Neurons and nerve impulses
The nervous system is the body's fast communication network, sending electrical signals at up to about 120 metres per second.
Organisation of the nervous system
The central nervous system (CNS) is the brain and spinal cord, which integrate information. The peripheral nervous system (PNS) is the nerves connecting the CNS to the body. Its sensory (afferent) division carries signals in; its motor (efferent) division carries commands out, split into the somatic system (voluntary, to skeletal muscle) and the autonomic system (involuntary, to organs).
Neuron structure
A neuron has a cell body containing the nucleus, dendrites that receive input, and a single axon that carries impulses away to axon terminals. Many axons are wrapped in myelin, made by Schwann cells in the PNS and oligodendrocytes in the CNS. Gaps in the myelin are nodes of Ranvier.
Glial cells
Glia support neurons. Astrocytes regulate the chemical environment and help form the blood-brain barrier; microglia act as immune cells; ependymal cells line brain cavities and help make cerebrospinal fluid; oligodendrocytes and Schwann cells form myelin. Most brain tumours arise from glia, since mature neurons rarely divide.
The resting membrane potential
At rest, the inside of a neuron is negative relative to the outside, about -70 millivolts. The sodium-potassium pump moves 3 sodium ions out and 2 potassium ions in per ATP, and the membrane leaks potassium more than sodium. This sets up a stored electrical gradient, like a charged battery.
The action potential
When a stimulus pushes the membrane to threshold (about -55 mV), voltage-gated sodium channels open and sodium rushes in: depolarisation, reaching about +30 mV. Sodium channels then close and potassium channels open, so potassium flows out: repolarisation. A brief overshoot (hyperpolarisation) follows before the resting state returns.
All-or-none and refractory periods
An action potential either fires fully or not at all; a stronger stimulus produces more frequent impulses, not bigger ones. Immediately after firing, a refractory period prevents another impulse, which keeps signals travelling one way along the axon.
Conduction speed
In unmyelinated axons the impulse spreads continuously and slowly. In myelinated axons it jumps from node to node (saltatory conduction), much faster. Wider axons also conduct faster. In multiple sclerosis, the immune system destroys CNS myelin, slowing or blocking signals.
Synaptic transmission
At a chemical synapse, the arriving impulse opens calcium channels in the axon terminal; calcium triggers vesicles to release neurotransmitter into the synaptic cleft. The neurotransmitter binds receptors on the next cell, opening channels that excite or inhibit it. It is then broken down or taken back up, ending the signal.
Key takeaways
The CNS integrates; the PNS has sensory and motor divisions, with somatic and autonomic branches. Neurons have dendrites, a cell body, and an axon, often myelinated. The resting potential is about -70 mV. Action potentials are all-or-none: sodium in, then potassium out. Myelin enables fast saltatory conduction. Synapses use calcium-triggered neurotransmitter release.
Flashcard
Brain and spinal cord
Check your understanding
What causes the rapid depolarisation phase of an action potential?