8. THE REGULATION OF BREATHING
317
sentative of activity passing down the motor nerves from its origins
within motor neurons of brain stem nuclei. The fact that motor activity
is not synchronous in all these final pathways but occurs in different
phases of the breathing cycle in a very regular and specific way underlines
the complexity of the respiratory oscillator. This mechanism must be
capable of maintaining an appropriate basic rhythm and in addition be
able to initiate activity of different duration and pattern to individual
muscles at different times in a breathing cycle. The processes which lead
up to the complex and highly integrated discharges in the respiratory
musculature can be considered in two convenient but not totally independent categories. In the first place it is necessary to understand
the interaction of large numbers of neural elements, most of them situated centrally, whose function is to initiate the basic respiratory process
and produce the final coordinated motor pattern. The aggregation of
neurons within the medulla oblongata involved in this function is usually
referred to as the respiratory center, although implications in such
a term of a fixed group of neurons with precise and restricted anatomical
location are unfortunate (Hughes and Shelton, 1962; Wang and Ngai,
1964). In addition, there are many processes involved in respiratory
homeostasis. That is to say, the basic process must be modified in an
appropriate way to meet the demands of changes in internal state such
as the transition from exercise to rest or of changes in external environment of great range and variety. The second category is thus one of feedback processes which ensure that the motor pattern produced by the
center is appropriate to the conditions. The two categories will be
considered in turn.
A. Experimental Techniques for Investigating
Central Respiratory Mechanisms
Investigations of neural coordinating mechanisms rely in general
upon the three classical methods of destruction, stimulation, and detection of activity. Their use in experiments on the central coordination of
respiration in a variety of animals has led to the present position in
which a great deal of information (and controversy) exists about the
large-scale arrangement and interaction of neuron groups, but very
little is known about the role of individual neurons; for example, expiratory and inspiratory groups which are functionally, if not morphologically, distinct form the conceptual basis of many hypotheses about the
medullary respiratory center in mammals. In some cases they are considered adequate to coordinate rhythmic breathing ( Salmoiraghi, 1963) ;
317
sentative of activity passing down the motor nerves from its origins
within motor neurons of brain stem nuclei. The fact that motor activity
is not synchronous in all these final pathways but occurs in different
phases of the breathing cycle in a very regular and specific way underlines
the complexity of the respiratory oscillator. This mechanism must be
capable of maintaining an appropriate basic rhythm and in addition be
able to initiate activity of different duration and pattern to individual
muscles at different times in a breathing cycle. The processes which lead
up to the complex and highly integrated discharges in the respiratory
musculature can be considered in two convenient but not totally independent categories. In the first place it is necessary to understand
the interaction of large numbers of neural elements, most of them situated centrally, whose function is to initiate the basic respiratory process
and produce the final coordinated motor pattern. The aggregation of
neurons within the medulla oblongata involved in this function is usually
referred to as the respiratory center, although implications in such
a term of a fixed group of neurons with precise and restricted anatomical
location are unfortunate (Hughes and Shelton, 1962; Wang and Ngai,
1964). In addition, there are many processes involved in respiratory
homeostasis. That is to say, the basic process must be modified in an
appropriate way to meet the demands of changes in internal state such
as the transition from exercise to rest or of changes in external environment of great range and variety. The second category is thus one of feedback processes which ensure that the motor pattern produced by the
center is appropriate to the conditions. The two categories will be
considered in turn.
A. Experimental Techniques for Investigating
Central Respiratory Mechanisms
Investigations of neural coordinating mechanisms rely in general
upon the three classical methods of destruction, stimulation, and detection of activity. Their use in experiments on the central coordination of
respiration in a variety of animals has led to the present position in
which a great deal of information (and controversy) exists about the
large-scale arrangement and interaction of neuron groups, but very
little is known about the role of individual neurons; for example, expiratory and inspiratory groups which are functionally, if not morphologically, distinct form the conceptual basis of many hypotheses about the
medullary respiratory center in mammals. In some cases they are considered adequate to coordinate rhythmic breathing ( Salmoiraghi, 1963) ;