The Nervous System: Organization, Neurons and Reflexes

Faster than thought

You touch a hot frying pan and your hand jerks away before you even feel the pain. A goalie's glove snaps up to a puck she barely saw. A doctor taps just below your kneecap and your leg kicks without your permission. All of these are the work of your nervous system, the fastest communication network in your body.

The endocrine system sends slow "group texts" through the blood. The nervous system sends rapid, targeted messages along neurons, cell to cell, in milliseconds. In this lesson you will see how the nervous system is organized and how a simple reflex works. In the next two lessons you will zoom in to the electrical signal itself and the junctions between neurons.

Organization of the nervous system

• Central nervous system (CNS): the brain and spinal cord. It receives information, integrates it and sends out commands.
• Peripheral nervous system (PNS): all the nerves connecting the CNS to the rest of the body. It has:
• a sensory (afferent) division, carrying signals to the CNS from receptors;
• a motor (efferent) division, carrying signals from the CNS to effectors. The motor division has two parts:
• the somatic nervous system, which controls skeletal muscles (mostly voluntary);
• the autonomic nervous system, which controls cardiac muscle, smooth muscle and glands (involuntary). It is divided into the sympathetic and parasympathetic divisions, which usually have opposite effects.

A nerve is not the same as a neuron. A nerve is a bundle of many axons wrapped in connective tissue, like a cable full of wires.

Neurons and glial cells

Every neuron has three main parts:

• Dendrites: short, branched extensions that receive signals from other neurons or receptors.
• Cell body (soma): contains the nucleus and most organelles.
• Axon: a single long extension that carries the impulse away from the cell body, ending in axon terminals that pass the signal to the next cell. Some axons, such as those running from your spinal cord to your toes, are about a metre long.

Many axons are wrapped in a myelin sheath, layers of fatty membrane from glial cells (Schwann cells in the PNS, oligodendrocytes in the CNS). The small gaps between myelin segments are the nodes of Ranvier. Myelin insulates the axon and makes impulses travel much faster, as you will see in the next lesson. In the CNS, myelinated axons make up the white matter; cell bodies and unmyelinated parts make up the grey matter.

There are three functional types of neurons:

• Sensory neurons carry signals from receptors to the CNS.
• Interneurons (association neurons), found only in the CNS, connect neurons with each other and do the "processing".
• Motor neurons carry signals from the CNS to effectors (muscles and glands).

Neurons in the PNS can sometimes regrow a damaged axon, with help from Schwann cells, if the cell body is intact. Neurons in the CNS have much less ability to regenerate, which is why spinal cord injuries are so serious and an active area of research.

The brain

• Cerebrum: the largest part, divided into left and right hemispheres joined by the corpus callosum. Its outer layer, the cerebral cortex, is folded to fit a large area into the skull. It has four lobes: frontal (planning, decisions, voluntary movement, speech production), parietal (touch, temperature and body position), temporal (hearing, language understanding, memory) and occipital (vision).
• Cerebellum: at the back, below the cerebrum; coordinates movement, balance and posture.
• Brainstem: the midbrain, pons and medulla oblongata. The medulla controls breathing rate, heart rate and blood pressure, as you have seen in several lessons.
• Thalamus: a relay station that sorts incoming sensory information and sends it to the right part of the cortex.
• Hypothalamus: below the thalamus; the homeostasis control centre for temperature, water balance, hunger and thirst, and the link to the pituitary gland.

Reflexes: the shortcut

A reflex is a rapid, automatic response to a stimulus that does not need conscious thought. Its pathway is a reflex arc:

receptor → sensory neuron → interneuron (in the spinal cord) → motor neuron → effector

When you touch a hot pan, heat receptors in your skin start impulses in sensory neurons. In the spinal cord, these connect to interneurons, which connect directly to motor neurons going back to the arm muscles. The muscles contract and your hand pulls away. Because the signal does not need to go to the brain and back, the response is very fast. At the same time, interneurons send a signal up the spinal cord to the brain, so you feel the pain and know what happened a moment later.

The knee-jerk (patellar) reflex is even simpler: the sensory neuron connects directly to the motor neuron in the spinal cord, with no interneuron. Doctors test reflexes like this to check that the nerves and spinal cord are working.