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Sensitivity: responding to stimuli
All living organisms must be able to detect changes in their environment and respond appropriately. Changes in the environment are called stimuli (singular: stimulus). A stimulus may be in either the external environment (outside the organism) or the internal environment (inside the organism). Sensitivity, the ability to respond appropriately to stimuli, is one of the characteristic features of life. Each organism has its own specific type of sensitivity that improves its chances of survival. A single-celled amoeba, for example, can move away from a harmful stimulus such as very bright light, and move towards a favourable stimulus such as food molecules, but it can only distinguish between a limited number of different stimuli.
In an amoeba, the detection of the stimulus and the response to the stimulus must both take place in a single cell. However, in large multicellular animals such as mammals, stimuli are detected in sense organs, and organs that respond are called effectors. The sense organs and effectors may be in quite different parts of the body. In addition, responses usually involve the coordinated actions of many different parts of the body. To achieve this coordination, one part of the body must be able to pass information to another part. In mammals, there are two major systems that convey information: the nervous system and the endocrine (hormonal) system.
The nervous system
Nervous systems range from the simple nerve nets of jellyfish and sea anemones, which have no brain and relatively few interconnections, to the nervous system of humans, with brains of staggering complexity. The human brain contains many millions of cells, each of which may communicate with thousands of other nerve cells. Their interconnections form circuits which enable us to control our muscles, think, remember, and even study our own brains.
All the various animal nervous systems are fast-acting communication systems containing nerve cells, neurones, which convey information in the form of nerve impulses (electrochemical changes). Neurones take various forms but each has a cell body, containing a nucleus, and nerve fibres, long extensions that transmit nerve impulses rapidly from one part of the body to another. Fibres carrying impulses away from the cell body are called axons; those carrying impulses towards the cell body are called dendrons. Apart from the main nerve fibre, there may be small dendrons (dendrites) extending from the cell body.
In mammals, sensory neurones carry messages from peripheral sense organs to a central nervous system (CNS) consisting of the brain and spinal cord. The CNS acts as an integration centre and processes information from many sources. Motor neurones convey instructions from the CNS to effector organs (mainly muscles and glands).
A mammalian motor neurone can convey information rapidly over considerable distances; for example, a single nerve impulse may be transmitted from the spinal cord to the feet in a few milliseconds. These fast-conducting neurones are enclosed along most of their length by a thick insulating material called the myelin sheath. The myelin sheath is produced by special supporting cells called Schwann cells. The sheath is essentially a series of cell membranes, each produced by a Schwann cell and wrapped many times around the axon. Gaps between the membranes of each Schwann cell, called the nodes of Ranvier, are the key to the fast transmission of nerve impulses.
Fast transmission enables mammals to respond almost instantaneously to stimuli. Nerve impulses can be directed along the nerve fibres to specific points in the body so that responses can be very localised.