Organisation
The central nervous system (CNS) is the brain and spinal cord. The peripheral nervous system (PNS) is all the nerves outside them. The PNS has a sensory (afferent) division carrying information in and a motor (efferent) division carrying commands out. The motor division includes the somatic system (voluntary skeletal muscle) and the autonomic system, whose sympathetic branch prepares the body for action ("fight or flight") and whose parasympathetic branch favours rest and digestion.
The neuron
A neuron has dendrites that receive signals, a cell body containing the nucleus, and a single axon that carries signals away to axon terminals. Myelin, a fatty sheath, insulates many axons and makes conduction faster because the impulse jumps between gaps (saltatory conduction). Myelin is made by Schwann cells in the PNS and oligodendrocytes in the CNS. Other glia include astrocytes and microglia.
Electrical signalling
At rest the inside of a neuron is negative, around -70 millivolts, because of the sodium-potassium pump and leaky potassium channels. When a stimulus reaches threshold, voltage-gated sodium channels open, sodium floods in, and the cell depolarises. Then sodium channels close and potassium channels open, so potassium leaves and the cell repolarises. The action potential travels along the axon in an all-or-none way.
Synapses
At the axon terminal, arrival of the impulse opens calcium channels. Calcium entry makes vesicles release neurotransmitter into the synaptic cleft. The chemical binds receptors on the next cell, causing a response.
Brain regions
The cerebrum handles thought, sensation and voluntary movement. The thalamus relays sensory input. The hypothalamus regulates temperature, hunger and hormones. The cerebellum coordinates smooth movement and balance. The medulla oblongata contains centres for heart rate and breathing.
Worked walk-through: a withdrawal reflex
Suppose a person's hand touches a hot pan and pulls away before they feel pain.
- Receptor: temperature and pain endings in the skin are stimulated.
- Sensory neuron: carries the impulse into the spinal cord.
- Integration centre: an interneuron in the spinal cord connects to motor neurons, bypassing the brain.
- Motor neuron: carries the command out.
- Effector: arm flexor muscles contract and the hand withdraws.
Signals also travel up to the brain, which is why pain is felt a moment later. Because the arc loops through the spinal cord, the response is fast and automatic.