Chapter3

Part 1
　The nervous system is composed of numerous neurons and glial cells. Neurons have four distinctive compartments: dendrites, for receiving signals from other neurons; the cell body, which contains complex apparatus and receives signals and integrates them; the axon, which is responsible for conducting impulses away from the cell body to other neurons and effectors; and nerve terminals, for the release of neurotransmitters at synapses.
　Neurons have three passive electrical properties that are important to electrical signaling: the resting membrane resistance, the membrane capacitance and the intracellular axial resistance. These passive properties influence the speed at which an action potential is conducted along the axon.
　Nerve fibres can be myelinated or unmyelinated. An action potential (AP) generated on the axon hillock and conducted along the axons is termed a nerve impulse. The characteristics of AP conduction are: physiological integrity as an all-or-nothing signal, isolation, bi-direction and relatively indefatigability. The nerve fibers can be divided into different types according to their electrical properties and their diameters. The conduction velocity of an AP depends mainly on the diameter of a nerve fiber and, if present, the thickness of myelin.
Part 2
　Information transmitted in CNS is mainly in the form of nerve impulses or action potentials (AP) through a succession of neurons, one after another, passing through special structures called synapses. There are two major types of synapse: the chemical synapse and the electrical synapse.
　Almost all of the synapses used for signal transmission in the human CNS are chemical synapses. The first neuron releases neurotransmitter at a presynaptic ending and this transmitter acts on receptors in the postsynaptic membrane of the next neuron to change its potential. When synaptic transmission causes a depolarization of the postsynaptic potential this is called an excitatory postsynaptic potential (EPSP), whilst a hyperpolarizing response of the postsynaptic membrane is termed an inhibitory postsynaptic potential (IPSP).
　Excitation of the postsynaptic neuron generates an action potential (AP) on the axon hillock if the membrane potential here reaches the threshold after summation of all the postsynaptic potentials. Conversely, inhibition of synaptic responses can be produced by two ways: postsynaptic inhibition (hyperpolarization and increased conductance of the membrane) and presynaptic inhibition (depolarization of the presynaptic terminal).
　The best known transmitters are rapidly acting low molecular weight substances, including acetylcholine (Ach), norepinephrine (NE), 5-hydroxytryptamine (5-HT), dopamine (DA), γ-aminobutyric acid (GABA), glycine and glutamate.
　Electrical synapses that consist of gap junctions are characterized by direct open fluid channels that conduct electricity from one cell to the next.
　The characteristics of the signal transmission at chemical synapses are: one-way propagation, synaptic delay, summation, change of excitatory rhythm, susceptibility to changes of internal environment and easy fatigue.
Part 3
　An essential component of nervous regulation is reflexactivity. Its structural base is a reflex arc which consists of 5 parts: receptor, afferent fiber, reflex center, efferent fiber and effector. There is an information stream flowing regularly from receptor to effector during reflex activity. R eflex can be divided into monosynaptic and polysynaptic ones, according to the structure of reflex center. Reflex center means the neuronal pools in different areas of the CNS, which regulate certain physiological functions. Information signals can be diverged, converged, or prolonged after passing through a neuronal pool. The general characteristics of reflex are: a final common path, change of excitatory rhythm, after-discharge and habituation or sensitization.
Part 4
　Sensation and perception begin in receptor cells that are sensitive to one or another kind of stimuli. Most sensations are identified with a particular type of stimulus. Stimuli initiate a special receptor potential which is then coded into the afferent impulses transmitted from the end of afferent fibers to their first synapse in the central nervous system.
　The pathways of somatic sensations include three relay neurons that link the receptors at the periphery with the spinal cord (or brain stem), thalamus, and cerebral cortex. The thalamus is an important second relay stage for all the somatic sensations, those neurons related to sensation send information to the cerebral cortex through two projecting pathways: specific projection system and non-specific projection system. The cerebral cortex related to sensation analysis includes somatic sensory area Ⅰ and somatic sensory area Ⅱ.There are several important rules of the somatic sensory area Ⅰ? for analyzing input sensory signals from the thalamus.
Part 5
　The motor system is composed of three control levels including spinal cord, brain stem, and motor cortex, as well as another two modulatory areas, the cerebellum and the basal ganglia.
　Spinal cord is the lowest motor center containing αand γ motor neurons. Theαmotor neurons innervate skeletal muscles and trigger the motor reflexes whereas,the γmotor neurons regulate the sensitivity of the muscle spindles. The major motor reflexes with centres in the spinal cord are stretch reflexes, which include tendon reflex and muscle tonus and flexor reflexes.
　The brain stem includes facilitatory and inhibitory areas to regulate muscle tone and also sends several descending pathways relaying the information from cerebellum and basal ganglia to spinal cord to regulate or coordinate motor movement. By contrast, the cerebellum and basal ganglia have no direct descending pathways to spinal cord, but are important in planning and regulating motor movement through connections with motor cortex and brain stem.
　Motor cortex includes primary motor cortex, premotor cortex and supplementary motor cortex. It controls and regulates motor movements through the pyramidal system and extrapyramidal system. There are several important rules for primary motor cortex to control and regulate motor reflexes and voluntary movement.
Part 6
　Visceral activities are regulated by a visceral nervous system that includes peripheral and central parts. The peripheral part of visceral nervous system controlling the visceral activities is called the vegetative nervous system, which is divided in turn into sympathetic, parasympathetic and enteric nervous systems. There are several differences between the peripheral systems for controlling skeletal movement and visceral activities. And there are some important characteristics for sympathetic and parasympathetic systems to regulate visceral reflexes, including innervation by both systems for most visceral organs.
　The central system to control visceral activities includes the different neuronal pools in spinal cord, brain stem, hypothalamus and limbic system. The hypothalamus is an important higher centre for the regulation of visceral activities.
Part 7
　We know little about the mechanisms of the higher functions of the brain. In this section we introduce the language centre of humans, most information coming from either clinical pathology or the advanced study of normal human brains with fMRI and PET. Learning and memory is another higher function of the human brain and also of the functions of the nervous systems of lower animals. The study of neuronal mechanisms of learning and memory made great progress during the last century.
　The sleep and wakefulness cycle is one of biological rhythmicity. Sleep has two phases according to the characteristics of the EEG, slow wave sleep (SWS) and rapid eye movement sleep (REMS or paradoxical sleep). Sleep is an active physiological process and has its specific nerve centres and neurotransmitters.
　EEG is the record of the spontaneous activities of the brain, while the evoked potentials of the brain are activities produced by stimulating specific pathways.