structure. Brain functions are distributed across networks, vascular territories cross the boundaries we use to learn the lobes, and a lesion can interrupt a connecting pathway rather than the cortical area where a function is processed. Anatomy helps you understand and communicate what you are seeing. Diagnosis still depends on the wider clinical picture, appropriate examination, investigation and senior or specialist assessment.

For nursing practice, a new focal neurological deficit or sudden change in speech, language, consciousness or other neurological function should never be explained away because it does not fit a neat textbook pattern. Recognise the change, complete the assessment appropriate to the situation and escalate promptly according to the clinical context and local pathway.

References

Basinger, H. and Hogg, J.P. (2023) ‘Neuroanatomy, Brainstem’, StatPearls. Treasure Island, FL: StatPearls Publishing.

Bui, T. and Das, J.M. (2023) ‘Neuroanatomy, Cerebral Hemisphere’, StatPearls. Treasure Island, FL: StatPearls Publishing.

National Institute for Health and Care Excellence (NICE) (2019, updated 2023) Suspected neurological conditions: recognition and referral. NG127. London: NICE.

Purves, D., Augustine, G.J., Fitzpatrick, D. et al. (eds.) (2001) Neuroscience. 2nd edn. Sunderland, MA: Sinauer Associates.

Vachha, B.A., Massoud, T.F. and Huang, S.Y. (2022) ‘Anatomy of the cerebral cortex, lobes, and cerebellum’, Neuroimaging Clinics of North America, 32(3), pp. 463–473. doi:10.1016/j.nic.2022.04.008.