NERVOUS SYSTEM

NERVOUS SYSTEM: TRUE OR FALSE?

HUMANS ONLY USE 10% OF THEIR BRAIN OR LESS False!

THE BRAIN USES 20% OF THE ENERGY WE CONSUME EACH DAY True!

MENTAL ABILITIES ARE SEPARATED INTO DISCRETE LEFT AND RIGHT “HALVES”

False!

NEW NEURONS CANNOT BE CREATED False!

THE BRAIN MAKES UP 2% OF OUR BODY WEIGHT True!

THE BRAIN DOES NOT CHANGE AFTER GROWTH THAT OCCURS DURING ADOLESCENCE

False!

A NERVE IS A SINGLE CELL False!

THE BRAIN USES 50% OF THE OXYGEN WE BREATHE INFalse!

THE LONGEST CELLS IN THE HUMAN BODY ARE IN THE SCIATIC NERVE

True!

THE HUMAN BRAIN IS FIRM AND GREY False!

TYPES OF NERVOUS SYSTEM CELLS

Functions of the Nervous System:

WHAT IS THE NERVOUS SYSTEM? WHAT ARE THE FUNCTIONS OF THE NERVOUS SYSTEM? A system of cells, tissue, and organs that

regulate the body’s responses to external and internal stimuli.Communication between organ systems.Provides information about environmental

conditions to all internal organs.Translates environmental stimuli to messages

understood by the cells External stimuli: environmental factors that influence

metabolic changes in a cell or physiological changes in tissues and organs.

Internal stimuli: cell secretions used to communicate info about a cell’s jobs and needs.

NEURONS & NEUROGLIA Neural tube Stem cells

Neurons & neurogliaNeurons:

excitable cells receive, interpret, and transmit external and

internal stimuli.Neuroglia:

maintain the excitability & health of neurons. Don’t take part in communication.Supportive.

TYPES OF NERVOUS SYSTEM CELLS: NEURAL CREST CELLS

• Derived from the neural tube

• Bidirectional communication with neuroglia and neurons.

• Play a role in the development of the nervous system.

ANATOMY OF A NEURON Categorized by their cell anatomy and

mode of communication

ANATOMY OF A NEURON Common features:

Axon: Long process Extends from cell body from the axon hillock Transfers impulses to the terminus. Usually one per neuron. Some have branches (collaterals) that reach out to

other neurons. Job: initiate the electrical signal that will be

transmitted from the axon to glands, muscles, other neurons.

ANATOMY OF A NEURON CONT’D…

Terminus: Releases neurotransmitters—transmit

info from one neuron to another. Cells must possess neurotransmitter

receptors if they are to respond to the stimulus

ANATOMY OF A NEURON

Cell body (soma): contains nucleus & organelles, ER and Golgi bodies that produce specialized enzymes and secretions needed for nerve cell communication.

Dendrite: antennae. Receive stimuli from several sources

Terminus: Releases

neurotransmitters

Axon: long process that comes off the body; transfers impulses to the terminus. Job: transmit electrical signal to glands, muscles, other neurons.

Axon Hillock: where the axon originates

SYNAPSE

Neurons don’t directly touch the cells with which they communicate

Form a synapse: the junction where an impulse is transmitted from one neuron to another.

ANATOMY OF A SYNAPSE

1. Synaptic Cleft: the space between the terminus of one neuron and the dendrites of another.

2. Pre-synaptic neuron: produces the neurotransmitter

3. Post-synaptic neuron: receives the neurotransmitter

4. Neurotransmitter: most pre-synaptic neurons produce 1 kind.

5. Receptor: Post-synaptic neurons an have a variety of neurotransmitter receptors.

SHAPES OF NEURONS TELL US ABOUT THEIR FUNCTIONS

NEUROGLIA AND STEM CELLS Make up bulk of cells in the nervous system Closely associated with neurons High lipid content

White in appearanceVulnerable to improper diet

Many types…

TYPES OF NEUROGLIA Astrocytes Ependymal cells Microglia Oligodendrites Radial glia Satellite cells Schwann cells

ASTROCYTES A.k.a. macroglia Largest class Star-shaped, w/ many branches, or feet Often associate w/ blood vessels

Control types of materials that pass from blood to neurons

Protects neurons from harmful agentsCreates blood-brain barrierMostly found in brain, spinal cord

EPENDYMAL CELLS Primary secretory cells Line cavities of brain, spinal column Produce cerebrospinal fluid (CSF)

Bathes, nourishes, protects brain, spinal cord

Cilia help circulate CSF

MICROGLIA Highly variable Found throughout nervous system Many carry out phagocytosis,

removing infectious agents, repair damage

Others produce secretions that maintain neuron health, assist in healing

Malfunctions often produce disorders

OLIGODENDRITES Large, w/ numerous branching

processes Wrap around axons of neurons

Form an insulating cover (myelin sheath) Found only in brain, spinal cord Speeds up nerve transmission

RADIAL GLIA Found in developing nervous system Provide framework for growing

interconnections In adults, assist maintenance of brain

and eyesCommunicate “needs” of these cells

SATELLITE CELLS Small, numerous Cover surface of neurons outside brain,

spinal cord Help maintain chemical environment May help w/ nerve cell repair

SCHWANN CELLS Form myelin sheath around axons of

neurons outside of brain, spinal cord Gaps between cells called nodes of

RanvierHelp speed transmission

NEURON PHYSIOLOGY

Functions of the Nervous System:

GUIDING QUESTIONS1. How do neurons communicate with one

another?2. What are the two types of

neurotransmitters?3. What has to happen to a neuron before it

generates or propagates an action potential?

4. What are the four stages of an action potential? Describe the changes that occur within the cell during each stage.

5. In what order do action potentials travel through a neuron?

HOW DO NEURONS COMMUNICATE WITH ONE ANOTHER? Neurons transmit the excitable response

that started in dendrites/cell body down through the axon

Cytoplasm must be modified to communicate the excited state to other cells…

Neurons transmit information to other cells by exploiting the action potential effect

WHAT HAS TO HAPPEN TO A NEURON BEFORE IT GENERATES OR PROPAGATES AN ACTION POTENTIAL? Must maintain an excitable condition called

resting potential.Chemically unstable conditionSodium ion concentration higher outside cell

than inside Creates a diffusion potential; sodium “wants” to

enterPotassium ions higher inside cell than outside

A.k.a. a “salty banana”

Sodium/potassium pump maintains this potential

FIRST LOOK: ACTION POTENTIAL ANIMATION Animation

WHAT ARE THE FOUR STAGES OF AN ACTION POTENTIAL? Debatable… some have 6 phases

DepolarizationRepolarizationHyperpolarizationRecovery phase

PHASE 1: DEPOLARIZATION Cytoplasm’s charge starts at -70 mV Dendrites receive stimulus from a.

another cell or b. the environment Sodium channels open, allowing rapid

influx If enough channels open, cytoplasm’s

charge reaches -55 mV = thresholdRequired for an action potential to

propagate, or travel, across the cell membrane

ALL-OR-NOTHING! At threshold, more Na+ channels open Charge of cytoplasm increases to +30

mV

Each depolarized segment of axon depolarizes the adjacent segment… like falling dominoes

PHASE 2: REPOLARIZATION Sodium channels close, and potassium

channels open K+ ions diffuse outward, causing the

cell’s interior to become more negative Neuron is becoming repolarized.

HYPERPOLARIZATION Repolarization is rapid! Cell moves past resting potential (-70

mv) and overshoots, reaching -90 mV.

This is hyperpolarization…

HYPERPOLARIZATION K+ gates on K+ channel proteins are

slow to close, allowing this hyperpolarization

Why does this occur?1. Prevents neuron from becoming

stimulated during repolarization period2. Prevents action potential from

travelling both forward AND backward… becomes a unidirectional signal.

= REFRACTORY PERIOD

RECOVERY PHASE Sodium/Potassium pumps return cell to

resting potential (Na+ outside, K+ inside)

Some cells send a second impulse before recovery is complete = tetany

ANOTHER ANIMATION Click here

SALTATORY CONDUCTION Action potentials relatively slow (5 25

m/second) To increase velocity, neurons’ axons are

myelinated. Reduces amount of membrane that must be

depolarizedStimulus “jumps” from node to node10 120 meters/second!

WORD CHALLENGE!

Axon Cytoplasm Dendrite Depolarization Diffusion Potential Hyperpolarization Influx K+ gated ion

channels K+ ion Na+ gated ion

channels Na+ ion Na+/K+ pump Outflux Refractory period Repolarization

Working with your partner, write a “story” that describes an action potential.

NEURON-TO-NEURON COMMUNICATION When the terminus depolarizes, calcium

ions diffuse into terminusStimulates movement of vesicles towards

terminal knobs Vesicles fuse w/ cell membrane, releasing

contentsThese vesicles contain neurotransmitters

Neurotransmitters diffuse across synaptic cleft, binding to matching receptors on post-synaptic neuron

4 STAGES OF NEUROTRANSMITTER COMMUNICATION 1. Synthesis and storage of

neurotransmitters 2. Neurotransmitter release 3. Neurotransmitter binding to post-

synaptic receptors 4. Inactivation of neurotransmitters

Synthesis occurs in nerve cell body, transferred to terminus

Inactivation occurs by degradation or reuptake (for recycling)… many drugs affect these processes

NEUROTRANSMITTER SUMMARY

TWO TYPES OF NEUROTRANSMITTERS Chemical signals that transfer action

potential from affector (sensory neuron receptor) to an effector (motor neuron, muscle, gland)

Can be excitatory or inhibitory Excitatory: helps depolarize post-

synaptic neuron (move interior closer to threshold)

Inhibitory: hyperpolarize post-synaptic neuron (move interior farther from threshold)

MAJOR CLASSES OF NEUROTRANSMITTERS Amino acids:

Usually in brain, spinal columnAspartate, gamma-aminobutyric acid

(GABA) (inhibitory), glutamate (excitatory), glycine (inhibitory)

Catecholamines: Excitatory; made from tyrosineEx. Epinephrine, norepinephrine, dopamine

(both excitatory/inhibitory)Associated w/ stress

MAJOR CLASSES OF NEUROTRANSMITTERS Cholinergics: Excites muscle cells;

made from dietary fats, other metabolic compoundsAcetylcholine most common

Monoamines: Related to catecholaminesSerotonin (made from tryptophan)

Inhibits catecholamine NT’sHistamine: associated w/ pain sensations,

stress

TYPES OF NEURON COMMUNICATION

Function of the Nervous System

TYPES OF SYNAPSE ARRANGEMENTS Key term: Innervate = supply a body part

w/ nervous stimulation Ex: Gland, muscle, neuron

Types of neural pathways (focus on the term!) Axo-dendritic synapse: terminus

dendrite connection Axo-somatic synapse: terminus nerve

cell body connection Axo-axonic synapse: terminus axon

connection Reverberating pathway (brain)

REVERBERATING PATHWAYS Neurons can stimulate themselves

repeatedly until another stimulus stops it

Linked to important pathways in brainEmotions, learning, memory

Breakdown in these pathways leads to disordersEx. Epilepsy (uncontrolled excitatory

activity)

EPSP VS. IPSP Type of communication that takes place

between two neurons also significant:Excitatory postsynaptic potential

(EPSP) = action potential generated In some pathways, may require multiple,

simultaneous EPSP’s to create an action potential

Inhibitory postsynaptic potential (IPSP) = action potential prevented Hyperpolarizes the membrane

Many neurons have both EPSP and IPSP connections – allows decision-making in brain!

REFLEXES

Functions of the Nervous System:

INNERVATION SEQUENCE Sensory neuron (w/ receptor)

interneuron (in spinal cord) motor neuron

OR… Afferent neuron interneuron Efferent

neuron

WHAT IS A REFLEX? Instantaneous, involuntary response to

a stimulus No intervention/conscious control

required Neurons arranged in a reflex arc

HOW DO REFLEXES BEGIN? Stimulus excites an affector

Carry out physiological job = transductionConvert a stimulus (touch/pain) into a

message that can be relayed to cellsPart of sensory nerve’s dendritesTransfers response to interneuron, which

relays information to motor neuron Motor neuron stimulates effector, which carries

out task of the reflex

Interneurons communicate w/ brain= certain reflexes can be “trained”, like

urination and bowel movements

PATHOLOGY OF THE NERVOUS SYSTEM

Function of the Nervous System:

TYPES OF NERVE CELL DISEASES Infectious: causes by microorganisms Degenerative: progressive

deterioration of a cell/tissue Congenital: embryological/maturation

errors Toxicological: poisons that affect cell

metabolism/communication Traumatic: injuries resulting

INFECTIOUSMost common: bacterial Release toxins into blood

Can inflame, kill neurons, neuroglia Affect neuron communication

Ex. Botulism – toxin blocks action of acetylcholine Produces flaccid paralysis (no muscle contraction)

Ex. Tetanus – toxin enhances acetylcholine Prevents muscle relaxation

Endotoxins: produced as bacteria replicate, die Cause immediate death to neuroglia and

neurons

ENDOTOXINS Commonly cause diseases Examples

Encephalitis – inflammation of brainMeningitis – inflammation of membranes

surrounding brain, spinal cord Fungal toxins similar to those from

bacteria

NEUROTROPHIC MICROBES Enter and infect nervous system cells Varied:

ProtistaViruses: herpes, rabiesViroidsPrions: Mad cow/BSE/Creutzfeldt-Jakob

Kill cells outright/produce inflammation Carried by mosquitoes, biting insects

DEGENERATIVE DISORDERSAmylotrophic lateral sclerosis (ALS) a.k.a. Lou Gehrig’s disease

Faulty mitochondria Gradual loss of muscle function

DemyelinationLoss of neuroglia around axons, bodies of

neuronsCauses: metabolic, loss of blood flow

Results in slower neural impulses, eventual degeneration

Ex: Multiple sclerosis

CONGENITAL Krabbe’s disease

Lack enzyme (galactosylceramide beta-galactosidase) that prevents accumulation of toxic wastes in nerve cells

Buildup of harmful fats Abnormal neuron functioning, diminished

neuroglia maturation

Hirschsprung’s diseaseAffects large intestine neuronsNerve cells stop growing during

development, causing loss of function of LI

TOXINS Variety of sources:

LeadArsenic, cyanide (pesticides) – block cellular

respiration, disabling neuronsTetrodotoxins: inhibits flow of sodium into

nerve cells

TRAUMATIC INJURY Neurons cannot be replaced once they

die* Injured neurons can be repaired

Intact neuroglia must be nearbyCan replicate if only a small number are

killedRebuild damaged components of neuronsRedirect axons to original positions

Encouraged by growth factors

Stem cells show promise

AGING OF THE NERVOUS

SYSTEM

WHY NO REPLACEMENT OF DAMAGED NEURONS? Mitosis is rare!

Cells are so specialized, to divide would mean de-differentiating!

Remember neurons originate from stem cells, not other neurons

= Neurons and neuroglia stay with you throughout life

= They accumulate damage over your lifespan

METABOLISM-DAMAGE CONNECTION The higher the cell’s metabolism, the

greater the buildup of metabolic “oxidizing” byproductsThese come from mitochondriaCan alter DNA metabolic errors that can be fatal

Alcohol, drug abuse, smoking, air pollution accelerate cell aging

GO WITH THE FLOW… As one ages, consistent blood flow to

tissues is lost Neurons are highly susceptible to this

High metabolic needsObtain nutrients, ions for action potentialsMaterials needed for NT’s

Become less responsive to stimuliGlands, muscles, neurons

TONIC FOR THE SOUL? Loss of tonic control

= regular nerve communication with glands, muscles

Without tonic control…Lose mobilityLoss of balance, postureLose muscle mass

INCREASE IN LATENCY Due to increased age Refractory period longer = fewer action potentials

Slows down impulses to muscles, delays sensory communication to brain, body

Wastes collect: plaques, tanglesAmyloid proteins = plaqueTangles = changes in cell’s cytoplasm,

changing shape Lipofuscin = fatty, brown pigment that

builds up; indicator of nerve cell pathology