20.2Central Nervous System and Control
General nature of sensation
It is thought that animals formed the central nervous system, which is packed densely with neurons, within the head in order to enhance signal processing efficiency. Invertebrates have ganglion nodes in the abdomen and adoral zone to process information from their eyes located on the head, antennae and tentacles, and chemical receptor sensory organs, whereas vertebrates have an even more sophisticated central nervous system. In higher animals the swelling at the front end of the neural tube differentiates into the forebrain, midbrain, and hindbrain. Functionally, areas of the brain handle different roles such as reception of input (from sensory organs), information processing, behavioral control, etc. The brains of higher vertebrates have countless more neurons than their spinal cords do, and are tasked with overseeing the entire nervous system.
Figure 20-8 Assignment of signals
A mechanism in which multiple sensory receptors function simultaneously and signals from numerous receptor cells reacting to the same stimulus are selected and collected by a small number of secondary sensory neurons. Olfactory glomeruli (right diagram) in the olfactory bulb are where cells carrying out such signal selection and collection work concentrate. One olfactory glomerulus receives nervous inputs from average 26,000 olfactory cells. Dozens of secondary neurons collect these signals and send them to the olfactory center.
Photoreceptive mechanism of visual cells
Pigment epithelium is located just outside the retina, under which three main types of cells–visual, bipolar, and ganglion–are laminated toward the interior (Column Figure 20-1). There are two types of visual cells, rod and cone, which react to light with different wavelength and intensity. Important features related to visual functions such as identification of color and adaptation to light and dark can be well explained by the respective characteristics of visual cells.
Molecular level studies on the rod-type visual cell have revealed its outstanding light signal amplification functions. Retinal molecules, which are light receptors, can react to the energy of only a few photons (smallest unit of light energy) and change its molecular conformation. This change in structure then triggers a change in the structure of the light receptor protein rhodopsin. The modified rhodopsin then activates a type of G protein called transducin one after the other. At this point, the light signal is believed to be amplified 100 to 1000 times. The activated transducin divides into α, β, and γ subunits. The α subunits then bind with a cyclic guanosine monophosphate (cGMP)-degrading enzyme and activate it. This activated enzyme breaks down 10,000 to 100,000 molecules of cGMP every second. Thus, high sensitivity can be achieved even with a weak light signal by such amplification actions using this series of chemical reactions.
The rod type visual cell has cation channels (allow Na+ and other cations such as K+, Ca2+, and Mg2+ to pass), which open when bound to cGMP; however, as cGMP is broken down according to the volume of received light signals, these cation channels become more closed (higher frequency of closed state) by strong light stimuli. This change is manifested as hyperpolarization of the membrane potential (membrane potential changes to negative side). As a result, the Ca2+ channel dependent on the membrane potential existing in the synapse closes (open when in the depolarized state with no light). This causes the intracellular Ca2+ concentration to drop, leading to the decrease in the amount of neurotransmitters (glutamic acid) released from visual cells. Signals from visual cells that detect light are sent to the bipolar cells through these complex amplification and nerve transmission processes and then are transmitted to the ganglion cells and brain. The Ca2+ flowing into the rod visual cells through these cation channels have separate functions. They activate the phosphorylation of rhodopsin and inhibit the activation of transducin. They are also known to inhibit the cGMP synthesis speed. These functions are involved in the mechanism that finely tunes light sensitivity such as light adaptation and dark adaptation.
Figure 20-9 Evolution of central nervous system
Side view (vertical section in humans) of the central nervous system. The cerebrum at the surface derives from the part called telencephalon. In amphibians and reptiles, the cerebrum is small and has a smooth surface. In these animals, the part called the optic lobe (i.e., optic tectum or colliculus superior) carries out integration of most sensory information input, rather than the cerebrum. It is an important part controlling output of commands to effectors. In birds and mammals, the cerebrum developed to make up a large part of the brain and became responsible for the same functions as the optic lobe. This may be due to the increased needs for information processing with the development of senses such as smell and vision in these animals. In humans, the neocortex makes up a very large area of the brain. The paleocortex and primitive cortex portion such as olfactory brain and hippocampus thought to have functioned from ancient times were pushed inside the brain with the development of the neocortex.
All vertebrae share the same basic brain structure (Figure 20-9). The most posterior portion, connecting the spinal cord and brain, is the hindbrain, consisting of the cerebellum, pons, and medulla oblongata . This is an important area that controls autonomic functions such as respiration, circulation, urination and vomiting. Damage to this area is fatal. The cerebellum joins the cerebral cortex, spinal cord and nerve fibers and integrates motor activity. It receives input on posture information such as muscle tension and joint sensation as well as visual and auditory information and aids muscular contraction control of the motor cortex to maintain posture, carry out positional control in space, and enable swift and highly efficient limb motion. Birds are known to have a well-developed cerebellum for complicated flight control and high-speed posture maintenance. The cerebellum is also deeply connected to the acquisition of motor skills.
The midbrain forms the hypothalamus, which is the nerve center involved in the control of various basic motions in living, such as posture control, eyeball movement, and gait motion, as well as in homeostasis. In humans, the midbrain and hindbrain (pons and medulla oblongata) control basic functions for survival. The combined area is called the brainstem.
The area before the optic lobe (in humans, this is equivalent to the optic tectum and colliculus superior) is the forebrain (Figure 20-9, left). In fish and amphibians, the optic lobe makes up a large part of the brain. This portion is linked to many regions of the brain by neurons. Inputs from various sensory organs are integrated here, and projected as inter-correlated positional information. In fish and amphibians, it serves as a vital brain region controlling various instinctive behaviors.
Functions of neocortex
Figure 20-10 Structure of cerebral neocortex
Six-layer structure parallel to the surface. Some neuron cells are seen to be stained black in Golgi silver staining. The Nissl technique stains only the cell body while the Weigert’s technique stains only the myelinated nerve fiber. (Excerpted from Neurological Anatomy in Relation to Clinical Medicine (3rd ed.) (Brodal A.). Oxford University Press 1981, p789.
The cerebrum is a part of the central nervous system derived from the forebrain. In lower vertebrates such as the frog, no marked behavioral changes are noted even if the cerebrum is surgically removed. However, in birds and mammals, the cerebrum has developed to serve as an important information processing site instead of the optic lobe. There are important neuronal cell layers in the cerebral cortex; these layers are folded to increase the surface area to diversify functions (Figure 20-10). The cerebrum in mammals has developed so large covering up the diencephalon (thalamus, hypothalamus, see Figure 20-9), and midbrain, which play a significant role in lower vertebrates. The cerebral cortex can be divided into the old cortex, which was already formed during the age of lower vertebrates, and new cortex, which differentiated later. The old cortex is the area that derives from the forebrain first in the development process, and consists of the paleocortex including the olfactory brain, hippocampus, primitive cortex of the corpus callosum gray matter, and limbic cortex.
One major feature of the cortex is the localization of functions. Different regions in the neocortex are only responsible for specific information processing functions. Some regions only receive input from sensory receptors. The part of the neocortex receiving input from sensory nerves is called the primary sensory cortex. There are also other sensory cortices that process the information received and output it elsewhere. The types of sensations most important to an animal differ by species. For example, in humans, they include vision and tactile sensation of the hand; to rats, they include olfaction and tactile sensation of the whiskers. The more important a particular sensory cortex is to a species, the greater relative area it will have in the cerebral cortex. In other words, the relative size of each area in the cerebral cortex corresponds with the complexity of information processing being carried out there. Consequently, it is thought that the various functions of the neocortex were not achieved by creating diverse purpose built neuron connections which differ by processing functions, but by increasing the number of sets of neuron circuits with the same structure (Figure 20-10).
Some of the other areas of the cerebral cortex function as motor cortex, performing only movement control. Movement of the various skeletal muscles is controlled by motor neurons extending toward the spinal cord. In lower mammals, the sensory cortex and motor cortex alone make up most of the cerebral cortex (Figure 20-11). However, primates like humans have a well developed area referred to as the association cortex, which belongs to neither the sensory or motor cortex. This area handles such roles as integration of information between multiple sensations, memory, guessing, thought, and communication with other individuals.
The localization of functions in the motor and sensory cortices can be understood by accurately tracking the input and output tracks of neurons. However, it has been difficult to map the locality of higher functions such as language, recognition, and behavior. Previously, scientists thought that various higher functions are achieved as the comprehensive ability of the brain as a whole; however, by studying the accurate correlation between behavioral disorders and damaged brain areas in patients with brain damage, locality is gradually being mapped. Broca's area, which is a motor speech area, was discovered from aphasic patients who were unable to talk although they understood the words of others. From patients who lost the ability to understand spoken words, a sensory language area called Wernicke's area was discovered. Locality can also be studied from records on electrical activity and local electrical stimulation. Today, regionality of the cerebral cortex can be studied by precise imaging of brain activity using technologies like functional magnetic resonance imaging (fMRI) and positron emission tomography (PET), and slowly data on the function localization map of the human brain is being accumulated. We have been wrong to think that various sensations and all mental activities rely solely on a particular brain area. However, many functions are realized via neuronal networks in the brain and by the connection of neurons, which are distant from each other.
Figure 20-11 Localization of functions in cerebral cortex
In the cerebral cortex, input signals from the sensory receptors are received at the primary sensory cortex such as visual area, auditory area, gustation area, etc. Areas of the cerebral cortex related to motor control and expression are called primary motor and higher motor cortices. Various information processing is carried out in areas other than primary sensory cortex and primary motor cortex. These areas are called association areas and they make up a large portion of the cerebral cortex in primates.
Hierarchical structure of neocortex
Along with the locality of cerebral functions, the hierarchical structure is also a major feature of the brain. The following section discusses this hierarchical structure using the visual information processing mechanism of primates as an example.
Various visual information processes are carried out in the retina, but most of its signals are transmitted to the primary visual cortex via structures called the optic nerve and lateral geniculate body (Figure 20-12). Some routes do not pass through this lateral geniculate body, including those projecting to the cerebral cortex via the superior colliculus (Figure 20-9) (related to unconscious visual functions) and visions that are projected to regions other than the cerebral cortex (such as pupillary reflex and circadian rhythm). All of the images that we are aware of as vision, however, are signals that arrive at the primary visual area via the lateral geniculate body.
The cerebral cortex consists of layered structures that are very much alike but have different roles. There are the primary sensory areas that process the simplest functions projected from sensory cells and higher sensory centers performing more complex activities such as extraction of features, determination of stimulus type, and referencing. These areas are located in layers adjacent to each other and stimulus information is exchanged regularly, eventually allowing higher information processing to be carried out. The ability of the brain to carry out various information processes in the same region, functioning as “hardware,” can sometimes be likened to the computer, which also performs varied activities using different programs running on one CPU. However, one major difference between the human brain and computer is that the brain is able to carry out parallel processing, as well as fuzzy processing that is incorrect at times.
Understanding mechanisms of learning and memory are very important for studying the relationship between the functions of the central nervous system and behaviors of animals. Presently, not all molecular and cellular mechanisms involved in learning and memory are known. One example of the simplest learning ability of animals can be explained by the synaptic plasticity of neurons. For example, sea hares draw in their gills when exposed to mechanical stimuli; however, this reaction disappears when the stimulus is repeated. This is because the motor neural synapse involved in this synaptic pathway gradually decreases the efficiency of neural transmission to the gill muscle. This is thought to be due to intracellular Ca2+ and pH changes caused by continued neuronal excitation, which weakens the responsive characteristics of receptor channels. There are small protrusions called spines on the surface of neuron dendrites and these form synaptic junctions with other neurons. Spines are thought to shrink, elongate, and change their thickness to change the efficiency of neural transmission in the synapse. Such changes in the excitation and inhibitory transmission efficiency in synapses are called synapse plasticity. This is thought to be closely connected to the mechanism of short-term memory, which remembers shapes, words, and numbers in a short period of time of several minutes.
On the other hand, long-term memory is memory that is stored in the cerebral cortex semi-permanently and the hippocampus in the limbic system plays an important role in the formation of long-term memory. A patient, who had to have the hippocampus surgically removed to treat a brain disorder, experienced no problems with normal short-term memory and motor functions, but lost the ability to form long-term memory after the operation. Moreover, animal studies have revealed that changes such as synaptic long-term potentiation (LTP) and long-term depression (LTD) occur easily in the hippocampus. Therefore, changes in the synapse transmission efficiency caused by LTP or LTD are thought to be related to the formation of long-term memory, although many aspects of this mechanism still remain unclear today.