NERVOUS SYSTEM

NERVOUS FUNCTIONS• Body’s master controlling and communicating

system• Three functions– Sensory input• Gathers information

from sensory receptors

– Integration• Processes and interprets

sensory input

– Motor output• Activates effector organs to cause a response

Nervous System Organization

ORGANIZATION

Two Principal Parts of the System• Central nervous system (CNS)– Brain and spinal cord– Integrating and command center• Interprets sensory input• Dictates motor responses

• Peripheral nervous system (PNS)– Nerves extending from brain and spinal cord– Carry impulses to and from the CNS

PERIPHERAL DIVISIONS

Two Functional Subdivisions of the PNS• Sensory division– “afferent division”– Nerve fibers conveying impulses to the CNS

• Somatic afferent fibers convey impulses from the skin, muscles, and joints

• Visceral afferent fibers convey impulses from visceral organs• Motor division– , “efferent division”– Nerve fibers conveying impulses from the CNS

ORGANIZATION

HISTOLOGY

• Nervous system consists mainly of nervous tissue• Highly cellular– e.g., <20% extracellular space in CNS

• Two principal cell types– Neurons

• Excitable nerve cells that transmit electrical signals

– Supporting cells• Smaller cells surrounding and wrapping neurons• “Neuroglia”

NEURONS• Nerve cells• Structural units of nervous system

– Billions are present in nervous system• Conduct messages throughout body

– Nerve impulses• Extreme longevity

– Can function optimally for entire lifetime• Amitotic

– Ability to divide is lost in mature cells– Cannot be replaced if destroyed

• Some (very few) exceptions• e.g., stem cells present in olfactory epithelium can produce new neurons

– Stem cell research shows great promise in repairing damaged neurons• High metabolic rate

– Require large amounts of oxygen and glucose

Neurons

Axon of anotherneuron

Axon of anotherneuron

Cell BodyCell BodyDendritesDendrites

AxonAxon

MyelinSheathMyelinSheath

Dendrites of another neuron

Dendrites of another neuron

Collins I4 lines

• Based on the diagram, what do you think each part does to receive and pass along an impulse toward the brain

Agenda11/3/11 – Day 1

• Take more notes• HW- vocab

NEURONS• Generally large, complex cells• Structures vary, but all neurons have the same basic

structure– Cell body– Slender processes

extending from cell body

– Plasma membrane is site of signaling

NEURON CELL BODY• Most neuron cell bodies are located in the CNS– Protected by bones of skull or vertebral column

• Clusters of cell bodies in the CNS are termed “nuclei”

• Clusters of cell bodies in the PNS are termed “ganglia”

NEURON CELL BODY• Major biosynthetic (control) center of neuron• Other usual organelles present except CENTRIOLES

-Why?• What do centrioles

do?

NEURON PROCESSES

• Extend from the neuron’s cell body

• Two types of neuron processes– Dendrites– Axons

NEURON PROCESSESTypical Dendrite• Short, slender, branching extensions of cell body– Generally hundreds clustering close to cell body– Most cell body organelles also present in dendrites

• Main receptive / input regions– Large surface area for

receiving signals from other neurons

– Convey incoming messages toward cell body

NEURON PROCESSESTypical Axon• Single axon per neuron• The axon forms from the narrowing of the cell body.

The region between the large cell body and the axon is the “axon hillock”

• Sometimes very short• Sometimes very long– e.g., axons controlling

big toe are 3 – 4 feet long

NEURON PROCESSESTypical Axon• Single axon may branch along length• “Axon collaterals” extend from neurons at ~ 90o angles• Usually branches

profusely at end– 10,000 or more

terminal branches is common

– Distal endings termed “axonal terminals”

NEURON PROCESSESTypical Axon• Conducting component of neuron• Generates nerve impulse• Transmits nerve

impulses away from cell body towards the

axonal terminals

NEURON PROCESSESTypical Axon terminal• Axonal terminals are secretory component of neuron• Sequence of events– Signal reaches terminals– Membranes of vesicles fuse with

plasma membrane– Neurotransmitters released– Neurotransmitters interact

with either other neurons or effector cells• Excite or inhibit

VocabularyEither in flash card form OR in list• CNS

• PNS• Neuron• Stimulus• Afferent division• Efferent division• neuroglia

• Amitotic• Dendrite• Cell body• Axon• Axon terminal• Ganglia• Nuclei (in terms of

clusters)

Collins I2 lines

• What is the difference between the PNS and the CNS?

Agenda11/4/11 -- Day 2

• Remember quiz 11/9• Take notes• Complete labeling and coloring of neuroglia• HW-complete ALL vocab terms

MYELIN SHEATH

• Whitish, fatty covering the axons of many neurons

• Protects and electrically insulates fibers• Increases speed of nerve impulse transmission– Some axons and all dendrites are unmyelinated

MYELIN SHEATH• In PNS, Schwann cells Continually wrap around the axon

of a neuron– Result is many concentric layers of plasma membrane

surrounding the axon– Thickness depends on number of wrappings

• Nucleus and most of cytoplasm exist as a bulge external to the myelin sheath

Myelin sheath and schwann cells

Node of Ranvier

Schwann Cells

MYELIN SHEATH• Adjacent Schwann cells on axon do not touch

each other– Gaps in sheath occur at regular intervals • “Nodes of Ranvier”

– Axon collaterals can emerge at these nodes

MYELIN SHEATH

• In CNS, there are both myelinated and unmyelinated axons

• Oligodendrocytes, not Schwann cells, form CNS myelin sheaths– Numerous processes that can coil around

numerous (up to 60) axons at once

NEUROGLIA

• “Nerve glue”• Six types of small cells associated with neurons– 4 in CNS– 2 in PNS

• Several functions– Supportive scaffolding for neurons– Electrical isolation of neurons– Neuron health and growth

CNS NEUROGLIA

• Astrocytes• Microglia• Ependymal cells• Oligodendrocytes

CNS NEUROGLIAAstrocytes

• Anchor neurons to capillary blood supply• Facilitate nutrient delivery to neurons– (blood astrocyte neuron)

CNS NEUROGLIAMicroglia• Small ovoid cells; thorny looking• Transform into macrophage– Phagocytize microorganisms, debris– (Cells of immune system cannot enter the CNS)

CNS NEUROGLIAOligodendrocytes• Wrap processes tightly around thicker neuron

fibers in CNS– Makes “Myelin sheath”– Insulating covering

CNS NEUROGLIAEpendymal Cells• Line central cavities of brain and spinal cord• Many are ciliated– Beating helps circulate cerebrospinal fluid cushioning brain

and spinal cord

PNS NEUROGLIASchwann cells• Surround and form myelin sheaths around larger

neurons of PNS– Functionally similar to oligodendrocytes

PNS NEUROGLIA

• Satellite cells– Surround cell bodies of PNS ganglia

HW- Vocab Terms

• Myelin sheath• Schwann cells• Nodes of ranvier• Oligodendrocytes• Neuroglea• Astrocyte• Microglia• Ependymal cell• Satalite cell

MYELIN SHEATH

• White matter– Regions of the brain and spinal cord containing

dense collections of myelinated fibers

• Gray matter– Regions of the brain and spinal cord containing

mostly nerve cell bodies and unmyelinated fibers

NEURON CLASSIFICATION

• Structural classification based upon number of processes– Multipolar neurons– Bipolar neurons– Unipolar neurons

• Functional classification based upon direction nerve impulse travels– Sensory (afferent) neurons– Motor (efferent) neurons– Interneurons (association neurons)

NEURON CLASSIFICATION

Structural Classification• Multipolar neurons

– Three or more processes– Most common neuron type

in humans• (> 99% of neurons)

• Bipolar neurons– Two processes – axon and

dendrite– Found only in some special

sense organs • e.g., retina of eye

– Act as receptor cells

• Unipolar neurons– Single short process– Process divides into proximal

and distal branches• Distal process often associated

with a sensory receptor– “Peripheral process”

• Central process enters CNS– Most are sensory neurons in

PNS

Classification of neurons by shape

NEURON CLASSIFICATIONFunctional Classification• Sensory (afferent) neurons– Transmit impulses toward CNS

• From sensory receptors or internal organs

– Most are unipolar– Cell bodies are located outside

CNS• Motor (efferent) neurons– Carry impulses away from CNS

• Toward effector organs– Multipolar– Cell bodies generally located in

the CNS

• Interneurons – a.k.a., association

neurons– Lie between motor

and sensory neurons in neural pathways

– Shuttle signals through CNS pathways where integration occurs

– > 99% of neurons in body

– Most are multipolar– Most are confined

within the CNS

NEUROPHYSIOLOGY

• Neurons are highly irritable– Responsive to stimuli

• Response to stimulus is action potential– Electrical impulse carried along length of axon– Always the same regardless of stimulus– The underlying functional feature of the nervous

system

ION CHANNELS

Plasma membranes contain various ion channels• Passive channels (leakage channels)– Always open

• Active channels (gated channels)– Ligand-gated channels

• Open when specific chemical binds

– Voltage-gated channels• Open and close in response to membrane potential

– Mechanically-gated channels• Open in response to physical deformation of receptor

– e.g., touch and pressure receptors

MEMBRANE POTENTIALS• A voltage exists across the plasma membrane– Due to separation of oppositely charged ions

• Potential difference in a resting membrane is termed its “resting membrane potential”– ~ -70 mV in a resting

neuron– Membrane is

“polarized”

MEMBRANE POTENTIALS

• Neurons use changes in membrane potentials as signals– Used to receive, integrate, and send signals

• Changes in membrane potentials produced by– Anything changing membrane permeability to ions– Anything altering ion concentrations

• Two types of signals– Graded potentials

• Short-distance signals– Action potentials

• Long-distance signals

MEMBRANE POTENTIALS

Graded Potentials• Short-lived local changes in membrane potential– Either depolarizations or hyperpolarizations

• Cause current flows that decrease in magnitude with distance

• Magnitude of potential dependent upon stimulus strength– Stronger stimulus larger voltage change– Larger voltage change farther current flows

MEMBRANE POTENTIALSGraded Potentials• Triggered by change in neuron’s environment– Change causes gated ion channels to open

• Small area of neuron’s plasma membrane becomes depolarized (by this stimulus)

• Current flows on both sides of the membrane– + moves toward – and vise versa

MEMBRANE POTENTIALSGraded Potentials• Inside cell: + ions move away from depolarized area• Outside cell: + ions move toward depolarized area– (+ and – ions switch places)

• Membrane is leaky– Most of the charge is quickly lost through membrane– Current dies out after traveling a short distance

MEMBRANE POTENTIALS

Graded Potentials• Act as signals over very short distances• Important in initiating action potentials

MEMBRANE POTENTIALS

Action Potentials• Principal means by which neurons communicate– Brief reversal of membrane potential

• Total amplitude of ~ 100 mV (-70 +30)

– Depolarization followed by repolarization, then brief period of hyperpolarization

– Time for entire event is only a few milliseconds

• Events in generation and transmission of an action potential identical between neurons and skeletal muscle cells

ACTION POTENTIALS

ACTION POTENTIALS

• Not all local depolarizations produce action potentials

• Depolarization must reach threshold values– Brief, weak stimuli produce sub threshold

depolarizations that are not translated into nerve impulses

– Stronger threshold stimuli produce depolarizing events

ACTION POTENTIALS

• Action potential is all-or-nothing phenomenon– Happens completely or doesn’t happen

• Independent of stimulus strength once generated– Strong stimuli generate more impulses of the

same strength per unit time– Intensity is determined by number of impulses per

unit time

ACTION POTENTIALS

Multiple Sclerosis (MS)• Autoimmune disease mainly affecting young

adults• Myelin sheaths in CNS are gradually destroyed• Interferes with impulse conduction– Visual disturbances, muscle control problems,

speech disturbances, etc.

• Some modern treatments showing some promise in delaying problems

SYNAPSE• Nerve impulse reaches axonal terminal• Voltage-gated Ca2+ channels open in axon– Ca2+ enters presynaptic neuron

• Neurotransmitter is released via exocytosis– Vesicles fuse with axonal membrane

• Neurotransmitter binds to postsynaptic receptors• Ion channels open in

postsynaptic membrane– Result is excitation or

inhibition

SYNAPSE

• Binding of neurotransmitter to its receptor is reversible

• Permeability affected as long as neurotransmitter is bound to its receptor

• Neurotransmitters do not persist in the synaptic cleft– Degraded by enzymes associated with postsynaptic

membrane– Reuptake by astrocytes or presynaptic terminal– Diffusion of neurotransmitters away from synapse

NEUROTRANSMITTERS• More than fifty neurotransmitters identified• Most neurons make two or more– Can be released singly or together

Classification by Structure• Acetylcholine (ACh)• Biogenic amines• Amino acids• Peptides• ATP• Dissolved gases

Classification by Function• Excitatory/Inhibitory• Direct/Indirect