The brain and spinal cord
The brain contains about 86 billion neurons. Different regions specialise in movement, sensation, emotion, and thought.
Protecting the CNS
The brain and spinal cord are protected by bone (skull and vertebrae), three membranes called the meninges (dura mater, arachnoid mater, and pia mater), cerebrospinal fluid that cushions and nourishes, and the blood-brain barrier, which blocks many substances in the blood from entering brain tissue. Meningitis is inflammation of the meninges.
The cerebrum
The cerebrum, the largest part, has two hemispheres connected by the corpus callosum. Its folded outer layer, the cerebral cortex, is grey matter (cell bodies); beneath is white matter (myelinated axons). Each hemisphere mostly controls the opposite side of the body.
Lobes of the cortex
The frontal lobe handles planning, decision-making, personality, and voluntary movement (the primary motor cortex). The parietal lobe processes touch and body position (the primary somatosensory cortex). The temporal lobe handles hearing and memory; the occipital lobe handles vision. Language areas, such as Broca's (speech production) and Wernicke's (comprehension), are usually in the left hemisphere.
Deeper brain structures
The thalamus relays sensory information to the cortex. The hypothalamus controls body temperature, hunger, thirst, and the pituitary gland. The limbic system, including the hippocampus (forming new memories) and amygdala (fear and emotion), shapes emotion and memory. The basal nuclei help control movement; their dysfunction underlies Parkinson's disease.
The brainstem and cerebellum
The brainstem has three parts: the midbrain, pons, and medulla oblongata. The medulla controls heart rate, breathing, and blood pressure, so damage there is life-threatening. The cerebellum coordinates movement, balance, and posture and helps learn motor skills like riding a bike.
The spinal cord
The spinal cord runs from the brainstem to about the first lumbar vertebra. In cross-section, an H-shaped core of grey matter is surrounded by white matter tracts carrying signals up (sensory) and down (motor). Thirty-one pairs of spinal nerves branch off. Spinal cord injury causes loss of function below the injury level.
Reflexes
A reflex arc is a rapid, automatic response: a sensory receptor, a sensory neuron, an integration centre in the spinal cord, a motor neuron, and an effector. The knee-jerk (patellar) reflex involves just one synapse. Doctors test reflexes to check the health of specific nerves and spinal segments.
Worked example
A stroke damages the left frontal lobe near Broca's area and the motor cortex. The patient may have trouble producing speech and weakness on the right side of the body, because each hemisphere controls the opposite side and language is usually left-sided. Brain imaging pinpoints the damage.
Key takeaways
The CNS is protected by bone, meninges, cerebrospinal fluid, and the blood-brain barrier. The cerebrum's lobes handle movement, sensation, hearing, memory, and vision; each side controls the opposite body side. The thalamus relays, the hypothalamus regulates, and the limbic system handles emotion and memory. The medulla controls vital functions; the cerebellum coordinates movement. The spinal cord carries tracts and mediates reflexes.
Flashcard
Three protective membranes
Check your understanding
Damage to the occipital lobe would most likely affect: