Human Anatomy and Physiology Twelfth Edition Lecture 10 Fundamentals of the Nervous System and Nervous Tissue The Nervous System Master controlling and communicating system of body Cells communicate via electrical and chemical signals Rapid and specific Usually cause almost immediate responses Structural Organization of the Nervous System Structurally the nervous system is divided into 2 subdivisions 1 Central nervous system CNS includes the brain and spinal cord integration and control center 2 Peripheral nervous system PNS NS outside the CNS includes the cranial nerves and spinal nerves and ganglia Functions of the Nervous System The CNS and PNS perform 3 general functions 1 Collecting information sensory input 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 integration CNS determines what if any response is required 3 Responding to information motor output CNS initiates specific nerve impulses called motor output to effectors muscles or glands to react to changes in the body s environment Figure 11 1 The nervous system s functions Sensory input Integration Motor output Functional Divisions of the Peripheral Nervous System SENSORY AFFERENT DIVISION MOTOR EFFERENT DIVISION Central nervous system CNS Brain and spinal cord Integrative and control centers Peripheral nervous system PNS Cranial nerves and spinal nerves Communication lines between the CNS and the rest of the body Sensory afferent division Somatic and visceral sensory nerve fibers Conducts impulses from receptors to the CNS Motor efferent division Motor nerve fibers Conducts impulses from the CNS to effectors muscles and glands Somatic sensory fiber Skin Visceral sensory fiber Stomach Skeletal muscle Motor fiber of somatic nervous system Somatic nervous system Somatic motor voluntary Conducts impulses from the CNS to skeletal muscles Sympathetic division Mobilizes body systems during activity Autonomic nervous system ANS Visceral motor involuntary Conducts impulses from the CNS to cardiac muscles smooth muscles and glands Parasympathetic division Conserves energy Promotes house keeping functions during rest Sympathetic motor fiber of ANS Heart Parasympathetic motor fiber of ANS Bladder Figure 11 2 Levels of organization in the nervous system Structure Function Sensory afferent division of PNS Motor efferent division of PNS Cytology Histology of Nervous Tissue Highly cellular little extracellular space Tightly packed Two distinct types of cell populations Neurons nerve cells Electrically excitable cells that initiate transmit and receive electrical signals Neuroglia Glial Cells small cells that surround and wrap delicate Non excitable cells that support and protect neurons neurons 1 Neurons The basic structural unit of the nervous system Large highly specialized cells 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 Non mitotic CELL BODY DENDRITES Neuron Structure A neuron has 3 main structural regions 1 Cell body perikaryon soma 2 Dendrites 3 Axon AXON 1 Cell Body Also called the SOMA PERIKARYON Contains typical organelles such as Nucleus Nucleolus Mitochondria Free ribosomes and rough endoplasmic reticulum Nissl bodies chromatophilic substance Some contain pigments Neuron Processes Arm like processes extend from body Two types of processes Dendrites Axon Tracts Bundles of neuron processes in CNS Nerves Bundles of neuron processes in PNS 2 Dendrites Short processes that branch from the cell body Receptive input region of neuron Receive nerve impulses incoming messages and carry them into the cell body as graded potentials short distance signals 2 Dendrites In motor neurons 100s of short tapering diffusely branched processes Same organelles as in cell body In many brain areas fine dendrites specialized Collect information with dendritic spines Appendages with bulbous or spiky ends Neuron cell body Dendritic spine 3 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 to other cells The Axon Structure Long axons called nerve fibers Occasional branches axon collaterals Distal endings are called axon terminals or terminal boutons synaptic knobs One axon per cell arising from axon hillock Cone shaped area of cell body In some neurons the axon is short or absent In others most of length of cell Some axons are 1 meter long Structures Associated with Axons PRESYNAPTIC NEURON AXON 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 axon terminals terminal boutons Synaptic Cleft Synaptic Vesicles w Neurotransmitter POSTSYNAPTIC NEURON DENDRITE The Axon Functional Characteristics Conducting region of neuron Generates nerve impulses Transmits impulses along Carries on many conversations with different neurons at same time Axons lack rough ER and axolemma neuron cell membrane to axon terminal Secretory region Neurotransmitters released into extracellular space Either excite or inhibit neurons with which axons in close contact Golgi apparatus Relies on cell body to renew proteins and membranes Efficient transport mechanisms Quickly decay if cut or damaged Transport Along the Axon Molecules and organelles are moved along axons by motor proteins and cytoskeletal elements Movement in both directions Anterograde away from cell body Examples mitochondria cytoskeletal elements membrane components enzymes Retrograde toward cell body Examples organelles to be degraded signal molecules viruses and bacterial toxins Myelin Sheath Myelin sheath Outer collar of perinuclear cytoplasm of Schwann cell Axon Some axons are covered by a myelin sheath which is composed of myelin Whitish protein lipoid substance Segmented sheath around most long or large diameter axons Myelinated fibers Function of myelin Protects and electrically insulates Increases speed of nerve impulse axon transmission Nonmyelinated fibers conduct impulses more slowly Cross sectional view of a myelinated axon electron micrograph 24 000x Myelination in the PNS Formed by Schwann cells Wrap around axon in jelly roll fashion One cell forms one segment of
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