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Introduction to the Nervous System 2 From Introduction to the Nervous System 1, we want to get these concepts: At any moment of life, afferent neurons are bringing to the brain and spinal cord a variable wave of excitation. The wave is variable because different receptors are being stimulated at any particular moment in time. The receptors have their origin in stimuli that arise outside the body, e.g., heat, light, sound; and in stimuli that have their origin inside the body, e.g., pH, proprioception, pressure, pain. Afferent neurons are excitatory. Every synapse that they make with other neurons tends to excite those neurons. With one exception (the stretch reflex), the synapses of afferent neurons are with interneurons (defined for our purposes as neurons that are entirely within the CNS). Therefore, a single afferent neuron extends between a receptor and the CNS. Within the CNS, the interneurons are in a vast and complex array of connecting pathways. Different from afferent neurons, which are essentially all excitatory, interneurons may be excitatory or inhibitory to the neurons with which each synapses and it is in this way that the wave of excitation brought to the CNS yields a varying response by the effectors of the body. Note: An excitatory neuron will be excitatory at every one of its synapses. An inhibitory neuron will be inhibitory at every one of its synapses. The effectors of the body are muscle and gland. A single efferent neuron extends from the CNS to striated muscle fibers. A chain of two neurons extends from the CNS to autonomic effectors: smooth muscle, heart muscle, and gland. Activities such as consciousness, dreaming, thinking , mechanisms of attention, etc., take place within the CNS itself. They are mechanisms of interneurons, a part of, and have their effect on, interneuronal circuitry within the CNS. By having excitatory and inhibitory neurons, the interneuronal circuitry modulates the wave of afferent excitation and brings about the variable contraction of muscle and secretion of glands that constitute the actions of the living animal. What determines that a neuron reaches the excitatory state? A neuron effects its synapses with other neurons by terminal and collateral bulblike structures of its axon called boutons. Depending on its function a single interneuron may have synapses with a few neurons, with hundreds, or with thousands of other neurons. The body and dendrites of a neuron within the CNS are covered with synaptic boutons of both excitatory and inhibitory neurons. In any small area of the cell body or dendrites of a neuron, the boutons of many neurons will make synaptic contact. The post-synaptic neuron will reach the excitatory state only if a sufficient area of its cell membrane is depolarized within a sufficiently brief period of time. These two features, depolarization of the neuron’s cell membrane covering a sufficient area and the depolarization’s 1 occurring over a sufficiently brief period of time are designated spatial summation and temporal summation respectively. When spatial and temporal summation occur, threshold is reached and the excitatory state passes as a wave of depolarization over the entire cell membrane. The neuron will “fire”, that is the excitatory state will propagate from the body or dendrite of the cell to depolarize (excite) the entire cell. Reaching the axon of the cell, the excitatory state extends peripherally along the axon to its contact with other neurons or, in the case of efferent neurons that innervate striated muscle, to myofibers; in the case of the first neuron of the two-neuron autonomic efferent chain, to neurons in autonomic ganglia. Inhibitory neurons oppose this depolarization. At synapses of inhibitory neurons, the cell membrane is hyperpolarized; that is, made more resistant to excitation. bouton bouton dendrite of postsynaptic neuron Electron micrograph of synaptic boutons in contact with a dendrite. Note the synaptic vesicles. Photo is from W. J. Banks’ Applied Veterinary Histology; 1993 Mosby Probably no interneuron or efferent neuron can be brought to threshold by a single excitatory neuron. Each neuron, its cell body and dendrites covered with 2 boutons, is subject to excitatory and inhibitory stimulation. When conditions of time and proximity of excitation result in threshold stimulation, it “fires” and carries impulses (the excitatory state) to all of its synapses. If it is an excitatory interneuron, every one of these synapses will be excitatory. If it is an inhibitory interneuron, every one of these synapses will be inhibitory. If it is an efferent neuron to striated muscle, each of its neuroeffector synapses will be excitatory at the motor endplate. Consciousness is awareness of the stimulus. The total nature of consciousness is not fully known; but it is unquestioned that it results from excitation of cerebral cortical neurons (neurons in the outer layer of grey matter, the cortex, of the cerebral hemisphere). Therefore, stimuli that are consciously perceived have interneuronal pathways that excite neurons in the cerebral cortex. If the excitation of these cortical neurons results in action, that action is designated a response or a conditioned reflex. Such an action’s taking place due to the animal’s perception of stimuli is a learned response. Such actions are present only after the animal has learned the appropriate response. They are to be distinguished from pathways that result in action but have not reached the cerebral cortex. For example, the “patellar reflex” can be elicited in the unconscious animal. Such an action is a reflex; or it may be designated an unconditioned reflex. Reflexes, also designated unconditioned reflexes, are unlearned responses and are present as soon as the neuronal pathways are functional. Consider the racing horse. It is running hard when a proximal sesamoid bone of its right forelimb suddenly fractures. Pain and other stimuli result in increased inhibition to the motor neurons innervating antigravity muscles of its injured limb. The horse reduces or avoids entirely using the extensor muscles that permit the horse’s placing weight on the injured limb. The horse quickly learns that bearing weight on the limb increases the pain that it feels. Impulses arising from pain receptors pass by afferent neurons to the spinal cord (CNS) and synapse with interneurons. Interneurons in this case provide a number of different pathways but we shall consider only a few of them. The horse is aware of the pain; therefore, a pathway led to the cerebral cortex and the horse consciously withdrew the limb to avoid pain. Pain also gives rise to avoidance reflexes that would result in flexion of the limb even in the unconscious animal. Neurons innervating extensor muscles of the limb, antagonists of the flexors, would be subject to interneuronal inhibition. Memory, learning, thinking all require consciousness = excitation of cortical neurons. Dreaming is the excitation of cortical neurons without the external stimulus being present. Thinking appears to be (key words) an auditory phenomenon; that is, you “hear” (excitation of cortical neurons without the external stimulus being present) what you think. 3 Voluntary action is also called voluntary motor activity. It is the conscious activation of skeletal muscle. It is obviously the result of thinking and therefore must have its origin in the cerebral cortex. Its origin is by excitation of interneurons in an area of the cerebral cortex designated the motor cortex. All stimuli ultimately contribute to effector action. Those that are consciously appreciated utilize pathways that traverse the cerebral cortex and bring about action designated a response (conditioned reflex). Stimuli whose interneuronal pathways to effectors are limited to the lower parts of the brain or the spinal cord bring about effector action designated a reflex (unconditioned reflex). A stimulus can give rise to interneuronal pathways that lead to both a response and a reflex. For example, a bright light shines into the animal’s eye. The pupil constricts, a reflex. The animal attempts to turn its head, a learned response to the brightness of the light. The patellar reflex. This reflex takes place in the anesthetized animal. Therefore, it does not require consciousness. The anesthetized animal is not aware of the receptors stimulated to bring about this reflex; the animal is not aware that the reflex has taken place. The conscious animal is aware that it has carried out the reflex. This awareness is due to activation of muscle, tendon, and joint receptors (proprioceptors) and perhaps due to cutaneous receptors stimulated by bending and stretching of the skin. Of course, the conscious animal has also felt the tap of the reflex hammer. The effectors are chiefly (not entirely, as other muscles, chiefly the cranial part of the biceps femoris, are also stretched by the patellar tap) myofibers of the quadriceps femoris muscle. So, for every efferent neuron stimulated, probably several hundred myofibers contract. In spinal cord injury resulting in the interruption of descending motor pathways originating from the brain, the patellar reflex is exaggerated; it is stronger. This is due to loss of inhibition from certain descending motor pathways. In this case, it is easier for the stimulation provided by the patellar tap to bring about reflex contraction of the quadriceps (and the cranial part of the biceps femoris). Put another way, it is easier for efferent neurons innervating the affected muscles to reach threshold. A question. A dog has been trained to remain calm while a single rabbit crosses its path. The owner releases three rabbits in front of the dog; the dog fidgets but remains in place. The receptor by which the dog sees the rabbits is? The effectors to carry out running movements of the limbs are? The normal reaction of the untrained dog would be to chase the rabbits. This reaction, were it to take place, would involve a voluntary act of the dog brought about by circuitry that is within its cerebral cortex and resulting in descending excitation passing from neurons in its cerebral cortex to efferent neurons to the appropriate muscles of the dog’s limbs. The dog’s remaining in place while the rabbits are released must be due to inhibitory synapses ending on……? 4 Relatively few of the stimuli that stimulate receptors of the body are consciously perceived: visual stimuli; auditory stimuli; PTT = pain, tactile and temperature stimuli, proprioceptive stimuli (tell the length and rate of change in length of striated muscles, pressure and tension receptors in joints); taste; smell; force of gravity. Pathways proceeding from excitation of these receptors pass to specific areas of the cerebral cortex: visual cortex, auditory cortex, somatosensory cortex (ptt + proprioception), olfactory cortex. The cortical area to which interneuronal pathways of receptors for taste and the force of gravity pass are less clear. Figure is from Lehrbuch der Anatomie der Haustiere, Band IV; Nickel, Schummer, Seiferle, G. Böhme, Ed., 1992; verlag Paul Parey. Motor c. Somatosensory (PTT + proprioception) c. Visual c. Auditory c. Olfactory c. Canine Brain. Areas of the cerebral cortex to which interneuronal pathways pass from synapses with afferent neurons of the specified receptors. The motor cortex (Area motorica, Motor c.) is the area of the cortex from which interneuronal pathways lead to efferent neurons that supply striated muscle. A dog encounters a bear. The hair on the back of the dog’s neck stands up, its heart beats faster, its adrenal gland secretes norepinephrine and its blood glucose level rises… The stimulus is the look and smell of the bear, perhaps the noise that the bear makes. The reflex result is the contraction of smooth muscles of the hair follicles and increased rate of contraction of heart muscle, reflex stimulation of the adrenal medulla… The dog also will adopt a particular 5 expression and defensive posture, etc. This dog has never before encountered a bear. These actions must be a reflex or response? In examining the animal, only a few actions/activities are relatively easily determined… Consciousness and behavior; Standing attitude (posture) and ambulation (how the animal moves); Animal’s action or lack of it in response to pain, tactile, proprioceptive, visual, and auditory stimuli, and temperature; Facial expression, eye movement and the resting position of the eyes; Appearance of the pupil; Appearance of the tongue, ability to swallow; Postural, limb, and anal reflexes; Atrophy of muscle, integrity of muscle reflexes. Less easily: taste, olfaction. 6