As we mentioned at the end of the last section, nervous tissue has a role in sensing environmental conditions and coordinating the response of the animal to those conditions. In radially symmetrical animals, nervous systems are usually arranged as nerve nets, without significant centralization. With cephalization and bilateral symmetry, though, come the arrangement of sensors near the leading end of the body. In order to shorten the path length, brains (central nervous components) are placed in the head near the sensor cells.

In keeping with their function in transmitting signals from one place to another, nerve cells are long and thin. Like the wires they resemble in both structure and function, they physically connect distant places that need to communicate. In a resting nerve cell (neuron) there are gradients of two positively charged ions, sodium (Na+) and potassium (K+). These gradients are generated at the expense of ATP by the sodium-potassium pump (figure 32.7). It is the movement of sodium and potassium ions along their respective gradients that is the basis for the propagation of nerve impulses.

Because of the presence of other ions (see section 2.4 for a review) the inside of the nerve cell is negatively charged, and the outside is positively charged. This means that a voltage exists across the membrane (see figure 32.8). When a nerve impulse begins, gated sodium ion channels in the membrane open, allowing the sodium ions to move down their gradient into the cell. Since they are positively charged, they make the inside of the cell more positive than the outside, a reversal of the previous situation. This reversal, or depolarization, causes two other events. First, it causes the next set of gated sodium channels to open, moving the depolarization down the neuron. Second, it causes gated potassium channels to open, allowing potassium to flow out of the cell , and restoring the resting polarity (voltage) across the membrane. These events are well-presented in figure 32.3. After the impulse, the sodium-potassium pump reestablishes the gradients so that a new impulse can be sent.

The above description explains how impulses are transmitted along the length of a single nerve cell, but not how the information is sent to the target cell. This cell-cell communication requires a synapse, or cell - cell junction. When the impulse reaches the synapse, it causes the release of neurotransmitters into the synapse. These transmitter molecules are sensed by gated channel proteins on the recipient nerve or muscle cell (figure 32.10), and used to begin a new impulse if the postsynaptic cell is a neuron or a contraction if it is a muscle cell. It is important to remember that neurotransmitter molecules must be removed from the synapse to end the signal. In the case of acetylcholine at neuromuscular junctions, the transmitter is removed by an enzyme. Inhibition of this enzyme (by pesticides and nerve gas, for example) causes rapid death due to the inability to regulate and control muscle contraction. Other neurotransmitters like dopamine and serotonin are removed from the synapse by reuptake into the original nerve cell. Drugs like cocaine interfere with the reuptake, causing intense pleasure as dopamine remains in the synapse. Soon, though, the postsynaptic cell adjusts to the new situation, requiring higher levels of dopamine to be stimualted. This, combined with low dopamine levels in the presynaptic cell, leads to a situation where the individual is incapable of experiencing pleasure or even well being in the absence of the drug.

As an example of a sensor - integrator - effector system, consider figure 32.14. In this example, a stimulus is detected by stretch receptors (sensors) causing a nerve impulse to be sent to the spinal cord, a part of the central nervous system. The sensor neuron synapses with (among others) an interneuron in the spinal cord, which in turn synapses with a motor neuron, signalling the biceps to contract to compensate for the weight added to the bowl. For other sensory pathways (hearing, vision, taste, etc), Sensory neurons send signals to the cerebrum of the brain, which serves to integrate and process the signals and decide on a response.