BIOLOGICAL, PHYSIOLOGICAL AND HEALTH SCIENCES - SAMPLE CHAPTERS
Simo S. Oja
University of Tampere Medical School, Finland, and Tampere University Hospital, Finland
University of Tampere Medical School, Finland
Keywords: sensory receptors, sensory modalities, specific senses, motor control, associative and integrative functions, emotions
1. Introduction and Overview of the Nervous System
2. Sensory Functions
3. Motor Functions
4. Integrative Functions
This article presents an overview of the functions of the nervous system. The topics dealt with include peripheral receptors, processing of sensory information in the central nervous system, the functions of specific senses, the central control of movement, brain associative and integrative functions, generation of emotions and theories about learning and memory.
1. Introduction and Overview of the Nervous System
The nervous system is a communication network that allows information transfer and processing in the organism. Neurophysiology is a discipline that deals with this aspect of animal and human physiology. The nervous system—a vast complex array of cellular networks—is connected to virtually all parts of the organism. It handles millions of bits of information in a time unit. The information reception from the environment and from the organism itself through different channels, and its primary processing in the central nervous system, is called sensory physiology. Based on this flow of information we can react in an appropriate manner to environmental and bodily events. Another important role of the nervous system is to control various bodily activities, such as motor functions and regulation of the secretion of endocrine and exocrine glands. Most of these activities are also initiated by stimuli emanating from sensory receptors. The integrative functions of the nervous system comprise complex processing of sensory information, association and interpretation of different types of signals, information selection and storage for future use, advance planning, and elicitation of behavioral responses. The nervous system, the brain in particular, is the most complex tissue in the organism. The understanding of the complexity of nervous system functions is therefore one of the greatest challenges for humanity.
The nervous system is divided into two major parts: the central and peripheral nervous systems. The central nervous system can be divided into seven major regions, which exhibit distinctly different functional propensities:
The spinal cord, the most caudal part, receives sensory information mediated by the peripheral nerves from various receptors, exteroceptors located in the skin, and from proprioceptors in the joints, muscles, and tendons. The spinal cord also mediates motor impulses from the higher regions to muscles and possesses motor functions of its own (i.e., reflexes).
The medulla oblongata directly above the spinal cord mediates both sensory and motor signals, but also includes centers responsible for the regulation of many vital autonomic functions, such as circulation, respiration, and digestion.
The pons in the metencephalon, above the medulla, conveys motor signals from the cerebrum to the cerebellum and possesses specific nuclei for the regulation of autonomic functions as well.
The cerebellum is a dorsal expansion of the metencephalon with a heavily gyrated surface. It is connected to the brain stem with three peduncles. It modulates motor functions, being mainly responsible for the learning of motor skills.
The mesencephalon, rostral to the pons, controls many sensory and motor functions and coordinates visual and auditory reflexes.
The diencephalon consists of two major parts: the thalamus and the hypothalamus. The former is an important link mediating information from the lower parts of the nervous system to the cerebral cortex and the latter an important center for the regulation of visceral and autonomic functions, and secretion of the endocrine glands.
The cerebrum is the most rostral part, consisting of a pair of cerebral hemispheres. The relatively great mass of the cerebral hemispheres in comparison to other brain parts and, in particular, the expanded surface of the cerebral cortex, are characteristics of the human brain. The surface of the cerebrum—the cerebral cortex—is heavily gyrated in man, greatly expanding its surface area. The cerebral cortex is the major region for sensory coding, association, and interpretation, and is responsible for the performance of motor and integrative functions, and cognition. The basal ganglia, which participate in the regulation of motor functions, the hippocampus, obviously important for information storage and memory, and the amygdaloid nuclei, which underlie the generation of different emotional states, are located in the interior of the cerebral hemispheres.
All nerve cell processes outside the central nervous system, with their associated cellular elements, are included in the peripheral nervous system. Their main role is to connect the central nervous system to the peripheral organs from which information is received and/or the functions of which are regulated by the central nervous system. The autonomic nervous system is a part of the nervous system designed to control visceral functions of the body. It consists of elements from both the central and peripheral nervous system. The centers located in the spinal cord, brain stem, and hypothalamus activate the autonomic nervous system. However, the activation of the above centers is partly under the control of the limbic cerebral cortex. Peripheral ganglia are also essential parts of the autonomic nervous system. The autonomic nervous system comprises two major subdivisions: the sympathetic and parasympathetic parts. Their functions are, in most cases, the opposite of each other (see Autonomous Neural Regulation). (For the main cell types of the nervous system, neurons, and glial cells, see Structural Neurobiology. For the functions of neurons responsible for impulse generation, propagation, and transmission, see Neurons, Action Potentials, and Synapses. Impulse transmission from neuron to neuron is mediated electrically in the minority of synaptic junctions, while the majority are chemical in nature, employing specific substances as mediators—neurotransmitters—see Neurotransmitters and Modulators.)
2. Sensory Functions
2.1. Peripheral Receptors
A variety of sensory receptors mediate information from the environment and from the interior of the body to the central nervous system (see Table 1). Different forms of energy first evoke analogical changes in their membrane potentials (receptor potentials). These are then converted into digital signals—action potentials—which are propagated along the neuronal plasma membranes over long distances to reach their destination in the central nervous system. The sensory receptors are adapted to respond to one particular type of energy and therefore exhibit a whole spectrum of structural and functional characteristics. The particular form of energy to which a receptor is most sensitive is known as its adequate stimulus. For instance, the adequate stimulus for a touch receptor is mechanical force and for a photoreceptor is electromagnetic radiation—light. A receptor responds to its adequate stimulus at a much lower threshold than to other types of stimuli, but responses to unspecific stimuli commonly occur at high stimulus intensities. The stimulus-evoked excitation of receptors most often opens ion channels in their plasma membranes, resulting in depolarization (see Ionic Channels of the Excitable Membrane), but light impinging on photoreceptors closes ion channels and causes hyper polarization.
Table 1. Sensory receptors, their modalities and locations Notes: The nerve endings in the skin form histologically recognizable Pacini’s corpuscles, Merkel’s disks, Ruffini’s endings, Krause’s end-bulbs, and Meissner’s corpuscles. These structures seem to function as mechanoreceptors responding to tactile stimuli. As an example, the structure of a Pacinian corpuscle is shown in Figure1.
The receptors can be classified in different ways. One traditional classification divides them into teleceptors, which receive information from a distance, exteroceptors, which respond to stimuli near the body, and proprioceptors, which respond to stimuli from the interior of the body. Some receptors are located in specific sensory organs ("specific senses"), whereas others are more widely distributed in the organism. A number of receptors responsible for different sensory modalities are compiled in Table 1 with their adequate stimuli and locations in the body. Within certain limits the sensory sensation is approximately related to the logarithm of the magnitude of stimulus (the classic Feber-Wechner’s law). More accurately the intensity of sensation (R) is described by the equation R = KSA, in which S is the magnitude of stimulus and K and A constants. The frequency of action potentials generated approximately obeys a similar equation.
Adaptation is a characteristic propensity of many receptors. In rapidly adapting receptors, a stimulus produces only a short-lived response that rapidly fades out during continuous stimulation. In slowly adapting receptors, the response is a repetitive discharge on prolonged stimulation. The first type of receptor is adapted to detect fast changes and the latter type is best suited for the maintenance of constant responses to internal or environmental effects. The receptive field of a receptor defines the region in which stimulation elicits its activation. The receptive fields of adjacent receptors frequently partly overlap. In the sensory (and also motor) pathways, divergence at various second- or third-order levels provides a means to generate receptive fields at the upper levels of the central nervous system that are larger than those of the first-order afferent neurons. At the same time, convergence of other neurons yields a certain degree of overlap (see Figure 1). Inhibitory mechanisms are also very important for the accurate localization and discrimination of different stimuli. Recurrent axon branches from adjacent neurons in a pathway can inhibit each other, providing a phenomenon known as lateral inhibition. With the aid of lateral inhibition, the targets of motor pathways can be similarly focused.
Figure 1. Diverging (A) and converging (B) neural networks Notes: One primary afferent neuron is able to activate by means of divergence several second-order neurons, which then each in turn activate several third-order neurons. On the other hand, in the case of convergence, several first-order neurons target on one third-order neuron.
The best-characterized receptors are located in the skin where a variety of receptors have been described by histological methods. The specificity of these receptors to different types of stimuli is not yet fully established. Figure 2 shows the structure of a typical skin receptor: the Pacinian corpuscle. In the skin, the receptor density is generally directly related to the ability to locate stimuli accurately and to discriminate between two stimuli that are close to each other. The number of second-order and third-order neurons is correspondingly in proportion to the density of first-order afferent neurons. At the most central level in the cerebral cortex, the size of the areas representing the different parts of the body is proportional to the density of the first-order sensory neurons. A sensory homunculus drawn over the sensory cerebral cortex, therefore, does not reflect the relative sizes of various body parts, but the number of sensory receptors in them.
Figure 2. Structure of a Pacinian corpuscle Notes: The primary nerve ending is encapsulated with concentric onion-like lamellae of connective tissue, which function as amplifiers of the mechanical pressure applied to the receptor. The pressure dislodges the layers and that generates alterations in the nerve membrane potential (generator potentials), which then evoke a burst of digital action potentials in the first node of Ranvier inside the corpuscle.
2.2. Somatosensory Pathways
Sensory information acquired from different receptors is collected, analyzed, and integrated in the brain. The impulses from the skin receptors and proprioceptors are first transmitted along peripheral sensory nerves to the spinal cord. From the facial regions the pathways terminate in the brain stem. The receptive fields on the surface of the body have been mapped and they show well-defined organization of skin dermatomes (see Figure 3). The dorsal column (medial lemniscal) pathways (fasciculus gracilis and fasciculus cuneatus) in the spinal cord are the most straightforward routes from proprioceptors and fine-touch receptors to the brain (see Figure 4). The first-order axons in these pathways do not cross the midline at the level of the spinal cord but project at the same side (ipsilaterally) up to the dorsal column nuclei in the medulla oblongata. Only there, the axons of the second-order neurons cross to the other side (decussate), synapse in the thalamus with the third-order relay interneurons, and the pathway finally terminates in the contralateral cerebral cortex.
Figure 3. Dermatomes in a human bodyNotes: Each spinal nerve innervates a restricted region of the skin known as a dermatome. The letters and numbers refer to different spinal cord segments. C = cervical segment, T = thoracic segment, L = lumbar segment, and S = sacral segment. The segments are numbered in ascending order from the rostral to the caudal end. Note that the dermatomes are nicely arranged in series if the human body is illustrated in a quadrupedal position.
Figure 4. Locations of the major ascending afferent (sensory) tracts (left-hand side) and descending efferent (motor) tracts (right-hand side) in the spinal cord
The presence of three successive neurons in a sensory pathway is a general rule with only a few exceptions (e.g., the olfactory and auditory pathways). Along the whole course of these sensory pathways, the projections from different parts of the body and from different receptor types are more or less segregated. This is the basis for the ability to locate the origin of sensory stimulation and for the discrimination of various sensory modalities. The brain interprets the nature of a stimulus by identifying the specific pathway that propagates the information. The sensations reaching the cerebral cortex can be consciously detected. The vast majority of sensory signals remain unrecognized, already filtered during their course in the thalamic relay nuclei or discarded in the cortex as insignificant.
The anterolateral spinothalamic pathways mediate other sensory modalities, for example, warmth, cold, pain, and sexual sensations, through the thalamus to the cerebral cortex. Their axons cross to the opposite side in the same spinal cord segment into which they primarily enter. There are also a number of afferent sensory pathways that do not terminate in the cerebral cortex. The most important in the control of motor functions are those that convey impulses from proprioceptors to the cerebellum (i.e., the dorsal and ventral corticospinal tracts). Their axons are the thickest and their velocity of conduction the greatest among all tracts of the human body. This propensity emphasizes the importance of fast reception from the periphery of the knowledge of execution of motor activities initiated by the brain.
A population of specific receptors called nociceptors senses pain. Morphologically, they are naked nerve endings. Furthermore, a very strong stimulation of other sensory receptors is prone to provoke pain. The superficial pain originates from the body surface; pain arising from muscles, tendons, joints, and bone is deep pain. Both of them are collectively called somatic pain. Pain arising from the internal organs is visceral pain. Pain impulses are transmitted into the central nervous system by two fiber systems; one of them consists of relatively small myelinated fibers and the other of even thinner unmyelinated fibers. The former conduct impulses faster than the latter. This dichotomy explains the two kinds of pain: a sharp, localized sensation followed by a dull and diffuse pain. Deep pain and visceral pain are poorly localized. Visceral pain is also often felt in some somatic structure; a phenomenon known as referred pain. The attenuation and treatment of pain can be affected at several mutually nonexclusive sites: at the level of peripheral receptors, in the dorsal horn of the spinal cord where nociceptive fibers synapse with the dorsal root ganglion cells (the so-called "gate control of pain," and in the brain stem.
2.3. Processing of Visual Information
In the eye, the distribution of photoreceptors, cones, and rods varies markedly in the different parts of the retina. The cones are most densely packed in the center, in the macula lutea where the visual acuity is greatest. The rods predominate in the periphery. The cones are responsible for photopic vision—for the great visual acuity in bright light—whereas the rods give us scotopic vision at low levels of illumination. During dark adaptation the sensory threshold of the cones increases initially, followed by an increase in the sensitivity of the rods. The latter is about 1000 times more effective than the former. Light adaptation is a reverse process aided by the regulation of the iris aperture. Color vision is the task of the cones, owing to the presence of three types of visual pigments—rhodopsins—which differ with respect to their protein components and, therefore, absorb light quanta at different wavelengths. The rods possess only one type of rhodopsin molecule and therefore cannot discriminate between different colors. All visual information is already processed in the relatively complex neuronal networks of the retina. Lateral inhibition produced by horizontal and amacrine cells in the retina is an essential component in this process.
The visual pathways leave the retina from the blind spot in which the optic nerve originates. The optic nerves cross at the optic chiasma located over the adenohypophysis at the bottom of the brain. At this site, the tracts from the nasal side of the retina cross from one side to another, whereas the fibers from the temporal retina project to the ipsilateral side. The visual information is carried to the visual areas of the occipital cerebral cortex via the lateral geniculate bodies in the diencephalon. In them, the projections from the retina are spatially still well arranged. The geniculate bodies have a dual function in the processing of visual information. First, they project to the visual cerebral cortex. Second, they gate the transmission of signals (i.e., control how much signaling is allowed to pass further to the cortex).
Two visual pathways, magnocellular and parvocellular, project from the geniculate bodies to the visual cortex. The former carries signals for detection of movement, depth, and flicker, and the latter signals for color vision, shapes, texture, and fine detail. The primary visual area is located in the most occipital part of the cerebral cortex, surrounded by the secondary visual cortex (the visual association area). In the primary visual cortex, spatial projections from the retina are still preserved. The visual cortex is organized into millions of vertical columns through six microscopically distinct cell layers. These vertical columns have a diameter of 30–50 m m and are the functional units in visual information processing. The responses of different neurons in layer IV in the visual cortex are strikingly dissimilar. The so-called simple cells respond to bars of light, lines, and edges when they have specific orientations. The complex cells have likewise preferred orientations of linear stimuli but they respond maximally when a linear stimulus with an unaltered orientation is moving across the visual field. These cells form the so-called orientation columns with an approximate width of 1 mm. Sensitivity changes in a sequential manner throughout the whole 360 degrees, when moving from one orientation column to the next neighboring column. The primary visual cortex is responsible for the detection of simple contours and shapes, but the interpretation and analysis of complex visual information relies on the visual association area. It is located outside the primary visual area in the occipito-temporal cortex. For instance, in this area the shapes of letters, different objects, and their coloring and texture are deciphered.
The three pairs of extraocular muscles attached to the eyeballs control eye movements. The eyes are able to fix closely to objects in the visual fields, although they are moving. Two mechanisms control fixation movements. First, a person can fix his or her gaze voluntarily to an object. This is done by means of the control of a small cortical area located bilaterally in the premotor cortex. Second, the gaze can be held involuntarily on an object after initial fixation. In this latter process, controlled by specific areas in the secondary visual cortex, a phylogenetically old visual pathway through the superior colliculi is operative. This brain region occupies a central position in the control of eye movements, receiving information from the retina, visual cortex, and cerebellum. The Edinger-Westphal nucleus in the brain stem controls through the oculomotor nerve accommodation of the lens for far and near located objects. A contraction of the ciliary muscle allows relaxation of the lens ligaments and increases the lens curvature and refractive power for a close-up view. With advancing age, the lens becomes more and more stiff and the accommodation ability is gradually lost. The eye aperture—the iris—regulates the amount of light allowed to be shed on the retina. Sympathetic fibers that originate from the superior cervical ganglion regulate the iris diameter.
2.4. Auditory Sensations
The auditory system serves for the detection and analysis of sounds (i.e., pressure waves in air). The range of hearing in a young individual extends to sounds between 20 Hz and 16 kHz, but the higher frequencies are lost on aging. The threshold of hearing is lowest at about 3000 Hz. The normal conversation is in the range of 200–5000 Hz. The auditory system responds to sound intensities over a million-fold range of 120 dB. A silent whisper is about 35 dB and conventional speech around 65 dB. An exposure to sound pressures over 120 dB is painful and damages the sensory apparatus. An incoming sound wave causes back and forth vibrations in the eardrum between the external ear and middle ear. In the middle ear these vibrations are transmitted by means of the lever system of the ossicle chain to the oval widow. This membrane separates the air-filled middle ear from the fluid-filled inner ear. In the inner ear the auditory structures are located in the cochlea in a bony labyrinth. The cochlea is longitudinally divided into three compartments, the scala vestibuli and scala tympani filled with perilymph, and the scala media, which is located between them and contains endolymph. The sound waves traverse the cochlea to another membranous structure—the round window—between the inner ear and middle ear. This arrangement allows the fluid in the cochlea to transmit vibrations to the organ of Corti, which is the sensory region resting on the basilar membrane that separates the scala media from the scala tympani.
The traveling waves in the cochlea travel different distances from the oval window causing the basilar membrane to vibrate maximally at a certain point. This location is furthest from the oval window at the lowest sound frequencies. The sensory receptors are hair cells whose processes—stereocilia—bathe in the endolymph and the bases of the cells in the perilymph. There is a potential difference of about +85 mV between the endolymph and the perilymph owing to the active ion secretion into the former by the stria vascularis. Furthermore, the resting potential of the hair cells is –85 mV with respect to the perilymph. Therefore, the potential across the receptor membranes of the hair cells is as large as 170 mV, and the electrochemical gradients of several cations are correspondingly very pronounced. The hair cells generate action potentials owing to the imposed mechanical vibrations. When the stereocilia bend to one direction, the cation channels are opened and the resulting inward current depolarizes the cell. The bending of stereocilia to the opposite direction causes hyperpolarization. The frequency of action potentials generated is proportionate to the loudness of sound stimuli and the location of the maximal displacement of the basilar membrane is the major determinant for the pitch of a sound.
The afferent neural path from the ear is the eight cranial nerve—nervus acusticus—originating from the ventral and dorsal cochlear nuclei in the medulla oblongata. The auditory impulses pass through a variety of pathways to both the ipsilateral and contralateral auditory cortex in the temporal lobe of the cerebral hemispheres. The relay neurons are located in the superior olivary nucleus, inferior colliculus, and the medial geniculate nucleus in the thalamus. The complex pathways involve several sets of synapses, considerable crossing over, and intermediate processing of the information. There obtains, however, a spatial correlation between the sensory cells in the organ of Corti and the locations in the primary auditory cortex. As with the eye, this spatial correlation can be mapped, and, in this case, a tonotopic map constructed. In the auditory cortex the neurons are also organized in a columnar fashion with binaural processing and alternating patterns of summation and suppression columns. The secondary auditory cortex responsible for interpretation and analysis of acoustic signals surrounds the primary auditory area. The sound localization is already processed mainly at the level of the superior olivary nuclei. It is mainly based on the differences in sound intensities and sound wave phases reaching the two ears.
2.5. Taste and Smell
Taste and smell are chemical senses. They are related to each other. Both taste and smell receptors are chemoreceptors that are stimulated by molecules dissolved in the saliva or nasal mucus. They have some unique properties, being closely associated with emotional states introducing pleasant or unpleasant feelings, and sexual arousal. Although they are separate senses, they nevertheless often operate in concert. For example, the smell of food in fact contributes markedly to its taste.
The sense organs for taste are the taste buds located in humans in the mucosa of the epiglottis, palate, pharynx, and tongue. They are spheroid bodies containing approximately 50 cells of four types, surrounded by epithelial cells. For humans, there are four basic tastes: sweet, salt, sour, and bitter. Sweet is best tasted on the tip of the tongue, salt in the anterolateral parts, sour at the sides and bitter on its back. Bitter and sour are also tasted on the pharynx and epiglottis. However, the degree of localization depends somewhat on the concentration of the stimulating substance. The sour and salty qualities are directly mediated by means of changes in the ion channel functions, whereas sweet and bitter sensations require transduction processes involving second messenger systems. As a final common pathway, the influx of Ca2+ through voltage-dependent Ca2+ channels is enhanced, depolarizing the first-order sensory neuron.
The taste pathways from the taste buds travel through the seventh, ninth, and tenth cranial nerves to the nucleus of the tractus solitarius in the medulla oblongata. The axons of the second-order neurons terminate in contact with the third-order neurons in the relay nuclei of thalamus. The pathway finally ends in the postcentral gyrus of the cerebral cortex.
The olfactory system can discriminate a great variety of odors. Although humans cannot compete with some other species in the functional abilities of their olfactory system, they can discriminate more than 10 000 odors, many of them at very low concentrations in air. It has been estimated that the number of different odorant receptors and encoding genes is enormous, even in humans. The smell modalities are therefore numerous and are not easily amenable for classification. In general, the primary odors have been defined to represent seven different qualities: ethereal, floral, peppermint, camphoraceous, musk, pungent, and putrid.
The olfactory receptors are embedded in a specialized part of the nasal mucosa located in the roof of the nasal cavity near the septum. This part is normally rather poorly ventilated, hence sniffing, which causes an eddy of air currents over the olfactory mucous membrane, facilitating the detection of odors. The signal transduction process of the olfactory stimuli, it has been suggested, involves one or more odorant-binding proteins that concentrate the odorants to the receptors. The binding of odorants to the receptors activates the second messenger systems with a consequent opening of Na+ channels and depolarization of the cells. A salient feature of the process is a rapid adaptation to a constant stimulation.
The axons of the receptors penetrate from the nasal cavity through the sieve-like plate of the cribriform bone into the cranium. There they contact the primary dendrites of mitral cells in the olfactory bulbs, forming complex synapse structures called olfactory glomeruli. The axons of the mitral cells terminate in the olfactory cortex. This pathway is unique among the sensory pathways, because it goes directly into the cerebral cortex, before projecting to the thalamus or other brain parts. The probable reason is that the olfactory system is phylogenetically very old in comparison with the other sensory systems.
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