Organization and Cells of the Nervous System PDF

Week 3: Organization and Cells of the Nervous System Organization of the Nervous System To understand neurophysiology, it is necessary to appreciate the organization of the nervous system and the gross anatomic arrangement of structures. A comprehensive presentation of neuroanatomy would be the su...

Week 3: Organization and Cells of the Nervous System Organization of the Nervous System To understand neurophysiology, it is necessary to appreciate the organization of the nervous system and the gross anatomic arrangement of structures. A comprehensive presentation of neuroanatomy would be the subject of an entire text. Thus, in this chapter, the anatomy will be described briefly, as is appropriate for the physiologic context. The nervous system is composed of two divisions: the central nervous system (CNS), which includes the brain and the spinal cord, and the peripheral nervous system (PNS), which includes sensory receptors, sensory nerves, and ganglia outside the CNS. The CNS and PNS communicate extensively with each other. Further distinction can be made between the sensory and motor divisions of the nervous system. The sensory or afferent division brings information into the nervous system, usually beginning with events in sensory receptors in the periphery. These receptors include, but are not limited to, visual receptors, auditory receptors, chemoreceptors, and somatosensory receptors (touch, pain, and temperature). This afferent information is then transmitted to progressively higher levels of the nervous system and finally to the cerebral cortex. The motor or efferent division carries information out of the nervous system to the periphery. This efferent information results in contraction of skeletal muscle, smooth muscle, and cardiac muscle or secretion by endocrine and exocrine glands. To illustrate and compare the functions of the sensory and motor divisions of the nervous system, consider an example introduced in Chapter 2: regulation of arterial blood pressure. Arterial blood pressure is sensed by baroreceptors located in the walls of the carotid sinus. This information is transmitted, via the glossopharyngeal nerve (cranial nerve [CN] IX), to the vasomotor center in the medulla of the brain stem—this is the sensory or afferent limb of blood pressure regulation. In the medulla, the sensed blood pressure is compared with a set point, and the medullary vasomotor center directs changes in sympathetic and parasympathetic outflow to the heart and blood vessels, which produce appropriate adjustments in arterial pressure—this is the motor or efferent limb of blood pressure regulation. The CNS includes the brain and spinal cord. The organization of major structures of the CNS is shown in Figures 3.1 and 3.2. Figure 3.1 shows the structures in their correct anatomic positions. These same structures are illustrated schematically in Figure 3.2, which may prove more useful as a study tool. Week 3: Organization and Cells of the Nervous System The major divisions of the CNS are the spinal cord; brain stem (medulla, pons, and midbrain); cerebellum; diencephalon (thalamus and hypothalamus); and cerebral hemispheres (cerebral cortex, white matter, basal ganglia, hippocampal formation, and amygdala). Week 3: Organization and Cells of the Nervous System Spinal Cord The spinal cord is the most caudal portion of the CNS, extending from the base of the skull to the first lumbar vertebra. The spinal cord is segmented, with 31 pairs of spinal nerves that contain both sensory (afferent) nerves and motor (efferent) nerves. Sensory nerves carry information to the spinal cord from the skin, joints, muscles, and visceral organs in the periphery via dorsal root and cranial nerve ganglia. Motor nerves carry information from the spinal cord to the periphery and include both somatic motor nerves, which innervate skeletal muscle, and motor nerves of the autonomic nervous system, which innervate cardiac muscle, smooth muscle, glands, and secretory cells (see Chapter 2). Information also travels up and down within the spinal cord. Ascending pathways in the spinal cord carry sensory information from the periphery to higher levels of the CNS. Descending pathways in the spinal cord carry motor information from higher levels of the CNS to the motor nerves that innervate the periphery. Brain Stem The medulla, pons, and midbrain are collectively called the brain stem. Ten of the 12 cranial nerves (CNs III–XII) arise in the brain stem. They carry sensory information to the brain stem and motor information away from it. The components of the brain stem are as follows: The medulla is the rostral extension of the spinal cord. It contains autonomic centers that regulate breathing and blood pressure, as well as the centers that coordinate swallowing, coughing, and vomiting reflexes (see Chapter 2, Fig. 2.5). The pons is rostral to the medulla and, together with centers in the medulla, participates in balance and maintenance of posture and in regulation of breathing. In addition, the pons relays information from the cerebral hemispheres to the cerebellum. The midbrain is rostral to the pons and participates in control of eye movements. It also contains relay nuclei of the auditory and visual systems. Cerebellum The cerebellum is a foliated (“leafy”) structure that is attached to the brain stem and lies dorsal to the pons and medulla. The functions of the cerebellum are coordination of movement, planning and execution of movement, maintenance of posture, and coordination of head and eye movements. Thus, the cerebellum, conveniently positioned between the cerebral cortex and the spinal cord, integrates sensory information about position from the spinal cord, motor information from the cerebral cortex, and information about balance from the vestibular organs of the inner ear. Thalamus and Hypothalamus Together, the thalamus and hypothalamus form the diencephalon, which means “between brain.” The term refers to the location of the thalamus and hypothalamus between the cerebral hemispheres and the brain stem. Week 3: Organization and Cells of the Nervous System The thalamus processes almost all sensory information going to the cerebral cortex and almost all motor information coming from the cerebral cortex to the brain stem and spinal cord. The hypothalamus lies ventral to the thalamus and contains centers that regulate body temperature, food intake, and water balance. The hypothalamus is also an endocrine gland that controls the hormone secretions of the pituitary gland. The hypothalamus secretes releasing hormones and release-inhibiting hormones into hypophysial portal blood that cause release (or inhibition of release) of the anterior pituitary hormones. The hypothalamus also contains the cell bodies of neurons of the posterior pituitary gland that secrete antidiuretic hormone (ADH) and oxytocin. Cerebral Hemispheres The cerebral hemispheres consist of the cerebral cortex, an underlying white matter, and three deep nuclei (basal ganglia, hippocampus, and amygdala). The functions of the cerebral hemispheres are perception, higher motor functions, cognition, memory, and emotion. Cerebral cortex. The cerebral cortex is the convoluted surface of the cerebral hemispheres and consists of four lobes: frontal, parietal, temporal, and occipital. These lobes are separated by sulci or grooves. The cerebral cortex receives and processes sensory information and integrates motor functions. These sensory and motor areas of the cortex are further designated as “primary,” “secondary,” and “tertiary,” depending on how directly they deal with sensory or motor processing. o Primary areas are the most direct and involve the fewest synapses. o Tertiary areas require the most complex processing and involve the greatest number of synapses. o Association areas integrate diverse information for purposeful actions. For example, the limbic association area is involved in motivation, memory, and emotions. o Examples: The primary motor cortex contains the upper motoneurons, which project directly to the spinal cord and activate lower motoneurons that innervate skeletal muscle. The primary sensory cortices consist of the primary visual cortex, primary auditory cortex, and primary somatosensory cortex. Basal ganglia, hippocampus, and amygdala. o The basal ganglia consist of the caudate nucleus, the putamen, and the globus pallidus. They assist in regulating movement. o The hippocampus is involved in memory. o The amygdala is involved with emotions and communicates with the autonomic nervous system via the hypothalamus (e.g., effect of emotions on heart rate, pupil size, and hypothalamic hormone secretion). Week 3: Organization and Cells of the Nervous System Cells of the Nervous System Neurons, or nerve cells, are specialized for receiving and sending signals. The structure of neurons includes the cell body, or soma; the dendrites; the axon; and the presynaptic terminals (Fig. 3.3). Glial cells, which greatly outnumber neurons, include astrocytes, oligodendrocytes, and microglial cells; their function, broadly, is to provide support for the neurons. Week 3: Organization and Cells of the Nervous System Structure of the Neuron Cell Body The cell body, or soma, surrounds the nucleus of the neuron and contains the endoplasmic reticulum and Golgi apparatus. It is responsible for the neuron’s synthesis and processing of proteins. Dendrites Dendrites are tapering processes that arise from the cell body. They receive information and thus contain receptors for neurotransmitters that are released from adjacent neurons. Axon The axon is a projection arising from a specialized region of the cell body called the axon hillock, which adjoins the spike initiation zone (or initial segment) where action potentials are generated to send information. Whereas dendrites are numerous and short, each neuron has a single axon, which can be quite long (up to 1 meter in length). The cytoplasm of the axon contains dense, parallel arrays of microtubules and microfilaments that rapidly move organelles and vesicles (containing proteins and neurotransmitters synthesized in the cell body) from the cell body to the axon terminus. This process is called fast axoplasmic transport and involves moving mitochondria and vesicles along the microtubules via an ATP-dependent motor protein called kinesin. Cytoskeletal elements and various soluble proteins also move from the cell body down the axon by slow axoplasmic transport. Anterograde transport: Movement from the cell body to the axon terminus (both fast and slow). Fast retrograde transport: Movement of growth factors and membrane fragments from the axon terminus to the cell body. Axons carry action potentials between the neuron cell body and the targets of that neuron, either other neurons or muscle. Axons may be insulated with myelin (see Chapter 1), which increases conduction velocity; breaks in the myelin sheath occur at the nodes of Ranvier. Multipolar Neurons The most common type of neuron in the mammalian nervous system is multipolar, having a single axon and many dendrites originating from the cell body. Multipolar neurons vary in shape, in the length of their axons, and in the complexity of their dendritic tree. The extent of dendritic branching correlates with the number of synaptic contacts from other neurons. For example, the dendritic tree of a cerebellar Purkinje cell can have up to one million contacts! Week 3: Organization and Cells of the Nervous System Presynaptic Terminals The axon terminates on its target cells (e.g., other neurons) in multiple endings, called presynaptic terminals. When the action potential transmitted down the axon reaches the presynaptic terminal, neurotransmitter is released into the synapse. The transmitter diffuses across the synaptic cleft and binds to receptors on the postsynaptic membrane (e.g., of dendrites of other neurons). In this way, information is transmitted rapidly from neuron to neuron (or, in the case of the neuromuscular junction, from neuron to skeletal muscle). Glial Cells Glial cells occupy over half of the brain’s volume and function as support cells for neurons. Some glial cells of the adult brain have the properties of stem cells and thus can give rise to new glial cells or even new neurons. Astrocytes supply metabolic fuel, as lactic acid, to the neurons; they also synthesize neurotransmitters, secrete trophic factors that promote neuronal survival, modulate cerebral blood flow, and help maintain the brain’s extracellular K+ concentration. Oligodendrocytes synthesize myelin in the CNS; Schwann cells synthesize myelin in the PNS. Microglial cells proliferate following neuronal injury and serve as scavengers to remove cellular debris.