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CHAPTER 48 NERVOUS SYSTEMS Section A: An Overview Of Nervous Systems 1. Nervous systems perform the three overlapping functions of sensory input, integration, and motor output 2. Networks of neurons either intricate connections form nervous systems Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Chapter 48 Nervous system Nervous systems perform the three overlapping functions of sensory input, integration, and motor output A nervous system has three overlapping functions: (1) Sensory input: conduction of signals from sensory receptors: such as light detecting cells in the eyes to integration centers. (2) Integration : is the process by which the signal from sensory input is interpreted and associated with appropriate responses of the body. Integration is carried out in the central nervous system (CNS) which consists of the brain and the spinal cord. (3) Motor output: is the conduction of signals from the integration center, the CNS, to effector cells, the muscle cells or the glands. Fig. 48.1 The signals of the nervous system are conducted by nerves. Nerves: ropelike bundles of extensions of neurons tightly wrapped in connective tissue. The nerves that communicate motor and sensory signals between the CNS and the rest of the body are collectively called peripheral nervous system (PNS). Networks of neurons with intricate connections form nervous systems Neuron structure and synapses The structural and functional unit of the nervous sytem is the neuron (nerve cell) A neuron has a: 1) Cell body: contains the nucleus and other organelles. 2) Dendrites: short highly branched processes that receive incoming messages from other cells and carry this information toward the cell body. Fig. 48.2 3) Axon: usually much longer than dendrite. convey outgoing messages from the neuron to other cells. Axon hillock: the region of the axon where it joins the cell body Some axons are enclosed by an insulated layer called the myelin sheath Axon endings are called synaptic terminals, that contain chemical messengers called neurotransmitters (which conduct a signal across a synapse). The site of contact between a synaptic terminal and a target cell is called a synapse . A synapse: is the junction where one neuron (presynaptic neuron) communicate with another neuron (postsynaptic neuron) in a neural pathway or where a neuron communicates with a muscle or gland cell. Fig. 48.2 Nerve impulses are conducted along a neuron. Dendrite cell body axon hillock axon Fig. 48.2 Functional organization of neurons Functionally there are three classes of neurons: 1) Sensory neurons: convey information about the external and internal environment from sensory receptors to CNS 2) Interneurons: located within the CNS, integrate sensory input and motor output 3) Motor neurons: convey impulses from the CNS to effector cells A Simple Nerve Circuit – the Reflex Arc. The simplest type of nerve circuit regulates a reflex (or autonomic response) and is called a reflex arc. The simplest reflex arc require only two kinds of nerve cells: 1) Sensory neuron: conveys signals from a sensory receptor to a motor receptor 2) Motor neuron: which sends signals to an effector cell, a muscle or a gland cell that carries out the response. Human knee-jerk reflex (Figure 49.3): Knee jerk reflex the one that makes your leg jerk forward when the doctor hits your knee with a small hammer: Knee jerk reflex involves more than simple sensory/motor circuit. Contraction of the quadriceps (the front thigh muscle) is accompanied by inhibition of the back thigh muscles (flexor muscles) that flex the lower leg (pull it toward the body). This inhibition involves a second nerve circuit: The sensory neurons from the quadriceps form synapses not only with motor neurons but also with interneurons in the spinal cord. These interneurons inhibit motor neurons to the flexor muscles, preventing them from contracting. Fig. 48.3 The cell body of the sensory neuron is located outside the spinal cord in a structure called ganglion. A ganglion is a cluster of nerve cell bodies within the PNS. Cell bodies of motor neurons and interneurons are located in the gray matter of the spinal cord. Supporting Cells (Glia) These are essential for the structural integrity of the nervous system and for the normal functioning of the neurons. Glia outnumber neurons by tenfold to fiftyfold. There are several types of glia in the brain and in the spinal cord. (a) Astrocytes are found within the CNS. Provide structural and metabolic support. Induce the formation of tight junctions between cells lining the capillaries in the brain. This results in the formation of the blood-brain barrier, which restricts the passage of most substances into the brain. Like neurons, astrocytes communicate with one another via chemical signals. (b) Oligodendrocytes are found within the CNS. Form a myelin sheath by insulating axons. (c) Schawnn cells are found within the PNS. Form a myelin sheath by insulating axons Fig. 48.5 CHAPTER 48 NERVOUS SYSTEMS Section D1: Vertebrate Nervous Systems 1. Vertebrate nervous systems have central and peripheral components 2. The divisions of the peripheral nervous system interact in maintaining homeostasis 3. Embryonic development of the vertebrate brain reflects its evolution from three anterior bulges of the neural tube 4. Evolutionarily older structures of the vertebrate brain regulate essential autonomic and integrative functions Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Vertebrate Nervous Systems Vertebrate nervous systems have central and peripheral Figure 48.16 The nervous system of a vertebrate components In all vertebrates the nervous system is divided into the Central nervous system (CNS) and the Peripheral nervous system (PNS) (Figure 48.16) Figure 48.16 The nervous system of a vertebrate The nervous system Central Nervous System (CNS) Brain Spinal cord Peripheral Nervous System (PNS) Cranial Nerve 12 pairs Spinal Nerves 31 pairs (viscera) (skeletal muscle) Central nervous system (CNS): consists of the brain and spinal cord The brain provides the integrative power that underlies the complex behavior characteristic of all vertebrates. The spinal cord runs lengthwise inside the vertebral column, integrates simple responses (knee-jerk reflex) and conveys information to and from the brain. The central canal of the spinal cord is continuous with the fluid filled spaces, called ventricles, of the brain. These cavities are filled with cerebrospinal fluid (CSF), which is formed in the brain by filtration of the blood. It conveys nutrients, hormones and white blood cells across the blood-brain barrier, to different parts of the brain. It acts as a shock absorber, cushioning the brain. The brain and the spinal cord are surrounded by layers of connective tissues called meninges. In mammals cerebrospinal fluid circulates between the meninges providing an addition cushion for the brain. Axons in the CNS are located in bundles and their myelin sheaths give them a white appearance (white matter). White matter : is composed of bundles of myelinated axons The white matter is distinguishable from the gray matter which consists of dendrites, unmyelinated axons and nerve cell bodies. Gray matter : consists of unmyelinated axons, nuclei, and dendrites. The divisions of the peripheral nervous system interact in maintaining homeostasis Peripheral nervous system (PNS): everything outside the CNS. It consists of paired cranial and spinal nerves and associated ganglia (Fig. 48.17). The cranial nerves (12 pairs) originate in the brain and innervate organs of the head and upper body. The spinal nerves (31 pairs) originate in the spinal and innervate the entire body. Most of the cranial nerves and all of the spinal nerves contain both sensory and motor neurons; a few of the cranial nerves are sensory only (e.g. the olfactory and optic nerves). Fig. 48.17 The PNS can also be divided into: 1) The sensory division of the PNS is made up of the sensory or afferent (incoming) neurons that convey information to the CNS from sensory receptors that monitor the external and internal environment. 2) The motor division is composed of the motor or efferent (outgoing) neurons that convey signals from CNS to effector cells. The motor division is divided into: a) The somatic nervous system: carries signals to skeletal muscles, mainly in response to external stimuli. It is often considered voluntary, (but a substantial proportion of skeletal muscle movement is determined by reflexes). b) The autonomic nervous system: conveys signals that regulate the internal environment by controlling smooth and cardiac muscles and the organs of the gastrointestinal, cardiovascular, excretory and endocrine systems. This control is generally involuntary. The somatic and autonomic nervous systems often cooperate in maintaining homeostasis. e.g. in response to a drop in temperature, the hypothalamus of the brain signals the autonomic nervous system to constrict surface blood vessels, which reduces heat loss; at the same time the hypothalamus signals the somatic nervous system and causes shivering. The autonomic nervous system consists of two subdivisions that act on our body organs with opposing effects (Figure 48.18): i) sympathetic ii) parasympathetic When sympathetic and parasympathetic nerves innervate the same organ, they often have antagonistic (opposite) effects. Sympathetic Parasympathetic Correlates with arousal and energy generation A calming and a return to emphasis on self-maintenance functions. Increases heart beats Decreases heart rate The liver converts glycogen to glucose Decreases energy storage Bronchi of the lungs dilate and support increased gas exchange Constricts bronchi in he lungs Inhibits digestion Stimulates digestion Stimulates secretion of adrenaline Stimulates secretion of acetlycholine Fig.48.18 The nervous system Central Nervous System (CNS) Brain Spinal cord Peripheral Nervous System (PNS) Cranial Nerve 12 pairs Spinal Nerves 31 pairs (viscera) (skeletal muscle)