The Central Nervous System
Brain regions, cortex, diencephalon, brain stem, cerebellum, protection and the spinal cord
By the end of this unit you can…
- ✓Describe the embryonic brain vesicles and the adult brain regions they become
- ✓Locate the cerebral lobes and the functional areas of the cortex
- ✓Describe the basal nuclei, diencephalon, brain stem and cerebellum and their functions
- ✓Explain how the meninges, CSF and blood–brain barrier protect the brain
- ✓Describe spinal cord anatomy: gray horns, white columns and major tracts
- ✓Relate common CNS disorders to the structures affected
Key terms
Practice →1Brain development & regions
The CNS develops from the embryonic neural tube. Its anterior end forms three primary brain vesicles — prosencephalon (forebrain), mesencephalon (midbrain) and rhombencephalon (hindbrain) — which become five secondary vesicles and then the adult brain regions.
| Secondary vesicle | Adult structure |
|---|---|
| Telencephalon | Cerebrum (cerebral hemispheres) |
| Diencephalon | Thalamus, hypothalamus, epithalamus |
| Mesencephalon | Midbrain |
| Metencephalon | Pons and cerebellum |
| Myelencephalon | Medulla oblongata |
The adult brain has four main regions: cerebral hemispheres, diencephalon, brain stem (midbrain, pons, medulla) and cerebellum. Gray matter (cell bodies) is a central core around the ventricles, plus an outer cortex of gray matter in the cerebrum and cerebellum; white matter (myelinated tracts) lies between.
2Cerebral hemispheres
The cerebral hemispheres make up ~83% of brain mass. The surface is folded into ridges (gyri), shallow grooves (sulci) and deep grooves (fissures). The longitudinal fissure separates the two hemispheres; the transverse cerebral fissure separates the cerebrum from the cerebellum.
| Landmark | Separates |
|---|---|
| Central sulcus | Frontal lobe (precentral gyrus) from parietal lobe (postcentral gyrus) |
| Lateral sulcus | Temporal lobe from the parietal and frontal lobes |
| Parieto-occipital sulcus | Parietal from occipital lobe |
Five lobes: frontal, parietal, temporal, occipital and the deep insula. Each hemisphere has three regions: superficial cerebral cortex (gray matter), internal white matter, and basal nuclei (gray matter islands deep in the white matter).
| Area | Location | Function |
|---|---|---|
| Primary motor cortex | Precentral gyrus (frontal lobe) | Conscious control of skilled voluntary movement; pyramidal cells → corticospinal tracts; mapped as a motor homunculus |
| Premotor cortex | Anterior to precentral gyrus | Plans learned, patterned movements |
| Broca's area | Usually left frontal lobe | Motor speech production |
| Frontal eye field | Frontal lobe | Voluntary eye movements |
| Primary somatosensory cortex | Postcentral gyrus (parietal lobe) | Receives general sensation from skin and proprioceptors; spatial discrimination |
| Primary visual cortex | Occipital lobe | Receives visual information |
| Primary auditory cortex | Superior temporal lobe | Receives sound information |
| Olfactory & gustatory cortex | Medial temporal lobe / insula | Smell / taste |
| Wernicke's area | Posterior temporal lobe (left) | Understanding language |
| Prefrontal cortex | Anterior frontal lobe | Intellect, judgment, reasoning, personality, planning |
Lateralization: the left hemisphere usually dominates language, math and logic; the right handles visual-spatial skills, intuition, emotion and artistic skills. Each hemisphere controls the opposite side of the body (contralateral). Cerebral white matter: commissural fibers connect the hemispheres (largest = corpus callosum), association fibers connect areas within one hemisphere, and projection fibers connect the cortex with lower CNS centers.
Basal nuclei (caudate nucleus, putamen, globus pallidus) help start, stop and regulate the intensity of voluntary movements and inhibit unwanted ones.
3Diencephalon
- Thalamus
- 80% of the diencephalon. The gateway (relay station) to the cerebral cortex: sorts, edits and relays almost all sensory input (except smell) as well as motor and emotional information.
- Hypothalamus
- Below the thalamus; the main visceral control center for homeostasis: controls the ANS, body temperature, hunger and thirst, sleep–wake cycles, emotional responses (limbic system) and the endocrine system — it controls the pituitary gland and makes ADH and oxytocin.
- Epithalamus
- Contains the pineal gland, which secretes melatonin to regulate the sleep–wake cycle.
4Brain stem & cerebellum
| Region | Key features & functions |
|---|---|
| Midbrain | Cerebral peduncles (corticospinal tracts); cerebral aqueduct; superior colliculi (visual reflexes) and inferior colliculi (auditory reflexes); substantia nigra (dopamine — linked to Parkinson's) |
| Pons | Bridge of tracts between cerebrum, cerebellum and medulla; nuclei of cranial nerves V–VIII; helps regulate breathing rhythm |
| Medulla oblongata | Pyramids (corticospinal tracts) — decussation of the pyramids (crossing over); vital autonomic centers: cardiovascular center (heart rate, blood pressure) and respiratory center; also vomiting, hiccuping, swallowing, coughing, sneezing |
The reticular formation runs through the brain stem; its reticular activating system (RAS) keeps the cortex alert and filters out repetitive stimuli (~99% of sensory input is ignored).
The cerebellum (11% of brain mass) compares the intended movement with what is actually happening and coordinates smooth, precise, balanced body movements, posture and equilibrium. It has an outer cortex and inner white matter, the arbor vitae (“tree of life”), and connects to the brain stem through cerebellar peduncles.
5Functional brain systems
- Limbic system
- The emotional (affective) brain — includes the amygdala (fear, recognizing angry faces) and cingulate gyrus, plus the hippocampus (forming new long-term declarative memories). Links emotions to thought.
- Reticular formation
- Arousal of the cortex (RAS) and motor control of visceral organs.
Higher mental functions: sleep alternates between non-REM (stages 1–3, deepest in stage 3) and REM sleep (dreaming, eyes moving, skeletal muscles inhibited) roughly every 90 minutes; memory involves short-term memory (STM; ~7 items) and long-term memory (LTM), with the hippocampus consolidating declarative memories. Brain waves recorded by an EEG reflect electrical activity (alpha = relaxed awake, beta = alert, theta, delta = deep sleep).
6Protecting the brain
The brain is protected by bone (skull), meninges, cerebrospinal fluid and the blood–brain barrier.
| Meninx (superficial → deep) | Features |
|---|---|
| Dura mater | Strongest, two-layered (“tough mother”); layers separate to form dural venous sinuses (e.g. superior sagittal sinus); folds include the falx cerebri |
| Arachnoid mater | Web-like middle layer; subarachnoid space below it is filled with CSF; arachnoid granulations reabsorb CSF into the venous sinuses |
| Pia mater | Delicate (“gentle mother”); clings tightly to the brain surface, following every gyrus |
- 1Choroid plexus in lateral ventricles
- 2Interventricular foramen
- 3Third ventricle
- 4Cerebral aqueduct
- 5Fourth ventricle
- 6Subarachnoid space (and central canal)
- 7Arachnoid granulations
- 8Dural venous sinus
CSF is made by the choroid plexuses (capillaries + ependymal cells) at ~500 mL/day (only ~150 mL present at once). It gives the brain buoyancy (reduces its effective weight by 97%), protects from blows and nourishes it.
The blood–brain barrier — continuous capillaries with tight junctions plus astrocyte feet — strictly limits what enters brain tissue. Lipid-soluble substances (O₂, CO₂, alcohol, nicotine, anesthetics) pass freely; glucose and amino acids need carriers.
7The spinal cord
The spinal cord runs from the foramen magnum to about L1–L2, ending in the conus medullaris; below it, the bundle of nerve roots is the cauda equina (“horse's tail”). It provides two-way conduction between the brain and body and is a major reflex center. 31 pairs of spinal nerves emerge from it. There are cervical and lumbar enlargements where nerves to the limbs arise.
- Gray matter (butterfly / H-shape)
- Posterior (dorsal) horns — interneurons receiving sensory input. Anterior (ventral) horns — somatic motor neuron cell bodies. Lateral horns (thoracic & superior lumbar) — autonomic (sympathetic) motor neurons. Central canal in the middle.
- Dorsal & ventral roots
- Sensory (afferent) fibers enter via the dorsal root (their cell bodies are in the dorsal root ganglion); motor (efferent) fibers leave via the ventral root. Roots join to form a spinal nerve.
- White matter columns (funiculi)
- Posterior, lateral and anterior columns carry ascending (sensory) and descending (motor) tracts.
| Tract | Direction | Carries |
|---|---|---|
| Dorsal column (fasciculus gracilis & cuneatus) | Ascending | Fine touch, vibration, conscious proprioception |
| Spinothalamic tracts | Ascending | Pain, temperature, crude touch |
| Spinocerebellar tracts | Ascending | Unconscious proprioception to the cerebellum |
| Lateral corticospinal (pyramidal) tract | Descending | Voluntary skilled movements |
8Brain injuries & disorders
| Disorder | What happens |
|---|---|
| Concussion | Temporary alteration of function after a blow; usually reversible |
| Contusion | Permanent damage; bruising of brain tissue |
| Stroke (CVA) | Blood supply to part of the brain is blocked (ischemic — most common) or a vessel bursts (hemorrhagic) → neurons die. Signs: one-sided weakness/paralysis, speech problems (FAST: Face, Arms, Speech, Time) |
| Transient ischemic attack (TIA) | Temporary episode of reversible ischemia — a warning sign of stroke |
| Alzheimer's disease | Progressive dementia; β-amyloid plaques and neurofibrillary tangles (tau); shrinking of the brain; ACh deficit |
| Parkinson's disease | Degeneration of dopamine-releasing neurons of the substantia nigra → resting tremor, rigidity, slow movement; treated with L-dopa |
| Huntington's disease | Fatal genetic disorder; degeneration of basal nuclei and cortex → jerky involuntary movements, then dementia |