Homeostasis and Regulation

An important characteristic of animals is the ability to self-regulate the extracellular environment in which their cells are bathed and function. The extracellular environment is a buffer between the intracellular environment and the external environment of an animal, which consists of an aquatic or terrestrial environment in exchange with the atmosphere. These external environments can be highly variable with respect to their physical characteristics, which would affect the intracellular physiological processes necessary for animals to function. Therefore, some aspects of the intracellular environment of an animal are invariably kept different from their external environment. Consequently, an important role of homeo-stasis in animals is the regulation of aspects of the extracellular environment different from the external environment to provide an optimal internal environment in which the cells function.

Homeostasis is an underlying principle of animal physiology, and physiological systems are the means by which homeostasis is maintained. Homeostatic processes maintain the internal environment, although not all animals regulate all physiological variables to the same extent. Animals may conform with respect to some physiological variables, with the internal variable the same as for the external environment.

For both conformers and regulators, there is a range of environmental conditions over which the animal can survive. Beyond this range, conformers experience sufficient change in the internal environment that physiological processes no longer function effectively, and regulators can no longer regulate against the environmental gradient and their internal environment changes sufficiently to prevent normal physiological function.

Homeostasis does not necessarily require a regulatory mechanism. Equilibrium homeostasis and steady-state homeostasis are nonregulatory means by which an internal variable is kept constant; for example, a body fluid solute can remain relatively constant if the rate of excretion balances the rate of synthesis (Figure 1). Many other homeostatic mechanisms, however, require regulation to maintain constancy. Negative feedback control is the most common regulatory system whereby a change in a variable is detected by a sensor and then counteracted by a response from an effector organ that is opposite to the perturbation (Figure 1). Many physiological systems are controlled by several regulatory effectors, resulting in multiple control systems with greater overall precision of regulation. The nervous and endocrine systems are responsible for integrating physiological functions in an animal. They ensure

External environment

External environment Input

Stimulus

External environment

External environment Input

Equilibrium homeostasis

Output Steady-state homeostasis

Equilibrium homeostasis

Output Steady-state homeostasis

Receptor

External environment Disturbance

Integrating center

Physiological variable

Effector _I

Response Negative feedback homeostasis

Figure 1 Schematic of equilibrium, steady-state, and negativefeedback regulatory mechanisms.

that the physiological processes of different cells, tissues and organs occur in a controlled and coordinated manner, and result in whole-body homeostasis.

Nervous System

The nervous system integrates physiological functions and ensures that the physiological processes of different cells, tissues and organs occur in a controlled and coordinated manner, and result in whole-body regulation. It is responsible for coordinating rapid and precise responses to perturbations in the animal's internal and external environment by sensing changes in a physiological variable, integrating and interpreting the changes, and eliciting an effector response to counteract the change.

The nervous system consists of aggregations of two cell types, neurons that generate and conduct action potentials (a change in polarity of voltage across a cell membrane) and glial cells that are accessory cells which support and assist the function of neurons. Neurons can be classified as sensory (or afferent), inter (or internuncial) or motor (or efferent) and connect to one another, and to sensory or motor effector cells, via synapses (Figure 2). In primitive animals (and simple reflexes in complex animals) there is a direct connection between sensory and effector cells by a single motor neuron, resulting in a simple three-cell sensory-motor circuit. However, in more advanced animals additional interneurons between the sensory and motor neurons allows for much greater complexity, permitting more complex integration and interpretation of sensory information, sophisticated motor control, and the development of complex behaviors.

Stimulus

Sensory neuron

Interneuron

Motor neuron

Figure 2 Schematic sensory-motor neuron system consists of a sensory neuron input, integrative and interpretative interneuron, and an effector motor neuron.

Sensory neuron

Interneuron

Motor neuron

Figure 2 Schematic sensory-motor neuron system consists of a sensory neuron input, integrative and interpretative interneuron, and an effector motor neuron.

The most primitive nervous systems are nerve nets; they occur in coelenterates and some flatworms. The development of cephalization (a head region) led to the concentration of neurons at the anterior end of the animal, forming the brain, and nerve cords consisting of concentrated groups of neurons transmitting information to other regions of the body. The nervous system is most highly developed in vertebrate animals. Here the brain and spinal nerve cord form the central nervous system while the peripheral nervous system consists of many paired nerves that run from the spinal cord to the peripheral regions of the body. These transmit sensory information to the central nervous system and return motor commands to the peripheral effectors. The somatic nervous system innervates efferent organs under conscious control (e.g., skeletal muscle), while the autonomic nervous system innervates involuntary visceral organs (e.g., gut and heart).

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