Neurons and the Nervous System

The Human Body lesson gives a first tour of the nervous system. This lesson goes deeper into how its cells signal. The nervous system has three jobs: it senses changes, makes sense of them, and sends out a response through muscles and glands, the effectors (see SEER).

Its signalling cell is the neuron. Each neuron has a cell body, one or more dendrites that carry impulses toward the cell body, and a single axon that carries them away (see SEER). Signals pass down the axon to another neuron, a muscle cell, or cells in some other organ (see NINDS). Support cells called neuroglia, or glial cells, surround the neurons (see SEER).

Neurons come in three kinds, named by the way they carry impulses (see SEER):

• sensory, or afferent, neurons carry impulses from receptors to the brain and spinal cord;
• motor, or efferent, neurons carry them out to muscles and glands;
• interneurons lie entirely within the brain and spinal cord and link the other two.

At rest, the inside of a neuron is about 70 millivolts more negative than the outside. This is the resting potential, about −70 mV (see Queensland Brain Institute).

If a stimulus lifts the membrane to its threshold, about −50 mV, the neuron fires an action potential that lasts about one millisecond (see Queensland Brain Institute). Sodium channels open and Na+ rushes in, so the voltage rises. Then potassium channels open and K+ flows out, bringing it back down. A pump, the Na/K-ATPase, later restores the balance (see Grider and others). For each ATP it uses, it moves 3 Na+ out and 2 K+ in (see Pirahanchi and others).

The rise is all-or-none: once threshold is reached, it goes all the way (see Grider and others). Below threshold, nothing fires.

Just after a spike comes the refractory period. In the absolute refractory period, the sodium channels' inactivation gates are closed and no new action potential can start. In the relative refractory period that follows, potassium channels are still open, so a new spike needs a stronger stimulus. This is why an impulse only travels forward along the axon (see Pitt Medical Neuroscience).

Many axons are wrapped in myelin, a fatty sheath made by Schwann cells in the peripheral nervous system and by oligodendrocytes in the brain and spinal cord. The sheath has regular gaps called nodes of Ranvier, which are needed to pass the impulse along and set its speed (see Grider and others).

Voltage-gated channels sit at the nodes, so the action potential is made again at each node and seems to leap from one to the next. This is saltatory conduction. It is fast, and it saves energy, because the pump only has to restore ions at the nodes (see Pitt Medical Neuroscience).

Wider, more myelinated axons are faster (see Pitt Medical Neuroscience). The largest myelinated fibres, which run to skeletal muscle, conduct at 70 to 120 metres per second. Thin unmyelinated C fibres, which carry pain and temperature, manage only 0.5 to 2.5 metres per second (see Kiernan).

Neurons do not touch. They meet at a synapse, across a gap called the synaptic cleft, about 20 to 40 nanometres wide (see Queensland Brain Institute). When an impulse reaches the end of the axon, tiny sacs called vesicles release neurotransmitters into the cleft. These chemicals cross it and attach to receptors on the next cell (see NINDS). So the synapse turns an electrical signal into a chemical one, and back into an electrical one (see Queensland Brain Institute).

Some neurotransmitters make the next cell more active and are called excitatory. Others damp it down and are called inhibitory (see NINDS). Some examples (see NINDS):

• Acetylcholine, excitatory, controls muscle contractions.
• Glutamate is a major excitatory transmitter.
• GABA is inhibitory.
• Serotonin brings on sleep and is involved in temperature regulation.
• Dopamine is involved in mood and the control of complex movements.

A reflex is a response that needs no conscious thought, so it is faster than one that does. Many reflexes are handled in the spinal cord without going to the higher brain centres. In the withdrawal reflex, your hand moves away before you are aware of the pain (see SEER). The path is a reflex arc: receptor, sensory neuron, interneuron, motor neuron and effector (see SEER).

The central nervous system, or CNS, is the brain and spinal cord. The peripheral nervous system, or PNS, is the nerves and ganglia outside them (see SEER). It includes 12 pairs of cranial nerves and 31 pairs of spinal nerves (see SEER).

The motor side of the PNS has two parts (see SEER):

• the somatic nervous system, which supplies skeletal muscles and is under conscious control, so it is also called voluntary;
• the autonomic nervous system, which runs involuntary jobs.

The autonomic system controls heart rate, breathing rate, blood pressure and body temperature. Its sympathetic and parasympathetic divisions work together to maintain homeostasis (see SEER). The Homeostasis and Hormones lesson shows how nerves and hormones share that job.

The cerebrum is split into two hemispheres, joined by a thick band of nerve fibres, the corpus callosum. Nearly all signals cross over, so the right hemisphere mainly controls the left side of the body (see NINDS). Most processing happens in the cortex, the grey outer layer (see NINDS).

The hindbrain, which includes the brain stem and cerebellum, controls vital functions such as breathing and heart rate (see NINDS). The brain stem has three parts: the midbrain, pons and medulla (see SEER). The cerebellum coordinates movement, and the thalamus relays sensory impulses to the cortex (see NINDS and SEER).

Because synapses work by chemicals, chemicals can change them. Drugs that raise GABA levels in the brain are used to treat epileptic seizures. Many medicines for mental health conditions work by changing the action of dopamine (see NINDS).

The idea of electrical cells talking through chemicals took about 170 years to build.