The master controlling and The master controlling and communicating system of the communicating system of the bodybody

FunctionsFunctions Sensory input –Sensory input – monitor monitor

internal and external internal and external stimulistimuli

Integration – Integration – interpretation interpretation of sensory input of sensory input

Motor output –Motor output – response response to stimuli by activating to stimuli by activating effector organs effector organs

Central nervous system (CNS) Central nervous system (CNS) Consists of Brain and spinal cordConsists of Brain and spinal cord Controls entire organismControls entire organism Integrates incoming information and responsesIntegrates incoming information and responses

Peripheral nervous system (PNS)Peripheral nervous system (PNS) Link between CNS, body and environment Link between CNS, body and environment Consists of Spinal and cranial nervesConsists of Spinal and cranial nerves Carries messages to and from the spinal cord and brainCarries messages to and from the spinal cord and brain

Sensory (afferent) divisionSensory (afferent) division Sensory afferent fibers –Sensory afferent fibers –

carry impulses from skin, carry impulses from skin, skeletal muscles, and joints skeletal muscles, and joints (sensory receptors) to the (sensory receptors) to the brainbrain

Visceral afferent fibers –Visceral afferent fibers – transmit impulses from transmit impulses from visceral organs to the brain visceral organs to the brain

Motor (efferent) division Motor (efferent) division Transmits impulses from the Transmits impulses from the

CNS to effector organs CNS to effector organs (muscles, glands)(muscles, glands)

Two Divisions:Two Divisions:

Somatic nervous system Somatic nervous system (=Voluntary)(=Voluntary) Conscious control of skeletal Conscious control of skeletal

musclesmuscles Conducts impulses from the Conducts impulses from the

CNS to skeletal musclesCNS to skeletal muscles

Autonomic nervous system Autonomic nervous system (ANS= involuntary)(ANS= involuntary) Regulates smooth muscle, Regulates smooth muscle,

cardiac muscle, and glandscardiac muscle, and glands Subconscious or involuntary Subconscious or involuntary

controlcontrol

Sympathetic Nervous System (Thoraco-lumbar outflow)Sympathetic Nervous System (Thoraco-lumbar outflow) ““Flight or fright system”Flight or fright system” Most active during physical activityMost active during physical activity

Parasympathetic Nervous System (Cranial-sacral outflow)Parasympathetic Nervous System (Cranial-sacral outflow) Regulates resting or vegetative functions such as Regulates resting or vegetative functions such as

digesting food or emptying of the urinary bladderdigesting food or emptying of the urinary bladder

The two principal cell types of the nervous The two principal cell types of the nervous system are:system are:Neurons –Neurons – excitable cells that transmit excitable cells that transmit

electrical signalselectrical signals

Supporting (glial) cells –Supporting (glial) cells – cells that surround cells that surround and wrap neuronsand wrap neurons

Structural units of the nervous Structural units of the nervous systemsystem

Receive stimuli and transmit Receive stimuli and transmit action potentialsaction potentials

Long-lifeLong-life

AmitoticAmitotic

Have a high metabolic rateHave a high metabolic rate

Each neuron consists of:Each neuron consists of: BodyBody AxonAxon dendritesdendrites

Cell Body (soma or perikaryon)Cell Body (soma or perikaryon)

Contains the nucleus and a nucleolus & Contains the nucleus and a nucleolus & usual organellesusual organelles

Has no centrioles (amitotic nature)Has no centrioles (amitotic nature)

Has well-developed Nissl bodies (rough Has well-developed Nissl bodies (rough ER)ER)

Nissl bodies -Nissl bodies -primary site of protein primary site of protein synthesissynthesis

Contains an axon hillock –Contains an axon hillock – cone-shaped cone-shaped area from which axons arisearea from which axons arise

Short, branched Short, branched cytoplasmic extensions cytoplasmic extensions

They are the receptive, or They are the receptive, or input, regions of the input, regions of the neuron neuron

Electrical signalsElectrical signals are are conveyed conveyed toward the cell toward the cell bodybody

Slender processes of uniform Slender processes of uniform diameter arising from thediameter arising from the axon axon hillockhillock

Initial segment:Initial segment: beginning of beginning of axonaxon

Axoplasm :Axoplasm : Cytoplasm of axon Cytoplasm of axon

Axolemma :Axolemma : Plasma membrane Plasma membrane of axonof axon

Long axons are called Long axons are called nerve nerve fibersfibers

Usually there is only Usually there is only one one unbranched axonunbranched axon per neuron per neuron

Rare branches, if present, are Rare branches, if present, are called called axon collateralsaxon collaterals

Presynaptic (Axon) terminal –Presynaptic (Axon) terminal – branched terminus of an axonbranched terminus of an axon

Trigger zone:Trigger zone: site where action site where action potentials are generated; potentials are generated; axon axon hillockhillock and part of and part of axon nearest to axon nearest to cell bodycell body

APAP are conducted along the axons are conducted along the axons to axonal terminals and release to axonal terminals and release neurotransmittersneurotransmitters

APAP conduction conduction awayaway from from cell cell bodybody

Neurons can be classified by structure:Neurons can be classified by structure: Multipolar Multipolar Most common in both CNS & PNSMost common in both CNS & PNS Single axon, many dendrites (motor Single axon, many dendrites (motor

neurons and interneurons of CNS)neurons and interneurons of CNS) Bipolar Bipolar two processes (one axon and one two processes (one axon and one

dendrite)dendrite) Are sensory neurons found in the Are sensory neurons found in the

retina, olfactory nerveretina, olfactory nerve Unipolar Unipolar single short process extending from single short process extending from

cell bodycell body Divides into two branches and Divides into two branches and

functions as both dendrite and axon functions as both dendrite and axon (sensory neurons , dorsal root (sensory neurons , dorsal root ganglia)ganglia)

Neurons can be classified by function:Neurons can be classified by function: Sensory (afferent) —Sensory (afferent) — transmit impulses from receptors transmit impulses from receptors

toward the CNStoward the CNS

Motor (efferent) —Motor (efferent) — carry impulses from CNS to muscles and carry impulses from CNS to muscles and glandsglands

Interneurons (association neuronsInterneurons (association neurons) Link sensory and motor Link sensory and motor neurons within CNSneurons within CNS Make up 99% of neurons in bodyMake up 99% of neurons in body

NERVOUS SYSTEM CELL TYPESNERVOUS SYSTEM CELL TYPES

NEUROGLIA (Glial cells)NEUROGLIA (Glial cells) Supporting cellsSupporting cells Surround neuronsSurround neurons Non-conductingNon-conducting 6 types6 types 2 in PNS2 in PNS 4 in CNS4 in CNS

AstrocyteAstrocyte OligodendrocytesOligodendrocytes

Ependymal CellsEpendymal Cells

MicrogliaMicroglia

Satellite CellsSatellite Cells

Schwann CellsSchwann Cells

1) Astrocytes1) Astrocytes In CNS onlyIn CNS only

Anchor neurons to capillariesAnchor neurons to capillaries

Regulate what substances reach the Regulate what substances reach the CNS from the blood (CNS from the blood (blood-brain blood-brain barrierbarrier))

Regulate extracellular brain fluid Regulate extracellular brain fluid compositioncomposition

Pick up excess KPick up excess K++

Recapture released (recycle) Recapture released (recycle) neurotransmittersneurotransmitters

2) Ependymal Cells2) Ependymal Cells

CNS onlyCNS only

Line the cavities of the Line the cavities of the brain and spinal cordbrain and spinal cord

CiliatedCiliated

Circulate the Circulate the cerebrospinal fluid cerebrospinal fluid (CSF)(CSF)

3) Microglia3) Microglia

CNS onlyCNS only

Migrate toward injured Migrate toward injured neuronsneurons

Specialized macrophagesSpecialized macrophages

Phagocytize necrotic Phagocytize necrotic tissue, microorganisms, tissue, microorganisms, and foreign substances and foreign substances that invade the CNSthat invade the CNS

4) Oligodendrocytes4) Oligodendrocytes

CNS onlyCNS only

Wrap extensions Wrap extensions around neuron fibers around neuron fibers (axons)(axons)

Form myelin sheathForm myelin sheath

1) Schwann Cells or 1) Schwann Cells or NeurolemmocytesNeurolemmocytes

PNS onlyPNS only

Wrap around axons of Wrap around axons of neurons in the PNSneurons in the PNS

Forms myelin sheathForms myelin sheath

2) Satellite Cells2) Satellite Cells

PNS onlyPNS only

Surround neuron cell Surround neuron cell bodiesbodies

Provide support and Provide support and nutrients to neuronal cell nutrients to neuronal cell bodiesbodies

Protect neurons from Protect neurons from heavy metal poisons (lead, heavy metal poisons (lead, mercury) by absorbing mercury) by absorbing them them

Myelinated Axons:Myelinated Axons: Whitish, fatty (protein-lipid), Whitish, fatty (protein-lipid), segmented sheath around most long axonssegmented sheath around most long axons

Functions:Functions: Protect the axonProtect the axon Electrically insulate fibers from one anotherElectrically insulate fibers from one another Increase the speed of nerve impulse Increase the speed of nerve impulse

transmissiontransmission

Formed by Formed by Schwann cellsSchwann cells in the in the PNSPNS

In In CNSCNS formed by formed by oligodendrocytesoligodendrocytes

Nodes of Ranvier :Nodes of Ranvier : Gaps in the myelin sheath Gaps in the myelin sheath between adjacent Schwann cellsbetween adjacent Schwann cells

Unmyelinated Axons :Unmyelinated Axons : Schwann cell Schwann cell surrounds nerve fibers but coiling does not surrounds nerve fibers but coiling does not take placetake place

White matter –White matter – dense collections of myelinated fibers dense collections of myelinated fibers

Gray matter –Gray matter – mostly nerve cell bodies and unmyelinated mostly nerve cell bodies and unmyelinated fibersfibers

In brain:In brain: gray is outer cortex as well as inner nuclei; white gray is outer cortex as well as inner nuclei; white is deeperis deeper

In spinal cord:In spinal cord: white is outer, gray is deeper white is outer, gray is deeper

SynapseSynapse Junction between one neuron and Junction between one neuron and

anotheranother

Where two cells communicate with Where two cells communicate with each other each other

Presynaptic neuron –Presynaptic neuron – conducts conducts impulses toward the synapseimpulses toward the synapse

Postsynaptic neuron –Postsynaptic neuron – Cell that Cell that receive the impulse receive the impulse

Most are Most are axo-dendriticaxo-dendritic or or axo-somaticaxo-somatic

Electrical Synapses:Electrical Synapses: Are gap junctions that allow Are gap junctions that allow

ion flow between adjacent ion flow between adjacent cells by protein channels cells by protein channels called called ConnexonsConnexons

Not common in CNSNot common in CNS

Found in cardiac muscle and Found in cardiac muscle and many types of smooth muscle many types of smooth muscle Action potential of one cell Action potential of one cell causes action potential in next causes action potential in next cellcell

Chemical SynapsesChemical Synapses Most are this typeMost are this type

Neurotransmitter Neurotransmitter released from released from synaptic vesiclessynaptic vesicles of of presynaptic neuronpresynaptic neuron

NeurotransmitterNeurotransmitter binds to binds to receptorsreceptors on on postsynaptic membranepostsynaptic membrane

Binding of Binding of neurotransmitterneurotransmitter to to receptorreceptor permeability permeability change in change in postsynaptic membranepostsynaptic membrane

Released at chemical synapsesReleased at chemical synapses

In response to AP Voltage-In response to AP Voltage-regulated calcium channels openregulated calcium channels open

Ca2+ diffuse into presynaptic Ca2+ diffuse into presynaptic terminalterminal

And causes synaptic vesicles to And causes synaptic vesicles to fuse with presynaptic membranefuse with presynaptic membrane

This fusion releases This fusion releases neurotransmitter into the neurotransmitter into the synaptic synaptic cleftcleft via via exocytosisexocytosis

When bound to receptors on When bound to receptors on postsynaptic neuron, the postsynaptic neuron, the neurotransmitter can either neurotransmitter can either exciteexcite or or inhibitinhibit the the postsynaptic neuronpostsynaptic neuron

Resting neurons maintain a difference in Resting neurons maintain a difference in electrical charge inside and outside cell electrical charge inside and outside cell membrane = membrane = RESTING MEMBRANE RESTING MEMBRANE POTENTIAL (RMP)POTENTIAL (RMP)

The inside of the resting neuron is The inside of the resting neuron is negatively charged, the outside is negatively charged, the outside is positively charged. positively charged.

Concentration of KConcentration of K++ higher inside than higher inside than outside celloutside cell

NaNa++ higher outside than inside higher outside than inside

RMPs vary from -40 to RMPs vary from -40 to -90mV in different neuron types-90mV in different neuron types

When bound to receptors on the When bound to receptors on the postsynpaticpostsynpatic neuron membrane: neuron membrane: Causes the opening of positive Causes the opening of positive

ion channelsion channels

Sodium ions enter rapidlySodium ions enter rapidly

RMP becomes more positiveRMP becomes more positive

This positive change in the RMP This positive change in the RMP is called is called depolarizationdepolarization

This brings the neuron closer to This brings the neuron closer to firingfiring

• A positive change in the A positive change in the RMPRMP– Caused by influx of Caused by influx of

positive ionspositive ions

– Causes the inside of Causes the inside of the cell membrane to the cell membrane to become less negativebecome less negative

– Depolarization spreads Depolarization spreads to adjacent areasto adjacent areas

When bound to receptors on the When bound to receptors on the postsynaptic membranepostsynaptic membrane:: Make the membrane more Make the membrane more

permeable to negative ions (usually permeable to negative ions (usually ClCl--))

As negative ions rush into the As negative ions rush into the neuron, the RMP becomes more neuron, the RMP becomes more negativenegative

The negative change in the RMP = The negative change in the RMP = hyperpolarizationhyperpolarization

Brings the neuron farther from firingBrings the neuron farther from firing

• A negative change in A negative change in RMPRMP

• Usually caused by Usually caused by influx of influx of chloride ionschloride ions

• Decreases the Decreases the likelihood of the likelihood of the neuron firingneuron firing

• Short changes in the RMP in Short changes in the RMP in small regions of the membranesmall regions of the membrane

• Can be positive or negative Can be positive or negative (depolarize or hyperpolarize the (depolarize or hyperpolarize the membrane)membrane)

• Alone, not strong enough to Alone, not strong enough to initiate an impulseinitiate an impulse

• summate or add onto each othersummate or add onto each other

• Together, can trigger a nerve Together, can trigger a nerve impulse (action potential)impulse (action potential)

EPSP (Excitatory Postsynaptic EPSP (Excitatory Postsynaptic Potential)Potential)

When depolarization occurs, response is When depolarization occurs, response is

stimulatorystimulatory

& graded potential is called & graded potential is called EPSPEPSP

Binding of a neurotransmitter on the Binding of a neurotransmitter on the postsynaptic membrane postsynaptic membrane more positive more positive RMP, reaches threshold (depolarization RMP, reaches threshold (depolarization occurs)occurs)

producing an action potential and cell producing an action potential and cell

responseresponse

IPSP (Inhibitory Postsynaptic Potential)IPSP (Inhibitory Postsynaptic Potential)

When hyperpolarization occurs, When hyperpolarization occurs,

response is inhibitory response is inhibitory

& graded potential is called & graded potential is called IPSP IPSP

Binding of the neurotransmitter on the Binding of the neurotransmitter on the postsynaptic membrane postsynaptic membrane more negative more negative RMP RMP (hyperpolarization)(hyperpolarization)

Decrease action potentials by moving Decrease action potentials by moving membrane potential farther from membrane potential farther from thresholdthreshold

40 to 50 Known Neurotransmitters40 to 50 Known Neurotransmitters

Acetylcholine (ACh) Acetylcholine (ACh)

Norepinephrine (NE)Norepinephrine (NE)

GABAGABA

DopamineDopamine

SerotoninSerotonin

Action Potential = Nerve ImpulseAction Potential = Nerve Impulse

Consists ofConsists of: Depolarization

Propagation

Repolarization

If depolarization reaches threshold (usually a positive If depolarization reaches threshold (usually a positive change of 15 to 20 mV or more), an action potential is change of 15 to 20 mV or more), an action potential is triggeredtriggered

The positive RMP change causes electrical gates in the The positive RMP change causes electrical gates in the axon hillock to openaxon hillock to open

Sudden large influx of sodium ions causes a reversal in the Sudden large influx of sodium ions causes a reversal in the membrane potential (becomes approx. 100mV more membrane potential (becomes approx. 100mV more positive)positive)

Begins at the axon hillock and travels down the axonBegins at the axon hillock and travels down the axon

Chemically gated channels – open with binding of a specific neurotransmitter

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

Chemically GatedChemically Gated(on dendrite or soma)(on dendrite or soma)

Voltage GatedVoltage Gated(on axon hillock and axon)(on axon hillock and axon)

Movement of the action potential down the axolemma

voltage-gated sodium channels open in response to positive RMP change

Restoration of the RMP back to it’s negative state

A repolarization wave follows the depolarization wave

3 factors contribute to restoring the negative RMP: Sodium (Na+) gates close (it no longer

enters) Potassium (K+) gates open, potassium

rushes out Sodium/potassium pump kicks in

An active process: requires cellular energy

Actively pumps 3 sodium (Na+)

ions out of the cell and 2 potassium (K+) ions in

Potassium leaks back out

Period of time when electrical sodium gates are open

From beginning of action potential until near end of repolarization

No matter how large the stimulus, a second action potential cannot be produced

The interval following the absolute refractory period when:

Sodium gates are closedPotassium gates are openRepolarization is occurring

A stronger-than-threshold stimulus can initiate another action potential

A single EPSP cannot induce A single EPSP cannot induce an action potentialan action potential

EPSP’s can add together or EPSP’s can add together or SUMMATE to initiate an SUMMATE to initiate an action potentialaction potential

Spatial Summation Large numbers of axon

terminals stimulate the postsynaptic neurons simultaneously

Temporal SummationOne or more presynaptic neurons transmit impulses in rapid fire succession

An action potential is an “all or none” phenomenon

When threshold is reached, the action potential will occur completely

If threshold is not reached, the action potential will not occur at all

Occurs only in myelinated axons

Depolarization wave jumps from one node of Ranvier to the next

Results in faster nerve impulse transmission

A nerve impulse in the presynaptic neuron causes release of neurotransmitter into synaptic cleft

Neurotransmitter binding to receptors on postsynaptic neuron dendrite or soma cause certain chemically gated ion channels to open

If Na+ channels open: Rapid influx of Na+ ions (depolarization) A small positive graded potential occurs (EPSP) If RMP changes in a positive direction by 20mV (or reaches the

threshold), voltage gated sodium channels in the axon hillock open Sodium rushes in at the axon hillock resulting in an action potential As the positive ions get pushed down the axon, more voltage

gated sodium channels open and the depolarization continues down the axon (propagation)

The process of restoring the negative RMP begins immediately following the depolarization wave (repolarization)

The larger the axon diameter, the faster the impulse travels

Myelinated axons conduct impulses more rapidly

Fiber Types: Type A fibers

Large diameter axon with thick myelin sheath

Impulse travels at 15 to 150 m/sec. Sensory and motor fibers serving skin,

muscles, joints Type B fibers

Intermediate diameter axon, lightly myelinated

Impulse travels at 3 to 15 m/sec, Part of ANS

Type C fibers Small axon diameter, unmyelinated Slow impulse conduction (1 m/sec. or less) Part of ANS

Organization of neurons in CNS varies in complexity Convergent pathways: many

converge and synapse with smaller number of neurons. E.g., synthesis of data in brain

Divergent pathways: small number of presynaptic neurons synapse with large number of postsynaptic neurons. E.g., important information can be transmitted to many parts of the brain

Oscillating circuit: outputs cause reciprocal activation