How the nervous system sends and sorts signals

There are two ways to split it.

By location (anatomy): the central nervous system (CNS) is the brain and spinal cord. The peripheral nervous system (PNS) is everything else, mainly nerves (bundles of axons) and ganglia (clusters of neuron cell bodies outside the CNS).

By job (function): the somatic nervous system handles conscious sensation and skeletal muscle, the muscle you can move on purpose. The autonomic nervous system (ANS) works without conscious control. It runs smooth muscle, cardiac muscle, and glands, usually to keep conditions inside the body steady (homeostasis).

The ANS has two divisions that pull organs in opposite directions. The sympathetic division is fight-or-flight: it raises heart rate and breathing rate, sends more blood to skeletal muscles, and turns digestion down. The parasympathetic division is rest-and-digest: it slows the heart and supports digestion, largely through the vagus nerve. Organs like the heart get input from both, and the balance sets what the organ does.

A neuron is the cell that carries signals. Supporting cells called glial cells surround and support neurons.

• Dendrites are branches that receive input from other neurons.
• The cell body (soma) is the main part of the cell; the dendrites and the axon extend from it.
• The axon is the single long fiber that carries the signal away from the cell body toward a target. Incoming signals are added up where the axon begins (the initial segment), and an action potential starts there if the total reaches threshold.
• Myelin is a fatty, insulating wrap around many axons.
• Axon terminals end in small swellings that release chemical messengers.

Information flows one way: dendrites, cell body, axon, axon terminals. Areas packed with cell bodies look darker (gray matter); areas packed with myelinated axons look white (white matter).

At rest, the inside of a neuron is negative, about -70 mV (the resting membrane potential). The sodium-potassium pump uses ATP to move Na+ out and K+ in.

1. A stimulus makes the membrane less negative. If it reaches threshold, about -55 mV, voltage-gated Na+ channels open.
2. Na+ rushes in, and the inside swings positive, to about +30 mV. This is depolarization.
3. Voltage-gated K+ channels open and K+ flows out, bringing the voltage back down. This is repolarization.
4. K+ keeps leaving briefly, so the voltage dips below -70 mV (hyperpolarization) before returning to rest.

An action potential is all-or-none. If threshold is reached, it fires fully; if not, nothing happens. A stronger stimulus does not make a bigger one.

Myelin is not continuous: the small gaps between its segments are the nodes of Ranvier. In myelinated axons the signal jumps from one node to the next (saltatory conduction), which is faster than the continuous conduction of unmyelinated axons.

At the synapse: when the action potential reaches the axon terminal, voltage-gated calcium (Ca2+) channels open. Calcium entering the terminal makes vesicles fuse with the membrane and release neurotransmitter into the synaptic cleft. It crosses the gap and binds receptors on the next cell.

Depending on the receptor, the next cell is pushed toward threshold (excitatory) or away from it (inhibitory). The neurotransmitter is then cleared by enzymes or taken back up. Acetylcholine (ACh) is the neurotransmitter between motor neurons and skeletal muscle. Glutamate is mainly excitatory; GABA and glycine are mainly inhibitory.

The corpus callosum is the band of white matter that lets the left and right hemispheres communicate.

A reflex is a fast, automatic response that does not wait for the brain. Touch a hot stove and your hand jerks back:

1. Receptors in the skin detect the heat and tissue damage.
2. A sensory neuron carries the signal through the dorsal (posterior) root into the spinal cord.
3. Inside the spinal cord, the signal is passed on at a synapse. This is the processing step.
4. A motor neuron in the ventral (anterior) horn sends the command out.
5. The effector, here the biceps brachii, contracts and pulls the hand away.

At the same time, a branch of the sensory signal acts through an interneuron. It inhibits the motor neuron to the opposing muscle (the triceps brachii). That keeps the triceps from contracting and slowing the withdrawal.