Nervous System

The Reflex.

The simplest complete reaction of the system is a reflex. There are many reflexes which are extremely complex, being built up of a number of simpler reflexes combined. A reflex is a reaction started by the environment acting as a stimulus upon some nerve which communicates the excitement to other nerves by connections with them in the central nervous organ. The ex citement so generated and transmitted finally travels outward from the central organ by one or more of the efferent nerves and through these reaches muscles or glands producing in them its final effect. The muscles and glands are from this point of view termed effector organs. The reaction is therefore "reflected" from the central organ. The nerve structures which include its tra jectory are spoken of as a nervous arc. The whole purpose of the central nervous organ is to bring afferent neurones into touch with efferent neurones. The whole purpose of reflex arcs is to bind one part of the organism to another part in such a way that what the environment is doing to the organism at one place may appropriately call forth or restrain movement or secretion in the muscles or glands wherever situated in the organism.

Receptor Cells.

There is one condition for the due perform ance of these reactions which is not provided by the nervous system itself. The afferent neurones are not in most cases so constituted as to be excitable themselves directly by the environ ment—for instance, they cannot be stimulated by light. Their amenability to the environment, their sensitization to environ mental agencies, is effected by special cells adjunct to their peripheral ends. These cells form organs called receptors. They are delicately adapted to be stimulated by this or that particular agent and are classifiable into various species, so that each species is easily excited by a particular agent which is "adequate" for it, and is inexcitable or excitable only with difficulty by agencies of other kinds. Thus in the skin some receptors are adapted for mechanical stimuli (touch) and not for thermal stimuli, while others (cold spots, warm spots) are adapted for thermal stimuli and not for mechanical. As far as it is known each afferent neurone is connected with receptors of one species only. The receptors thus confer upon the reflex arcs selective excitability.

Each arc is thus tuned to respond to certain stimuli, while other arcs not having that kind of receptor do not respond. The recep tors, therefore, while increasing the responsiveness of the organ ism to the environment, prevent confusion of reactions (inco ordination) by limiting to particular stimuli a particular re action.

Proprioceptors.

The system of neurones is thus made acces sible to the play of the external world acting on the body. And in addition to those receptors which are stimulated directly by the external world, are others lying within the mass of the organism itself, which are excitable by actions occurring in the organism itself. These are called proprioceptors. They are distributed pre ponderantly in the muscles and structures functionally adjunct to muscle, such as joints, ligaments, fasciae, etc. The reactions both

passive and active induced in such motor structures, reflexly by environmental stimuli, tend therefore secondarily to be followed and accompanied by reflex reactions initiated from proprioceptors.

Conduction.

The process by which the excitement generated in the afferent neurone travels along the reflex arc is known as conduction. Conduction along afferent and efferent nerves differs in some important respects from that obtaining in the nerve centre, i.e., in the piece of the central nervous system connecting the afferent nerve with the efferent nerve. In a nerve-trunk the excited state set up in it by a stimulus travels along its fibres as wave-like disturbance at a speed of about ion metres per second, and does not alter in intensity or speed in its travel. A nerve-trunk when excited at some point along its length transmits the "impulse," i.e., the wave-like excited state in both directions, i.e., both up and down each fibre, from the point stimulated. This is true whether the fibre is afferent or efferent. The speed of travel of the nervous impulse along the nerve-trunk is the same whether the nervous impulse is weak or intense. The nerve-trunk shows practically no delay in its response to an effective, even though weak, stimulus and its response ceases practically at once on ces sation of the exciting stimulus. When excited by repeated brief stimuli the rhythm of the response corresponds closely with that of the stimuli, even when the frequency of the latter is as high as Soo per second. With momentary stimuli a response even so brief as 20 can be given by the nerve-trunk. Finally, nerve-trunk conduction is singularly resistant to fatigue, to impoverished blood supply, and to many drugs which powerfully affect reflex actions.

Through the central nervous organ the travel of the impulse exhibits departure from these features. Its intensity is liable to be altered in transit. Its time of transit is much longer than for a similar length of nerve-trunk. Its direction of transmission be comes polarized, that _is, confined to one direction along the nervous path. To a strong stimulus the central reaction instead of being as brief as an impulse, may endure for a whole second or more. A stimulus not capable of evoking a response from a centre when applied once may by simple repetition become ef fective (temporal summation). It is in the grey matter that con duction differs in these respects from conduction in nerve-trunks. In the grey matter each afferent fibre breaks up into branching threadlets which ramify in various directions and terminate in close apposition with other neurones. The point of nexus of one neurone with another is termed the synapse. If synapsis occurs by contiguity and not homogeneous continuity, it is fair to suppose that at it the transmission of nervous impulses must be different from that observable in the homogeneous conducting threads of nerve fibres. The conduction must traverse some thing of the nature of a membrane.