It has already been pointed out that the simplest working unit of the nervous system is made up of two neurons, one motor, the other sensory. When the peripheral terminations of the sensory neuron are stimulated, the impulse travels to the sensory cell, is transferred to the motor cell, passes down the efferent axon, and elicits a response in the associated motor mechanism. This response, which is quite involuntary, is known as a simple reflex action, and the pathway the impulse has traversed from sensory periphery to responsive mechanism is termed a reflex arc or pathway.
The whole nervous system consists of series of reflex pathways of various degrees of complexity, and it is important, therefore, that the student should clearly understand in the first instance the mechanism of the simpler forms. It may be stated at once that a simple independent reflex are, consisting of only two neurons, does not exist in any of the higher forms of animal life. Each sensory neuron is connected not with one but with several motor neurons and frequently with several intermediate neurons.
Simple examples of reflex actions can be well studied in a frog, the brain of which has been destroyed: if, in such an animal, one toe be pinched the foot is immediately drawn away; even in this case it will be obvious that more than one muscle is thrown into action. In the human subject if an object suddenly approaches or touches the front of the eyeball, the eyelids rapidly close. This is known as the winking reflex, and is an example where a single muscle (composed, however, of many muscle fibres) is brought into play. This reflex demonstrates also the fact that, although the movement is involuntary, it may be associated with consciousness. Similarly, if the sole of the foot be tickled, whether the individual be awake and conscious, or asleep and unconscious, the leg is drawn up.
The simple reflex pathway consisting of two neurons is useful as a theoretical starting-point from which to study certain properties of reflexes. For the performance of a reflex action a measurable period of time is required; this is known as the reaction time and is calculated from the time a apDication of the stimulus till the commencement of the respQnse. A portion of this time is taken up by the transmission of the impulse along the nerves, and this portion can be calculated since the velocity of nerve impulses is knwon and the length of the arc can be measured. In every case, however, even when to this transmission time are added the latent periods of the receiving and responding mechanisms, the total is less than the reaction time. Therefore, is lost somewhere in the course of the reflex arc, and this may occur either in the cell bodies, in the finer processes, at the junction of the sensory with the motor neuron, or at all of these; in other words a certain amount of time is taken up in the reflex centre, and this is described as the reflex time or the reduced reaction time.
It has already been pointed out that the nerve cells may considerably modify impulses passing along their processes, and one important fact of this nature, specially related to the activity of the reflex centre, is that the reflex response to a stimulus does not cease immediately after the stimulus has ceased to act but may persist for some time.
In the higher types of reflex, where each sensory neuron is connected with several motor neurons, an entering peripheral impulse may put into action many responsive mechanisms. Indeed so closely interrelated are all the neurons that a single sensory impulse is theoretically capable of affecting all the motor cells of the central nervous system. That this conception is not entirely hypothetical is seen in cases of strychnine poisoning where the excitability of the neurons is greatly increased. In such cases the slightest stimulus, such as a faint noise, a draught of air, etc., suffices to throw the patient into general convulsions.
It is apparent then that an entering impulse must select definite paths amongst the network of interlacing fibres. Certain of these paths are determined in the lower forms of animal life as accumulated experiences of the most satisfatory reflex arcs absolutely essential to the preservation of existence, e.g. the reflexes which control the circulation and respiration. Other paths have been developed first as conscious habits; when the habit is fully established the element of consciousness sinks into the background, and, if brought forward at any subsequent time, proves a hindrance rather than a help; for example the awkward gait of an individual who is conscious of the fact that his method of walking is under observation.
In considering the complex type of reflex pathway, another action of the reflex centre becomes apparent. If a decapitated frog be suspended so that the limbs hang free, and areas on the skin be stimulated (usually by the application of small pieces of paper soaked in an irritant), purposeful efforts are made by the limbs to remove the stimulant. This necessitates the presence of some arrangement, independent of the brain, by which orderly, intentional movements can be carried out. Since the sensory arm of the arc communicates with several motor neurons, it follows that every motor neuron is brought under the influences of several sensory paths. Hopeless confusion and disorder in muscular movements is prevented by the power which the reflex centre has of inhibiting (i.e. blocking) certain pathways. In the ordinary reflex, therefore, only such impulses are allowed to pass as will produce an orderly response; hence these reflexes are designated co-ordinated reflexes.
To indicate briefly how closely interrelated are the groups of neurons, and how delicately balanced, two easily demonstrable reflex phenomena may be cited:
(1) a simple reflex may be the starting point for a succession of coordinated movements, e.g. the swallowing reflex, where a mass of food is driven down to the stomach by a series of regular progressive contractions of the gullet;
(2) if the sole of t1he foot of a sleeping person be tickled, the leg is drawn up and after an interval is again straightened out. The return to the straight position is not merely a passive falling back of the limb but is an orderly movement, showing that the first reflex has initiated a second reflex producing an opposite effect. A similar phenomenon occurs in the reflex movements of ordinary walking.
Human nervous system
The Animal Cell
Nerve Cells and Nerve Fibres
Nature of the Nerve Impulse
Specific Energy of Nerves
General Construction and Development of the Nervous System
The Spinal Cord
The Chief Fibre Systems of the Cerebro-Spinal Axis
The Areas of Localisation on the Cerebral Cortex
The Sense Organs
Human Brain Anatomy
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