Chapter 14 Outline Organization of the Nervous System Cytology of Nervous Tissue Myelination of Axons Axon Regeneration Nerves Synapses Neural Integration and Neuronal Pools Development of the Nervous System
Structural Organization of the Nervous System Structurally, the nervous system is divided into two subdivisions: 1. Central nervous system (CNS)— includes the brain and spinal cord 2. Peripheral nervous system (PNS)— includes the cranial nerves, spinal nerves, and ganglia
Structural Organization of the Nervous System Figure 14.1
Functional Organization of the Nervous System The CNS and PNS perform three general functions: 1. Collecting information—receptors are PNS structures that detect changes in the internal and external environment (sensory input) and pass the information on to the CNS 2. Processing and evaluating information—CNS determines what, if any, response is required 3. Responding to information—CNS initiates specific nerve impulses, called motor output, to effectors (muscles or glands) to react to changes in the body’s environment
Functional Organization of the Nervous System There are two functional divisions of the nervous system: Sensory nervous system 1. Sensory nervous system 2. Motor nervous system
Functional Organization of the Nervous System
Sensory Nervous System Sensory nervous system (afferent) receives sensory information from receptors in the PNS and transmits it to the CNS Subdivided into two systems: 1.Somatic sensory (voluntary)—touch, pain, pressure, vibration, and proprioception 2.Visceral sensory (involuntary)—impulses from viscera
Motor Nervous System Motor nervous system (efferent) sends impulses from CNS to muscles and glands Subdivided into two systems: 1.Somatic motor (voluntary)—impulses from the CNS that cause contraction of skeletal muscles 2.Autonomic motor (involuntary)— impulses from the CNS that regulate smooth and cardiac muscle and glands
Cytology of Nervous Tissue There are two distinct types of cells within the nervous system: 1. Neurons (nerve cells)—electrically excitable cells that initiate, transmit, and receive nerve impulses 2. Glial cells—nonexcitable cells that support and protect the neurons
Neurons The basic structural unit of the nervous system Conduct nerve impulses from one part of the body to another part They have the following features: 1.High metabolic rate 2.Extreme longevity 3.Nonmitotic
Neuron Structure A neuron has three main structural regions: 1.Cell body 2.Dendrites 3.Axon
Neuron Structure Figure 14.3
Cell Body Contains typical organelles such as: ─Nucleus ─Nucleolus ─Mitochondria !!!!!! ─Free ribosomes and rough endoplasmic reticulum (Nissl bodies)!!!!
Dendrites Short processes that branch from the cell body Receive nerve impulses and carry them to the cell body
Axon Neurons have either one axon or no axon at all. Neurons without an axon are called anaxonic. The region where the axon connects to the cell body is the axon hillock. Axons transmit nerve impulses away from the cell body and transmit information to other cells.
Structures Associated with Axons 1.Axon collaterals—side branches of the main axon 2.Telodendria—fine terminal extensions at the end of the axon and its collaterals 3.Synaptic knobs—expanded regions at the tip of telodendria
Structural Classification of Neurons Classified according to the number of processes emanating directly from the cell body of the neuron: 1. Unipolar—single, short process that branches like a T 2. Bipolar—two processes, one dendrite and one axon 3. Multipolar—many dendrites and a single axon, most common of all neurons
Structural Classification of Neurons Figure 14.4
Functional Classification of Neurons Functionally, neurons are classified according to the direction that the nerve impulse is traveling relative to the CNS: 1. Sensory (afferent)—transmit impulses from sensory receptors to the CNS 2. Motor (efferent)—transmit impulses from CNS to muscles or glands 3. Interneurons—facilitate communication between sensory and motor neurons
Functional Classification of Neurons Figure 14.5
Glial Cells=Neuroglias Sometimes referred to as neuroglia Found in both CNS and PNS Smaller than neurons and capable of mitosis Physically protect and nourish neurons More numerous than neurons Brain tumors are more likely to be derived from glial cells than neurons
Glial Cells of the CNS Figure 14.6
Glial Cells of the CNS There are four types of cells found in the CNS: 1. Astrocytes 2. Ependymal cells 3. Microglial cells 4. Oligodendrocytes See Table 14.4 for the function of each of these glial cells.
Cellular Organization in Neural Tissue Produce CSF BBB Regulate MyelinClean lady Repair-Myelin
Function of CNS Glial Cells
Astrocytes Most abundant glial cells in the CNS, whose functions include: 1. Helping to form the blood-brain barrier (BBB) 2. Regulating tissue fluid composition 3. Forming a structural network 4. Replacing damaged neurons 5. Assisting neuronal development
Astrocytes Figure 14.7
Ependymal Cells Ciliated cuboidal epithelial cells that line the ventricles of the brain and the central canal of the spinal cord In conjunction with other glial cells, the ependymal cells produce cerebral spinal fluid (CSF) and form the choroid plexus
Ependymal Cells Figure14.7
Microglial Cells Small cells that are motile Wander through the CNS and exhibit phagocytic activity, removing cellular debris from dead or dying cells
Microglial Cells Figure 14.7
Oligodendrocytes Associated with CNS axons only Wrap themselves around the axons like electrical tape wrapped around a wire Produce myelin, which is an insulator of electrical activity
Oligodendrocytes Figure 14.7
Function of PNS Glial Cells
Glial Cells of the PNS There are two types of cells found in the PNS: 1. Satellite cells 2. Neurolemmocytes=Schwann Cells See Table 14.4 for the function of each of these glial cells.
Satellite Cells Flattened cells arranged around neuronal cell bodies in ganglia Figure 14.7
Neurolemmocytes Also called Schwann cells Associated with PNS axons only Wrap themselves around the axons like electrical tape wrapped around a wire Produce myelin which is an insulator of electrical activity Same structure and function as oligodendrocytes
Neurolemmocyte Development Figure 14.8 1 Axon Neurolemmocyte starts to wrap around a portion of an axon. Nucleus Direction of wrapping 2 Neurolemmocyte cytoplasm and plasma membrane begin to form consecutive layers around axon.