13.2 The Nervous System

13.2 The Nervous System

The nervous system moves information fast enough for an animal to sense a change and answer it. The path through the topic is one cell, then the electrical impulse, then the chemical junction, then the organs and divisions that organize those cells.

A neuron and its resting membrane

Parts of a neuron

A neuron is a cell built to carry signals. Dendrites are branching processes that receive input from other cells or from sensory stimuli. The cell body contains the nucleus and most of the organelles, and it sums the inputs. The axon is the long process that carries an impulse away from the cell body. Many vertebrate axons are wrapped in myelin, a fatty sheath formed by glial cells. Myelin insulates the membrane so the impulse is regenerated at bare gaps called nodes of Ranvier, which speeds conduction. Some axons have no myelin and conduct more slowly. Synaptic terminals are the endings that pass the message to the next cell. Signal flow in a typical neuron is dendrite, cell body, axon, then terminal.

The resting membrane

Across the membrane of a typical neuron that is not firing, the inside is negative relative to the outside by about -70 mV. That resting membrane potential comes from ion gradients and from selective permeability. Sodium is more concentrated outside the cell. Potassium is more concentrated inside. The sodium-potassium pump keeps those gradients in place by moving sodium out and potassium in, and it spends ATP to do so. At rest the membrane lets potassium leak outward more readily than it lets sodium leak in. Potassium efflux, together with large negative ions trapped in the cell, leaves the interior negative. The exact voltage varies a little among cells. About -70 mV, inside negative, is the value to remember.

The action potential

An action potential is a brief reversal of membrane polarity that travels along the axon. It is all-or-none. Once the membrane reaches threshold, that patch of axon fires a full spike. A stronger stimulus does not make a taller spike. Intensity is carried by how often spikes occur, and by how many neurons are recruited, not by spike height.

The ion sequence is fixed. Voltage-gated sodium channels open, and Na+ rushes in down its gradient. The inside becomes less negative and then briefly positive. That rising phase is depolarization. The sodium channels then inactivate. Voltage-gated potassium channels open, and potassium leaves, restoring a negative interior. That falling phase is repolarization. Pumps and ongoing ion leaks later restore the original concentration gradients, but the rapid swing of the spike itself is the sodium entry followed by the potassium exit.

A short refractory period follows the spike, so the membrane that just fired cannot immediately fire again. The impulse continues toward the terminals instead of bouncing backward. Along the axon the spike is regenerated, so it does not fade with distance.

Chemical synapses

From electricity to a transmitter

At a chemical synapse the electrical signal becomes a chemical signal. The action potential arrives at the synaptic terminal and opens voltage-gated calcium channels. Calcium enters the terminal, and vesicles that store neurotransmitter fuse with the cell membrane. The transmitter leaves those vesicles by exocytosis and diffuses across the synaptic cleft, the narrow gap between the two cells. It binds receptor proteins on the next cell. Those receptors may open ion channels or start a slower intracellular cascade, so the next cell becomes more likely or less likely to fire. The first cell does not throw its action potential across the cleft as an electric current.

Acetylcholine is the transmitter at the neuromuscular junction, the synapse between a motor neuron and a skeletal muscle fiber. It binds receptors on the muscle cell, the membrane depolarizes, and contraction can follow. Acetylcholinesterase in the cleft breaks acetylcholine down so the fiber is not stimulated continuously.

A few cells are coupled by gap junctions, so ions can flow directly, but those electrical synapses are the exception. At a typical synapse the cleft stops the electrical spike, and exocytosis is how the transmitter leaves the vesicle.

Organization and a reflex

Central and peripheral paths

The central nervous system is the brain and the spinal cord. That is where most integration occurs. The peripheral nervous system is the network of nerves that connects the central nervous system with the rest of the body, including sense organs, skeletal muscles, and internal organs.

Three functional classes cover most pathways. Sensory neurons carry information from receptors toward the central nervous system. Interneurons lie inside the central nervous system and link one neuron to another. Motor neurons carry commands outward to effectors such as muscle fibers or glands. A sensation that leads to a movement uses that order: in, then connection, then out.

A reflex arc uses the same order for a rapid automatic response. A receptor detects the stimulus, a sensory neuron carries the signal in, and a motor neuron carries a command out to the effector. One or more interneurons often sit between them in the spinal cord, although some reflexes connect sensory and motor neurons more directly. Withdrawing a hand from a sharp or hot object does not wait for conscious planning in the cerebral cortex. The brain may be informed, but the withdrawal is already under way.

Sympathetic and parasympathetic divisions

Within the peripheral system, autonomic pathways control many involuntary organs and split into two divisions that usually oppose each other. The sympathetic division prepares fight-or-flight. Heart rate rises, airways widen, and blood flow is directed toward skeletal muscle. The parasympathetic division supports rest-and-digest. Heart rate slows, and digestive secretion and motility increase. Many organs receive both kinds of input, and the balance shifts with the situation. These are neural pathways, not names for bones or for the calcium store in the skeleton.