Fundamentals of the Nervous System Andnervous Tissue

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Fundamentals of the Nervous System andNervous Tissue

Nervous System

• Master control and communication system

• Has three overlapping functions

• Sensory receptors monitor changes inside and outside the body

• Change—a stimulus

• Gathered information—sensory input

Nervous System

• Processes and interprets sensory input

• Makes decisions—integration

• Dictates a response by activating effector organs

• Response—motor output

Basic Divisions of the Nervous System

• Central nervous system (CNS)

• Brain and spinal cord

• Integrating and command center

• Peripheral nervous system (PNS)

• Outside the CNS

• Consists of nerves extending from brain and spinal cord

• Cranial nerves

• Spinal nerves

• Peripheral nerves link all regions of the body to the CNS

• Ganglia are clusters of neuronal cell bodies

Sensory Input and Motor Output

• Sensory (afferent) signals picked up by sensor receptors

• Carried by nerve fibers of PNS to the CNS

• Motor (efferent) signals are carried away from the CNS

• Innervate muscles and glands

• Divided according to region they serve

• Somatic body region

• Visceral body region

• Results in four main subdivisions

• Somatic sensory

• Visceral sensory

• Somatic motor

• Visceral motor (autonomic nervous system)

Types of Sensory and Motor Information

Basic Divisions of the Nervous System

• Somatic sensory

• General somatic senses—receptors are widely spread

• Touch

• Pain

• Vibration

• Pressure

• Temperature

(receptors discussed in Chapter 14)

• Somatic sensory (continued)

• Proprioceptive senses—detect stretch in tendons and muscle

• Body sense—position and movement of body in space

• Special somatic senses (Chapter 16)

• Hearing

• Balance

• Vision

• Smell

• Visceral sensory

• General visceral senses—stretch, pain, temperature, nausea, and hunger

• Widely felt in digestive and urinary tracts, and reproductive organs

• Special visceral senses

• Taste

• Somatic motor

• General somatic motor—signals contraction of skeletal muscles

• Under our voluntary control

• Often called “voluntary nervous system”

• Branchial motor

• Typical skeletal muscle derived from somitomeres

•

• Visceral motor

• Regulates the contraction of smooth and cardiac muscle

• Makes up autonomic nervous system

• Controls function of visceral organs

• Often called “involuntary nervous system”

• Autonomic nervous system (Chapter 15)

Nervous Tissue

• Cells are densely packed and intertwined

• Two main cell types

• Neurons—transmit electrical signals

• Support cells (neuroglial cells in CNS)

• Nonexcitable

• Surround and wrap neurons

The Neuron

• The human body contains billions of neurons

• Basic structural unit of the nervous system

• Specialized cells conduct electrical impulses along the plasma membrane

• Nerve impulse (action potential)

The Neuron

• Other special characteristics

• Longevity—can live and function for a lifetime

• Do not divide—fetal neurons lose their ability to undergo mitosis; neural stem cells are an exception

• High metabolic rate—require abundant oxygen and glucose

• Neurons die after 5 minutes without oxygen

The Cell Body

• Cell body (soma)

• Perikaryon—around the nucleus

• Size of cell body varies from 5–140µm

• Contains usual organelles plus other structures

• Chromatophilic bodies (Nissl bodies)

• Clusters of rough ER and free ribosomes

• Stain darkly and renew membranes of the cell

•

• Neurofibrils—bundles of intermediate filaments

• Form a network between chromatophilic bodies

•

• Most neuronal cell bodies are

• Located within the CNS

• Protected by bones of the skull and vertebral column

• Ganglia—clusters of cell bodies

• Lie along nerves in the PNS

Structure of a Typical Large Neuron

Neuron Processes

• Dendrites

• Extensively branching from the cell body

• Transmit electrical signals toward the cell body

• Chromatophilic bodies—only extend into the basal part of dendrites and to the base of the axon hillock

• Function as receptive sites for receiving signals from other neurons

Neuron Processes

• Axons

• Neuron has only one

• Impulse generator and conductor

• Transmits impulses away from the cell body

• Chromatophilic bodies are absent

• No protein synthesis in axon

• Neurofilaments, actin microfilaments, and microtubules

• Provide strength along length of axon

• Aid in the transport of substances to and from the cell body

• Axonal transport

• Branches along length are infrequent

• Axon collaterals

• Multiple branches at end of axon

• Terminal branches (telodendria)

• End in knobs called axon terminals (also called end bulbs or boutons)

• Nerve impulse

• Generated at the initial segment of the axon

• Conducted along the axon

• Releases neurotransmitters at axon terminals

• Neurotransmitters—excite or inhibit neurons

• Neuron receives and sends signals

Synapses

• Site at which neurons communicate

• Signals pass across synapse in one direction

• Presynaptic neuron

• Conducts signal toward a synapse

• Postsynaptic neuron

• Transmits electrical activity away from a synapse

Types of Synapses

• Axodendritic

• Between axon terminals of one neuron and dendrites of another

• Most common type of synapse

• Axosomatic

• Between axons and neuronal cell bodies

Synapses

• Elaborate cell junctions

• Axodendritic synapses—representative type

• Synaptic vesicles on presynaptic side

• Membrane-bound sacs containing neurotransmitters

• Mitochondria abundant in axon terminals

• Synaptic cleft

• Separates the plasma membrane of the two neurons

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Fundamentals of the Nervous System andNervous Tissue

Classification of Neurons

• Structural classification

• Multipolar—possess more than two processes

• Numerous dendrites and one axon

• Bipolar—possess two processes

• Rare neurons

• Found in some special sensory organs

• Unipolar (pseudounipolar)—possess one short, single process

• Start as bipolar neurons during development

Functional Classification of Neurons

• Functional classification is

• According to the direction the nerve impulse travels relative to the CNS

• Types of neurons

• Sensory neurons

• Motor Neurons

• Interneurons

Functional Classification of Neurons

• Sensory neurons

• Transmit impulses toward the CNS

• Virtually all are unipolar neurons

• Cell bodies in ganglia outside the CNS

• Short, single process divides into

• The central process—runs centrally into the CNS

• The peripheral process—extends peripherally to the receptors

• Motor (efferent) neurons

• Carry impulses away from the CNS to effector organs

• Most motor neurons are multipolar

• Cell bodies are within the CNS

• Form junctions with effector cells

• Interneurons (association neurons)—most are multipolar

• Lie between motor and sensory neurons

• Confined to the CNS

Supporting Cells

• Six types of supporting cells

• Four in the CNS

• Two in the PNS

• Provide supportive functions for neurons

• Cover nonsynaptic regions of the neurons

Neuroglial in the CNS

• Neuroglia

• Glial cells have branching processes and a central cell body

• Outnumber neurons 10 to 1

• Make up half the mass of the brain

• Can divide throughout life

• Astrocytes are the most abundant glial cell type

• Sense when neurons release glutamate

• Extract blood sugar from capillaries for energy

• Take up and release ions to control environment around neurons

• Involved in synapse formation in developing neural tissue

• Produce molecules necessary for neuronal growth (BDTF)

• Propagate calcium signals involved with memory

• Microglia—smallest and least abundant glial cell

• Phagocytes—the macrophages of the CNS

• Engulf invading microorganisms and dead neurons

• Derive from blood cells called monocytes

• Ependymal cells

• Line the central cavity of the spinal cord and brain

• Bear cilia—help circulate the cerebrospinal fluid

• Oligodendrocytes—have few branches

• Wrap their cell processes around axons in CNS

• Produce myelin sheaths

Neuroglia in the PNS

• Satellite cells—surround neuron cell bodies within ganglia

• Schwann cells (neurolemmocytes)—surround axons in the PNS

• Form myelin sheath around axons of the PNS

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Fundamentals of the Nervous System andNervous Tissue

Myelin Sheaths

• Segmented structures composed of the lipoprotein myelin

• Surround thicker axons

• Form an insulating layer

• Prevent leakage of electrical current

• Increase the speed of impulse conduction

Myelin Sheaths in the PNS

• Formed by Schwann cells

• Develop during fetal period and in the first year of postnatal life

• Schwann cells wrap in concentric layers around the axon

• Cover the axon in a tightly packed coil of membranes

• Neurilemma

• Material external to myelin layers

• Nodes of Ranvier—gaps along axon

• Thick axons are myelinated

• Thin axons are unmyelinated

• Conduct impulses more slowly

• Oligodendrocytes form the myelin sheaths in the CNS

• Have multiple processes

• Coil around several different axons

Nerves

• Nerves—cablelike organs in the PNS

• Consists of numerous axons wrapped in connective tissue

• Axon is surrounded by Schwann cells

• You see many nerves in lab

• Nerves of brachial plexus

• Radial, axillary, median, musculocutaneous, ulnar

• Nerves of lumbosacral plexus

Nerves

• Endoneurium—layer of delicate connective tissue surrounding the axon

• Perineurium—connective tissue wrapping surrounding a nerve fascicle

• Nerve fascicles—groups of axons bound into bundles

• Epineurium—whole nerve is surrounded by tough fibrous sheath

Gray and White Matter in the CNS

• Gray matter

• Is gray-colored and surrounds hollow central cavities of the CNS

• Forms H-shaped region in the spinal cord

• Dorsal half contains cell bodies of interneurons

• Ventral half contains cell bodies of motor neurons

• Primarily composed of neuronal cell bodies, dendrites, unmyelinated axons

• Surrounds white matter of CNS in cerebral cortex and cerebellum

• White matter

• Lies external to the gray matter of the CNS

• Composed of myelinated axons

• Consists of axons passing between specific regions of the CNS

• Tracts are bundles of axons traveling to similar destinations

Integration Between the PNS and CNS

• The CNS and PNS are functionally interrelated

• Nerves of the PNS

• Information pathways to and from body periphery

• Afferent PNS fibers respond to sensory stimuli

• Efferent PNS fibers transmit motor stimuli from CNS to muscles and glands

Integration Between the PNS and CNS

• Nerves of the CNS

• Composed on interneurons that

• Process and receive sensory information

• Direct information to specific CNS regions

• Initiate appropriate motor responses

• Transport information from one area of the CNS to another

Reflex Arcs

• Reflex arcs—simple chains of neurons

• Explain reflex behaviors

• Determine structural plan of the nervous system

• Responsible for reflexes

• Rapid, autonomic motor responses

• Can be visceral or somatic

Five Essential Components to the Reflex Arc

• Receptor—site where stimulus acts

• Sensory neuron—transmits afferent impulses to the CNS

• Integration center—consists of one or more synapses in the CNS

• Motor neuron—conducts efferent impulses from integration center to an effector

• Effector—muscle or gland cell

• Responds to efferent impulses

• Contracting or secreting

Types of Reflexes

• Monosynaptic reflex

• Simplest of all reflexes

• Just one synapse

• The fastest of all reflexes

• Knee-jerk reflex

• Polysynaptic reflex

• More common type of reflex

• Most have a single interneuron between the sensory and motor neuron

• Withdrawal reflexes

Neuronal Circuits

• Diverging circuit—one presynaptic neuron synapses with several other neurons (divergence)

• Converging circuit—many neurons synapse on a single postsynaptic neuron (convergence)

• Reverberating circuit—circuit that receives feedback via a collateral axon from a neuron in the circuit

• Serial processing

• Neurons pass a signal to a specific destination along a single pathway from one to another

• Parallel processing

• Input is delivered along many pathways; a single sensory stimulus results in multiple perceptions

Simplified Design of the Nervous System

• Three-neuron reflex arcs

• Basis of the structural plan of the nervous system

• Similar reflexes are associated with the brain

• Sensory neurons—located dorsally

• Cell bodies outside the CNS in sensory ganglia

• Central processes enter dorsal aspect of the spinal cord

• Motor neurons—located ventrally

• Axons exit the ventral aspect of the spinal cord

• Interneurons—located centrally

• Synapse with sensory neurons

• Interneurons are neurons confined to CNS

• Long chains of interneurons between sensory and motor neurons

Disorders of the Nervous System

• Multiple sclerosis

• Common cause of neural disability

• An autoimmune disease

• Immune system attacks the myelin around axons in the CNS

• Varies widely in intensity among those affected

• More women than men are affected

• When men are affected, disease develops quicker and is more devastating

• Cause is incompletely understood

Neuronal Regeneration

• Neural injuries may cause permanent dysfunction

• If axons alone are destroyed, cells bodies often survive and the axons may regenerate

• PNS—macrophages invade and destroy axon distal to the injury

• Axon filaments grow peripherally from injured site

• Partial recovery is sometimes possible

Neuronal Regeneration

• CNS—neuroglia never form bands to guide re-growing axons and may hinder axon growth with growth-inhibiting chemicals

• No effective regeneration after injury to the spinal cord and brain

Nervous Tissue Throughout Life

• Nervous system develops from the dorsal ectoderm

• Invaginates to form the neural tube and neural crest

• Neural tube walls begin as neuroepithelial cells

• These cells divide and become neuroblasts

Nervous Tissue Throughout Life

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The CentralNervous System

The Central Nervous System

• Central nervous system

• The brain and spinal cord

• Directional terms unique to the CNS

• Rostral—toward the nose

• Caudal—toward the tail

The Spinal Cord

• Functions of the spinal cord

• Spinal nerves attach to it

• Provides two-way conduction pathway

• Major center for reflexes

• Location of the spinal cord

• Runs through the vertebral canal

• Extends from the foramen magnum to the level of the vertebra L1 or L2

• Conus medullaris

• The inferior end of the spinal cord

• Filum terminale

• Long filament of connective tissue

• Attaches to the coccyx inferiorly

• Cervical and lumbar enlargements

• Where nerves for upper and lower limbs arise

• Cauda equina

• Collection of spinal nerve roots

• Spinal cord segments

• Indicate the region of the spinal cord from which spinal nerves emerge

• Designated by the spinal nerve that issues from it

• T1 is the region where the first thoracic nerve emerges

The Spinal Cord

• Two deep grooves run the length of the cord

• Posterior median sulcus

• Anterior median fissure

White Matter of the Spinal Cord

• White matter

• Outer region of the spinal cord

• Composed of myelinated and unmyelinated axons

• Allow communication between spinal cord and brain

• Fibers classified by type

• Ascending fibers

• Descending fibers

• Commisural fibers

Gray Matter of the Spinal Cord and Spinal Roots

• Shaped like the letter “H”

• Gray commissure—contains the central canal

• Dorsal horns

• Consist of interneurons

• Ventral and lateral horns

• Contain cell bodies of motor neurons

Organization of the Gray Matter of the Spinal Cord

• Gray matter

• Divided according to somatic and visceral regions

• SS—somatic sensory

• VS—visceral sensory

• VM—visceral motor

• SM—somatic motor

Protection of the Spinal Cord

• Protected by vertebrae, meninges, and CSF

• Meninges

• Dura mater—a single layer surrounding spinal cord

• Arachnoid mater—lies deep to the dura mater

• Pia mater—innermost layer

• Delicate layer of connective tissue

• Extends to the coccyx

• Denticulate ligaments—lateral extensions of pia mater

Cerebrospinal Fluid

• Fills the hollow cavities of the brain and spinal cord

• Provides a liquid cushion for the spinal cord and brain

• Other functions:

• Nourishes brain and spinal cord

• Removes wastes

• Carries chemical signals between parts of the CNS

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The CentralNervous System

The Brain

• Performs the most complex neural functions

• Intelligence

• Consciousness

• Memory

• Sensory-motor integration

• Involved in innervation of the head

• Brain also controls:

• Heart rate, respiratory rate, blood pressure

• Autonomic nervous system

• Endocrine system

Embryonic Development of the Brain

• Brain arises from rostral part of the neural tube

• Three primary brain vesicles in 4-week-old embryo

• Prosencephalon—the forebrain

• Mesencephalon—the midbrain

• Rhombencephalon—the hindbrain

• Secondary brain vesicles

• Prosencephalon

• Divides into telencephalon and diencephalon

• Mesencephalon—remains undivided

• Rhombencephalon

• Divides into metencephalon and myelencephalon

• Structures of the adult brain

• Develop from secondary brain vesicles

• Telencephalon à the cerebral hemispheres

• Diencephalon à thalamus, hypothalamus, and epithalamus

• Metencephalon à pons and cerebellum

• Myelencephalon à medulla oblongata