Chapter 2
The Vertebrate Endocrine System
Communication among cells in multicellular animals is necessary for their overall
coordination and survival. Individual cells lying adjacent to one another make contact , either directly or with the aid of ions and metabolites, across the relatively
narrow fluid-filled extracellular spaces . Interdependent cells in large animals may,
however, be separated by distances of several metres, so that if appropriate coordination is to take place, special methods must be used. Such 'long distance' cell communication takes place in two main ways ; through nerve cells and by the secretions
of endocrine gland s. Nervous communication is characteristically rather faster than
endocrine, and attains its specific actions by utilizing discrete tissue pathways to
particular organs. Although endocrine communication is slower, once initiated,
its actions often may be more prolonged. It is thus peculiarly suited to integration
of met abolic, rather than the mechanical and sensory, processes which are more the
special province of the nerves . The pathway for endocrine communication is the
extracellular fluid, so that endocrine products can ultimately come in contact with
man y cells of the body. This results in a problem of specificity in communication,
that is principally solved by utilizing a host of different molecules, each with special
affinities for the physico-chemical characteristics of certain cells. Thus it is not surprising that mo re different endocrine hormones have been identified than neurotransmitter substances.
Neural and endocrine coordination systems are interdependent and interrelate
to each other. The correct function of all cells in the body, including the nerves
and endocrine glands, depends on adequate supplies of suitable metabolic substrates, and an optimal osmotic and ionic environment; properties that can be related
to the actions of both nerves and endocrine glands. The two systems exchange information , and influence each other's activity, particularly in the region of the hypothalamus and pituitary gland and in the adrenal medulla. Nerve impulses can
affect the rates of secretion of endocrine glands, while the hormones may influence
neural processes as in behaviour.
Individual cells are usually self contained autoregulated units which, providing
the y are situated in an optimal environmental solution, can maintain life. Cells in
tissue culture media thus may carry through their life cycle in perpetuity. Cells
can, however, usually onl y autoregulate under a rather limited range of ionic and
osmotic concentrations, whil e a constant supply of metabolic substrates, in suitable
form and at an adequate concentration, are also necessary. The endocrine-glands
playa predominant role in regulating such internal environmental conditions .
The pattern of endocrine coordination is in accord with the classical servo-control system (Fig. 2.1.) . The main elements of the relevant servo-system are : its sensitivity to change from a 'set point' value as conceived by the 'misalignment detector' which stimulates a ' cont roller' to feed 'controlled power' to a 'moto r'. The
'motor' has an output which can correct the misalignment, a change which then
'feeds back' to inhibit the 'cont ro ller' . The endocrine system by analogy is sensitive
to changes in osmotic, ionic, metabolic and substrate levels and these can affect
39
The Vertebrate Endocrine System
Communication among cells in multicellular animals is necessary for their overall
coordination and survival. Individual cells lying adjacent to one another make contact , either directly or with the aid of ions and metabolites, across the relatively
narrow fluid-filled extracellular spaces . Interdependent cells in large animals may,
however, be separated by distances of several metres, so that if appropriate coordination is to take place, special methods must be used. Such 'long distance' cell communication takes place in two main ways ; through nerve cells and by the secretions
of endocrine gland s. Nervous communication is characteristically rather faster than
endocrine, and attains its specific actions by utilizing discrete tissue pathways to
particular organs. Although endocrine communication is slower, once initiated,
its actions often may be more prolonged. It is thus peculiarly suited to integration
of met abolic, rather than the mechanical and sensory, processes which are more the
special province of the nerves . The pathway for endocrine communication is the
extracellular fluid, so that endocrine products can ultimately come in contact with
man y cells of the body. This results in a problem of specificity in communication,
that is principally solved by utilizing a host of different molecules, each with special
affinities for the physico-chemical characteristics of certain cells. Thus it is not surprising that mo re different endocrine hormones have been identified than neurotransmitter substances.
Neural and endocrine coordination systems are interdependent and interrelate
to each other. The correct function of all cells in the body, including the nerves
and endocrine glands, depends on adequate supplies of suitable metabolic substrates, and an optimal osmotic and ionic environment; properties that can be related
to the actions of both nerves and endocrine glands. The two systems exchange information , and influence each other's activity, particularly in the region of the hypothalamus and pituitary gland and in the adrenal medulla. Nerve impulses can
affect the rates of secretion of endocrine glands, while the hormones may influence
neural processes as in behaviour.
Individual cells are usually self contained autoregulated units which, providing
the y are situated in an optimal environmental solution, can maintain life. Cells in
tissue culture media thus may carry through their life cycle in perpetuity. Cells
can, however, usually onl y autoregulate under a rather limited range of ionic and
osmotic concentrations, whil e a constant supply of metabolic substrates, in suitable
form and at an adequate concentration, are also necessary. The endocrine-glands
playa predominant role in regulating such internal environmental conditions .
The pattern of endocrine coordination is in accord with the classical servo-control system (Fig. 2.1.) . The main elements of the relevant servo-system are : its sensitivity to change from a 'set point' value as conceived by the 'misalignment detector' which stimulates a ' cont roller' to feed 'controlled power' to a 'moto r'. The
'motor' has an output which can correct the misalignment, a change which then
'feeds back' to inhibit the 'cont ro ller' . The endocrine system by analogy is sensitive
to changes in osmotic, ionic, metabolic and substrate levels and these can affect
39