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Introduction to the Nervous System. t o be copied. Nervous Tissue & Homeostasis. excitable characteristic of nervous tissue allows for generation of nerve impulses (action potentials) that provide communication & regulation of most body tissue.
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Introduction to the Nervous System to be copied
Nervous Tissue & Homeostasis • excitable characteristic of nervous tissue allows for generation of nerve impulses (action potentials) that provide communication & regulation of most body tissue. • together with endocrine system: responsible for maintaining homeostasis
ENDOCRINE Differences in Nervous & Endocrine Control of Homeostasis NERVOUS rapid responder action potentials slow, prolonged response releases hormones
Structures of the Nervous System • total mass of 2 kg (~3% of total body mass) • Skull • Spinal Cord • Spinal Nerves • Cranial Nerves • Ganglia • Enteric Plexus • Special Senses & other Sensory Receptors
Functions of the Nervous System • 3 basic functions: • Sensory • Integrative • Motor
Sensory Function • sensory receptors detect internal & external stimuli • sensory (afferent) neurons carry this sensory information to spinal cord & brain thru cranial & spinal nerves
Integrative Function • integrate: process • nervous system takes information from sensory neurons & processes that information, analyzes it, stores some of it & makes decisions for appropriate responses • served by interneurons (connect 1 neuron to another neuron • Perception: • conscious awareness of sensory stimuli • occurs in brain
Motor Function • served by motor (efferent) neurons • carry info from brain/spinal cord effectors (muscle or gland) thru cranial or spinal nerves • results in muscles contraction or gland secreting
Histology of the Nerrvous System • 2 cell types • Neurons • Neuroglia
Neurons • nerve cells that possess electrical excitability: • ability to respond to a stimulus & convert it into an action potential • stimulus: any change in environment that is strong enough to initiate an action potential
Action Potential • electrical signal that propagates along surface of neurolema (membrane) • begins & travels due to movement of ions between interstitial fluid & inside of neuron thru specific ion channels • once begun it travels rapidly @ constant strength
Parts of Neuron: Cell Body • contains nucleus, cytoplasm, typical organelles, • + Nissl bodies clusters of RER • make materials for: • growth of neuron • regenerate damaged axons in PNS
Nerve Fiber • general term for any neuronal process or extension that emerges from cell body • most neurons have 2: • Dendrites • Axons
Dendrites • “little trees” • input portion of neuron • usually, short, tapering, highly branched • their cytoplasm contains Nissl bodies, mitochondria
Axon • propagates action potentials • another neuron • muscle fiber • gland cell
Parts of an Axon • joins cell body @ cone-shaped elevation: axon hillock • part of axon closest to hillock = initial segment • jct of axon hillock & initial segment where action potential arises so is called the trigger zone
Parts of an Axon • axoplasm: cytoplasm of an axon • axolemma: plasma membrane of axon • axon collaterals: side branches along length of axon (most @ 90°) • axon terminals: axon divides into many fine processes
Synapse • site of communication between 2 neurons or between a neuron & effector cell • synaptic end bulbs: tips of some axon terminals swell into bulb-shaped structures • synaptic vesicles: store neurotransmitter • many neurons have >1 neurotransmitter, each with different effects on postsynaptic cell
Axonal Transport • 2 types: • for moving materials from cell body axon terminals • slow • 1-5 mm/d • replenishes new axoplasm to developing or regenerating axons • fast • 200 – 400 mm/d • moves materials to/from cell body • organelles or membranes needed in axon terminal
Structural Classification Types of Neurons Functional Classification Sensory Interneurons Motor use # processes extending from cell body Multipolar neurons Bipolar neurons Unipolar neurons
Multipolar Neurons • several dendrites with 1 axon • includes most neurons in brain & spinal cord
Bipolar Neuron • 1 main dendrite & 1 axon • retina, inner ear, olfactory area of brain
Unipolar Neuron • are sensory neurons that begin in embryo as bipolar • during development axon & dendrite fuse then divide into 2 branches (both have characteristic structure & function of an axon) • 1 branch ends with dendrites (out of CNS) • 2nd branch ends in axon terminal (in CNS) • cell bodies of most found in ganglia
Pyramidal Cells • in cerebral cortex of brain
Neuroglia (Glia) • ~50% vol of CNS • “glue” • do not generate or propagate action potentials • multiply & divide in mature nervous systems • glioma: • brain tumors derived from glial cells • very malignant, grow rapidly
Glial Cells of the CNS • ASTROCYTES • OLIGODENDROCYTES • MICROGLIA • EPENDYMAL CELLS
Astrocytes • star-shaped • largest & most numerous of glial cells • functions: • physically support neurons • assist in blood-brain-barrier (bbb) • in embryo: regulate growth, migration, &interconnections between neurons • help maintain appropriate chemical environment for propagation of action potentials
Oligodendrocytes • “few trees” • smaller & fewer branches than astrocytes • Functions: • form & maintain myelin sheath on axons in CNS • 1 oligo. myelinates many axons
Microglia • small cells with slender processes giving off many spine-like projections • function: • phagocytes • remove cellular debris made during normal development • remove microbes & damaged nervous tissue
Ependymal Cells • single layer of cuboidal to columnar cells • ciliated & have microvilli • function: • line ventricles of brain & central canal of spinal cord • produce, monitor, & assist in circulation of cerebrospinal fluid (CSF) • form bbb
Neuroglial Cells of the PNS • Schwann cells • Satellite cells
Schwann Cells • functions: • myelinate axons in PNS • 1 Schwann cell myelinates 1 axon • participate in axon regeneration
Satellite Cells • flat cells that surround cell bodies of neurons in PNS ganglia • functions: • structural support • regulate exchange of materials between neuronal cell bodies & interstitial fluid
Myelination • myelin sheath: made up of multilayered lipid & protein (plasma membrane) covering • function: • electrically insulates axon • increases speed of nerve impulses
Nodes of Ranvier • gaps in myelin sheath • 1 Schwann cell wraps axon between nodes of Ranvier
Myelin • amount increases from birth to maturity • infant‘s responses slower & less coordinated as older child or adult in part because myelination is a work in progress thru infancy
Demyelination • loss of myelin sheath • see in disorders: • multiple sclerosis • Tay-Sachs • side effect of radiation therapy & chemotherapy
Gray Matter of the Nervous System • contains: • neuronal cell bodies • dendrites • unmyelinated axons • axon terminals • neuroglia
White Matter of the Nervous System • composed of: • myelinated axons