So far, the functioning of individual nerve cells and how they can be used collectively to form nervous systems has been described. Collectively, nervous systems provide information about what is happening in the immediate environment of an animal. This may either be the internal environment, e.g. the concentrations of particular ions in the body fluids, or the external environment, e.g. the salinity of the water in which a particular aquatic animal finds itself living. Equally, they must also be in a position to respond to this information regarding the state of their environment ― they must be able to initiate appropriate biological responses. This chapter deals with the mechanisms by which animals collect information from their environment and how they are able to respond to such information.
In order for animals to monitor change in their immediate environments, it is necessary that they are equipped with appropriate sensory receptors which allow them to ‘pick up’ the vast amount of information that bombards them. Information in this sense would include aspects of the environment, such as temperature, chemical composition, the presence of light and so on. The structure of sensory receptors varies widely. At the simplest level, they can simply be dendritic endings of neurons, usually unmyelinated neurons, as in the case of nociceptors or pain receptors. At their most complex, they involve the use of specialized nonneural receptors which pass ‘information’ onto neurons, as in the case of ‘hair cells’, which are the sensory receptors in the auditory and vestibular systems of vertebrates.
Classification of sensory receptors
There are two ways of classifying sensory receptors. The first of these is based on the type of stimulus, i.e. their modality. There are six basic types of sensory receptor in this classification: chemoreceptors, thermoreceptors, mechanoreceptors, photoreceptors, magnetoreceptors, electroreceptors. (Specific examples of these will be discussed later.)
The second means by which sensory receptors may be classified is by their location. Thus, there is a group of sensory receptors which monitor internal conditions, known as interoceptors, e.g. those monitoring ionic composition of body fluids, and another group which monitor external conditions, the exteroceptors, e.g. those concerned with reception of sound waves.
Before discussing movement of whole animals, it is worth considering the movement of substances within cells. All animal cells are dynamic structures, with organelles and other substances continually moving. However, it is worth remembering that order is superimposed upon this dynamic structure. This order comes from the cytoskeleton, which, as the name implies, is the internal skeleton of all animal cells. The cytoskeleton, which contributes to the shape of the cell, its movement, and movement within the cell is composed of three types of fibre: microfilaments, microtubules and intermediate filaments. Microtubules are polymers of a protein called tubulin and these elements make up the cilia and flagella which can be found in animal cells. Microfilaments are also proteinacious structures, formed from proteins called actins, sometimes together with other protein molecules, such as myosin. As will be seen later, these structures are important in making cells shorten ― the type of response which occurs when muscles contract. The final group are the intermediate filaments. These structures play a less dynamic role in cell movement and are probably important in stabilizing cell structure and resisting movement.
In virtually all cells, the cytoplasm of the cell is continually moving, a phenomenon called cytoplasmic streaming. It should be remembered that this is a directed movement, i.e. it does not occur randomly. A good example of this cytoplasmic movement is seen in axonal transport. Here, microtubules run along the entire length of the axon from the cell body to the axon terminals. Movement of substances occurs in both directions, from the cell body to the axon terminals and vice versa. Some neurotransmitters are transported from the cell body to the axon, known as anterograde transport. This may reach speeds of up to 40 cm day-1. Flow in the opposite direction, retrograde flow, carries material from the axon terminal back to the cell body for reprocessing and occurs at a much slower rate of about 8 cm day-1.
Amoeboid movement is a characteristic movement of both unicellular animals, such as Amoeba, and also the cells of multicellular animals. For example, white blood cells in vertebrates use amoeboid movement to leave the circulation and enter the tissues where they may become involved in inflammatory reactions, and during the early development of animals, many cells move to their final destination by such movement. Amoeboid movement by single-celled animals involves the animal extending the cell to form a pseudopodium (). The precise mechanism behind this movement is unclear, but it is thought to involve changes in the physical nature of the cell cytoplasm. The general cytoplasm of the cell is liquid in its nature and is known as plasmasol. However, around the periphery of the cell it is much more viscous in nature, and here it is called plasmagel. Pseudopodia are formed by the breakdown of the plasmagel cytoplasm in a particular region of the cell membrane. Positive pressure is generated in the main part of the cell by the interaction of microfilaments. There are two types of microfilament which form a network in the cytoplasm ― these are actin and myosin. Actin and myosin interact with each other in a similar manner to the interaction of actin and myosin in muscle contraction, described in site. This interaction forces the plasmasol past the breakdown of the plasmagel, which causes the cell well to bulge outwards at that point to form a pseudopodium. As the plasmasol enters the pseudopodium it changes into plasmagel and the pseudopodium is thus prevented from forming any further.