Your brain has the consistency of firm tofu, uses about 20% of your resting energy while making up about 2% of your body mass, and contains roughly 86 billion neurons. Yet it can be knocked out of action by a hard hit in a hockey game. The previous lesson showed how a single neuron fires and passes on a signal. This lesson zooms out to the whole central nervous system (CNS): how it is protected, which regions handle which jobs, how the spinal cord can act on its own in a reflex, how scientists image a living brain, and what happens when parts of the system fail. Each disorder makes sense once you know which cells or regions it damages.
Protecting the CNS: meninges and cerebrospinal fluid
Bone (the skull and vertebrae) is the first layer of protection. Under it, three membranes called the meninges wrap the brain and spinal cord: the tough outer dura mater, the web-like arachnoid mater, and the delicate pia mater clinging to the brain's surface. Between the arachnoid and pia is the subarachnoid space, filled with cerebrospinal fluid (CSF). CSF is made continuously from blood plasma by the choroid plexus in the brain's four hollow ventricles, about 500 mL a day, and is reabsorbed into the blood so only about 150 mL is present at once. It cushions the brain against the skull, makes it buoyant (a 1.4 kg brain effectively weighs under 50 g in CSF, so it does not crush its own lower surface), and carries away wastes. Because CSF bathes the CNS, a lumbar puncture below the end of the spinal cord samples it to diagnose meningitis, an infection of the meninges. Finally, the blood-brain barrier, capillary walls sealed by tight junctions, keeps most drugs and microbes in the blood out of brain tissue.
Brain regions and what they do
| Region | Main functions | If it is damaged |
|---|---|---|
| Frontal lobe | Planning, judgment, personality; primary motor cortex starts voluntary movement; Broca's area (usually left) produces speech | Personality change, poor impulse control (Phineas Gage, 1848); paralysis on the opposite side; halting speech |
| Parietal lobe | Somatosensory cortex receives touch, pain, temperature and body position; spatial awareness | Numbness on the opposite side; neglect of one side of space |
| Temporal lobe | Hearing (auditory cortex); Wernicke's area understands language; the hippocampus forms new long-term memories | Fluent but meaningless speech; inability to form new memories (patient H.M.) |
| Occipital lobe | Visual cortex interprets signals from the eyes | Blindness in part of the visual field even with healthy eyes |
| Cerebellum | Coordinates movement, balance and posture; motor learning | Clumsy, jerky movement and poor balance (why roadside sobriety tests check balance) |
| Thalamus / hypothalamus | Thalamus relays sensory input to the cortex; hypothalamus runs homeostasis and the pituitary | Sensory loss; failure of temperature, thirst or hormone control |
| Brainstem (midbrain, pons, medulla) | Midbrain: eye and startle reflexes; pons: relays signals, helps set breathing; medulla: breathing, heart rate, blood pressure | Often fatal: the vital centres stop |
The cerebrum's two hemispheres are joined by the corpus callosum, a thick band of axons. Most motor and sensory pathways cross over in the brainstem, so the left hemisphere controls and feels the right side of the body (contralateral control). At the Montreal Neurological Institute in the 1930s to 1950s, neurosurgeon Wilder Penfield stimulated the cortex of awake epilepsy patients and mapped which strip of cortex moved or felt each body part. The map is lopsided: the hands, lips and tongue get far more cortex than the back, because they have more receptors and finer muscle control. His McGill colleague Brenda Milner later showed, through patient H.M., that the hippocampus is needed to form new memories. The brain can also rewire: after a stroke, nearby regions can take over lost functions with therapy, a property called neuroplasticity.
The spinal cord and the reflex arc
In cross-section the spinal cord shows a butterfly of grey matter (cell bodies, dendrites, interneurons and synapses, where processing happens) surrounded by white matter (myelinated axons in tracts carrying signals up to and down from the brain). In the brain this is reversed: grey matter forms the outer cortex. Sensory neurons enter through the dorsal root (their cell bodies sit in the dorsal root ganglion) and motor neurons leave through the ventral root. A reflex is a rapid, automatic response routed through the spinal cord (or brainstem) without waiting for the brain to decide. In the stretch reflex (knee jerk), the sensory neuron synapses directly on the motor neuron: one synapse, the fastest possible arc. The withdrawal reflex is polysynaptic: interneurons excite the motor neurons to the flexor muscle and, through an inhibitory interneuron, relax the opposing extensor (reciprocal inhibition), so the limb does not fight itself. Interneurons also send the signal up the white matter to the brain, which is why you feel pain a moment after you have already moved.
- Effector: skeletal muscle
- One motor neuron runs from the CNS all the way to the muscle
- Releases acetylcholine; always excites the muscle
- Voluntary movements and spinal reflexes
- Effectors: smooth muscle, cardiac muscle and glands
- Two-neuron chain: preganglionic neuron synapses in a ganglion with a postganglionic neuron
- Sympathetic ends with norepinephrine, parasympathetic with acetylcholine; can excite or inhibit
- Involuntary control, coordinated by the hypothalamus and medulla
Imaging the living brain, and when the system fails
| Technique | What it detects | Strength | Limitation |
|---|---|---|---|
| EEG (electroencephalography) | Electrical activity of many cortical neurons, through scalp electrodes; brain waves such as alpha (relaxed) and delta (deep sleep) | Millisecond timing; cheap; diagnoses epilepsy and sleep stages | Poor at locating activity; misses deep structures |
| fMRI (functional MRI) | Blood oxygen level: active regions receive extra oxygenated blood, which changes the magnetic signal | Locates activity to a few millimetres anywhere in the brain | Measures blood flow, not neurons directly; lags activity by several seconds |
| CT and structural MRI | Anatomy: bleeding, tumours, shrinkage, lesions | CT is fast for emergencies; MRI shows soft tissue in fine detail | Show structure, not function; a concussion usually looks normal |
| Disorder | What goes wrong | Symptoms | Treatment and research |
|---|---|---|---|
| Concussion | The brain shifts and twists inside the skull, stretching axons and disrupting ion balance and metabolism | Headache, dizziness, confusion, memory gaps, light sensitivity | Rest, then gradual return to learning and sport; a second hit before healing is dangerous |
| Alzheimer's disease | Amyloid-β plaques outside neurons and tau tangles inside; neurons die, starting in the hippocampus | Progressive memory loss, confusion, personality change | Cholinesterase inhibitors ease symptoms; anti-amyloid antibodies modestly slow early disease |
| Parkinson's disease | Dopamine-producing neurons in the midbrain's substantia nigra die | Tremor at rest, stiffness, slow movement, shuffling walk | L-DOPA (crosses the blood-brain barrier and becomes dopamine); deep brain stimulation |
| Multiple sclerosis | The immune system attacks CNS myelin, leaving scarred lesions | Vision problems, numbness, weakness, fatigue; often relapsing | Immune-modifying drugs, including anti-CD20 monoclonal antibodies; Canada has one of the world's highest MS rates |