Chapter 10 Nervous System

The Nervous System Components Responsible for Subdivisions Brain, spinal cord, nerves, sensory receptors Responsible for Sensory perceptions, mental activities, stimulating muscle movements, secretions of many glands Subdivisions Central nervous system (CNS) Peripheral nervous system (PNS)

Central Nervous System Consists of Brain Spinal cord Brain and spinal cord Continuous with each other

Peripheral Nervous System Two subcategories Sensory or afferent Motor or efferent Divisions Somatic nervous system Autonomic nervous system (ANS) Sympathetic Parasympathetic Enteric

Nervous System Organization

Cells of Nervous System Neurons or nerve cells Receive stimuli and transmit action potentials Organization Cell body or soma Dendrites: Input Axons: Output Neuroglia or glial cells Support and protect neurons

Section 1 General Function of Neuron and Neuroglia I Neuron

Typical Neuron has 4 Regions Structure and Function Typical Neuron has 4 Regions Cell Body Dendrites Axon Presynaptic Terminals Each region is specialized for its particular function Information flows in a single direction

Neuron Cell Body Location Most are found in the central nervous system Gray matter – cell bodies and unmylenated fibers Nuclei – clusters of cell bodies within the white matter of the central nervous system Ganglia – collections of cell bodies outside the central nervous system

Functional Classification of Neurons Sensory (afferent) neurons Carry impulses from the sensory receptors Cutaneous sense organs Proprioceptors – detect stretch or tension Motor (efferent) neurons Carry impulses from the central nervous system

Functional Classification of Neurons Interneurons (association neurons) Found in neural pathways in the central nervous system Connect sensory and motor neurons

By function (connections) Sensory Motor Interneuron

Neuron Classification

Neuron Anatomy Extensions outside the cell body Dendrites – conduct impulses toward the cell body Axons – conduct impulses away from the cell body (only 1!)

Dendrites of Motor Neurons Short, tapering, and diffusely branched processes They are the receptive, or input, regions of the neuron Electrical signals are conveyed as graded potentials (not action potentials)

Axons: Structure Slender processes of uniform diameter arising from the hillock Long axons are called nerve fibers Usually there is only one unbranched axon per neuron Rare branches, if present, are called axon collaterals Axonal terminal – branched terminus of an axon

Axons: Function Generate and transmit action potentials Secrete neurotransmitters from the axonal terminals

Whitish, fatty (protein-lipid), segmented sheath around most long axons It functions in: Protection of the axon Electrically insulating fibers from one another Increasing the speed of nerve impulse transmission Myelin Sheath

Myelin CNS: oligodendroglia or oligodendrocytes PNS: Schwann cells ~

Nodes of Ranvier Gaps in the myelin sheath between adjacent Schwann cells They are the sites where collaterals can emerge Saltatory conduction

Myelin Sheath and Neurilemma: Formation Formed by Schwann cells in the PNS A Schwann cell: Envelopes an axon in a trough Encloses the axon with its plasma membrane Concentric layers of membrane make up the myelin sheath Neurilemma – remaining nucleus and cytoplasm of a Schwann cell

Myelin Sheath and Neurilemma: Formation Figure 11.5a-d

Unmyelinated Axons A Schwann cell surrounds nerve fibers but coiling does not take place Schwann cells partially enclose 15 or more axons

Axons of the CNS Both myelinated and unmyelinated fibers are present Myelin sheaths are formed by oligodendrocytes Nodes of Ranvier are widely spaced There is no neurilemma

2. Classification and Function of Nerve Fibers Function: conducting action potential 1)Characteristic  physiological integration (anesthetic and tetrodotoxin, TTX)  insulation,  two direction  no fatigue

2). Conducting velocities of AP propagation: 0.5~120m/s. The factors that influence the AP propagation: ~The diameter of NF: 0.2 -20 mm, The larger the diameter is , The faster A.P. propagates. ~ Myelin sheath: ~Temperature:

3). The general classification of NF  Electrical physiological classification (efferent nerve): A, A, A, A; B, C.  Morphological classification (afferent nerve): I, II, III, IV.

4) Axoplasmic transport The concept: Various organelles and materials must be moved from the cell body, where they are made, to the axon and its terminals in order to maintain the structure and function of the cell axon.  The mechanisms: Cytoskeletal filaments in the axon and cell body, which serve as the rails along which the transport occurs, are linked by proteins to the substances and organelles being moved.

 Anterograde axoplasmic transport ~ fast axoplasmic transport: 410mm/day, mitochondria, vesicles, secretory granule. ~ slow axoplasmic transport:1-12 mm/d, cytoskeletal elements & soluble proteins  Retrograde axoplasmic transport：Axon transport of certain materials are from the axon terminals to the cell body. 205mm/d, NGF, tetanic toxin, horseradish peroxidase (HRP).

Fig. Axopasmic transport Fig. The method of horseraidish peroxidase

5) Trophic action of the nerve to the target The motor nerve release some substance that has trophic action on the skeletal muscle The denervated muscle does not receive nerve signals and due to this, muscle atrophy begins. After two months, the muscle fibers degenerate and denervation atrophy follows. Fibrous tissue replaces the muscle

II Supporting Cells: Neuroglia Ependymal Cell Microglia Oligodendrocyte Astrocyte Martini, F. Fundamentals of Anatomy and Physiology, Fig 12-6.

Neuroglia of CNS Astrocytes Ependymal Cells Regulate extracellular brain fluid composition Promote tight junctions to form blood-brain barrier Ependymal Cells Line brain ventricles and spinal cord central canal Help form choroid plexuses that secrete cerebrospinal fluid (CSF)

Neuroglia of CNS Microglia Specialized macrophages Oligodendrocytes Form myelin sheaths if surround axon

Neuroglia of PNS Schwann cells or neurolemmocytes Satellite cells Wrap around portion of only one axon to form myelin sheath Satellite cells Surround neuron cell bodies in ganglia, provide support and nutrients