The cells and organs that receive internal and external stimuli are called receptors. They are constructed to receive certain kinds of stimuli and are categorized by this feature. Thus we have mechanoreceptors that respond to touch, sound and motion; photoreceptors which respond to light; and chemoreceptors that respond to chemicals and result in taste or smell sensations, etc. These receptors vary in complexity from single cells to complex organs.

Mechanoreceptors vary greatly. The skin contains specialized mechanoreceptor cells that react to touch and muscles have stretch receptors. The ear, a very complex receptor organ, has two distinct functions, balance and hearing. Both functions, however, also rely on mechanoreception. The sound is transmitted by air pressure changes to the eardrum, the inner ear ossicles (hammer, anvil and stirrup), and finally the oval window of the cochlea. The movement of the oval window causes the fluid inside the cochlea to move and, depending on the frequency of the sound, specialized ciliated cells are stimulated. The inner ear also contains three fluid filled canals (vestibular apparatus) which also contains ciliated cells. The stimulation of these cells signals the adjacent sensory neurons to let us know which direction we are moving and how fast.

The vertebrate eye is another example of a complex receptor organ. Our eyes are image forming but not all photoreceptors form images. The eye is designed to collect and focus light rays on the retina at the back of the eye. This is where the actual photoreceptor cells, the rods (black and white vision) and cones (color vision) are located.

The senses of taste and smell are both received by chemoreceptors. Both taste and smell require that the chemical stimuli they process are dissolved in fluid. The receptor cells of the tongue and nose are specialized to receive only certain chemicals. Thus the tongue can be mapped to show where sweet, sour, salty and bitter substances are detected.

When stimulated, all receptor cells and organs release a chemical transmitter which depolarizes the adjacent sensory neuron(s). These sensory neurons are part of the peripheral nervous system. If they lead into the brain, they are cranial nerves or if they lead into the spinal cord, they are spinal nerves. The peripheral nervous system consists of both sensory neurons ending in the brain or spinal cord and motor neurons originating in the brain or spinal cord and going out to effector cells or organs.

The nervous system is far superior in speed and selectivity to the endocrine system. It depends on a specialized system of nerve cells called neurons which receive and give instructions by means of electrical impulses directed over specific pathways. The neurons are highly differentiated cells and are similar in all animals. Their structure and function are intimately related. The resting neuron (like all cells) has a charge difference across its membranes due to the differential distribution of ions (thanks to the Na⁺ /K⁺ pump in cell membrane). When a neuron is stimulated, the resting potential gives way to the action potential and the membrane is depolarized. The depolarization travels in one direction down the axon of the neuron to the opposite end of the cell. At the "far" end, the neuron has small vesicles containing a neurotransmitter which is released when the membrane depolarization occurs there. When the neurotransmitter is released, it binds to receptors in the membrane of the next cell (either another neuron or an effector cell), and depolarizes it. If the cell is another neuron, the depolarization begins in that cell and travels along its axon. If the next cell is a muscle, the muscle cell membrane is depolarized (in a manner similar to the neuron) and a series of reactions occur within the muscle resulting in either contraction or relaxation of the muscle.

Neurotransmitters can be excitatory or inhibitory. Each muscle is enervated by both kinds. Endocrine or exocrine glands, the other kind of effector cells or organs, respond by releasing their product when stimulated by a motor neuron. It is interesting to note that even in the highly efficient nervous system, the ultimate messenger between cells is a chemical. So even here we see the reliance on a process very similar to the evolutionary older system of hormonal (chemical) communication!

The nervous system is divided into the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS consists of the brain and spinal cord. The neurons of the CNS are referred to as inter neurons. They make connections with many other neurons including neurons within the CNS as well as the sensory and motor neurons of the PNS. The neurons of the CNS send messages to many effector cells and receive messages from many receptor cells. It is their job to integrate the information received and coordinate the body's responses.

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