Nervous Tissue and Function

Nervous Tissue and Function Function of the Nervous System Organization (Structural and Functional) Supporting Cells of the Nervous System Anatomy of a Neuron Classification of Neurons by Function Graded and Action Potentials Myleination and MS Reflexes Synapses EPSPs and IPSPs Neurotransmitters

Functional Classification of the Peripheral Nervous System

Organization of the Nervous System

and reflexes

inhibitory stimulatory

Figure 11.2

Central nervous system (CNS)Brain and spinal cordIntegrative and control centers

Peripheral nervous system (PNS)Cranial nerves and spinal nervesCommunication lines between theCNS and the rest of the body

Parasympatheticdivision

Conserves energyPromotes house-keeping functionsduring rest

Motor (efferent) divisionMotor nerve fibersConducts impulses from the CNSto effectors (muscles and glands)

Sensory (afferent) divisionSomatic and visceral sensorynerve fibersConducts impulses fromreceptors to the CNS

Somatic nervoussystem

Somatic motor(voluntary)Conducts impulsesfrom the CNS toskeletal muscles

Sympathetic divisionMobilizes bodysystems during activity

Autonomic nervoussystem (ANS)

Visceral motor(involuntary)Conducts impulsesfrom the CNS tocardiac muscles,smooth muscles,and glands

StructureFunctionSensory (afferent)division of PNS Motor (efferent) division of PNS

Somatic sensoryfiber

Visceral sensory fiber

Motor fiber of somatic nervous system

Skin

Stomach Skeletalmuscle

Heart

BladderParasympathetic motor fiber of ANS

Sympathetic motor fiber of ANS

Nervous Tissue and Function Function of the Nervous System Organization (Structural and Functional) Supporting Cells of the Nervous System Anatomy of a Neuron Classification of Neurons Neuron Function Graded and Action Potentials Myleination and MS Reflexes Synapses EPSPs and IPSPs Neurotransmitters

Nervous Tissue: Support Cells (Neuroglia)

Astrocytes• Abundant, star-shaped cells

• Brace neurons

• Form barrier between capillaries and neurons

• Control the chemical environment of the brain

• Help transfer nutrients between capillaries and neurons

• Mop up and recapture potassium ions and neurotransmitters

"Star cells connect and protect"

Nervous Tissue: Support Cells Microglia

• Spider-like phagocytes

• Dispose of debris by phagocytosis

• Monitor neuron health "Tiny garbage spiders"

Brain orspinal cordtissue

Ependymalcells

Fluid-filled cavity

• Range in shape from squamous to columnar

• May be ciliated

• Line the central cavities of the brain and spinal column

• Separate the CNS interstitial fluid from the cerebrospinal fluid in the cavities

Ependymal Cells (literally, “wrapping garment” cells)

Nervous Tissue: Support Cells Oligodendrocytes

• Produce myelin sheath around nerve fibers in the central nervous system

"Oligos insulate"

Nervous Tissue: Support Cells Schwann cells

• Form myelin sheath in the peripheral nervous system

• Satellite cells

• Surround neuron cell bodies in the PNS

Schwann cells(forming myelin sheath)

Cell body of neuronSatellitecells

Nerve fiber

Nervous Tissue and Function Function of the Nervous System Organization (Structural and Functional) Supporting Cells of the Nervous System Anatomy of a Neuron Classification of Neurons Neuron Function Graded and Action Potentials Myleination and MS Reflexes Synapses EPSPs and IPSPs Neurotransmitters

Figure 11.4b

Dendrites(receptive regions)

Cell body(biosynthetic centerand receptive region)

Nucleolus

NucleusNissl bodies

Axon(impulse generatingand conducting region)

Axon hillockNeurilemma

Terminalbranches

Node of RanvierImpulsedirection

Schwann cell(one inter-node)

Axonterminals(secretoryregion)

(b)

(Rough ER)

Neuron Cell Body Names and Locations

Clusters of cell bodies Bundles of nerve fibers (neuronal processes)

CNS Nuclei Tracts White matter -dense myelinated fibers

Gray matter- unmyelinated fibers and cell bodies

PNS Ganglia Nerves

(bundles of axons)

Axons and Connection(s) to Other Neurons

Synapse

Nerve Fiber Coverings In the PNS, Schwann cells

produce myelin sheaths in jelly-roll like fashion

Nodes of Ranvier – gaps in myelin sheath along the axon

In the CNS, oligodendrocytes produce myelin sheaths and have no neurilemma (so cannot regenerate if damaged).

In multiple sclerosis (MS) the myelin sheaths are destroyed (autoimmunity) leading to poorly functioning muscles

Functional Classification of Neurons Sensory (afferent) neurons

• Carry impulses from the sensory receptors

• Cell bodies outside the CNS

o Cutaneous sense organs

o Proprioceptors – detect stretch or tension

From skin receptors, muscles and tendons

Motor (efferent) neurons

• Carry impulses from the central nervous system

• Cell bodies inside the CNS

Interneurons (association neurons)

• Found in neural pathways in the central nervous system

• Connect sensory and motor neurons

• Cell bodies inside the CNS

Nervous Tissue and Function Function of the Nervous System Organization (Structural and Functional) Supporting Cells of the Nervous System Anatomy of a Neuron Classification of Neurons by Function Graded and Action Potentials Myleination and MS Reflexes Synapses EPSPs and IPSPs Neuraltransmitters

Principles of Electricity Opposite charges attract each other

Energy is required to separate opposite charges across a membrane

Energy is liberated when the charges move toward one another

If opposite charges are separated, the system has potential energy

Definitions Voltage (V): measure of potential energy generated by

separated charges Potential difference: the voltage or difference in charge

measured between two points Current (I): the flow of electrical charge (ions) between

two points Resistance (R): hindrance to charge flow (provided by the

plasma membrane) Insulator: substance with high electrical resistance Conductor: substance with low electrical resistance Intracellular fluid (ICF) - cytoplasm of neuron Extracellular fluid (ECF) - fluid outside a neuron cell

Role of Membrane Ion Channels (Protein “Gates”) Two main types of ion channels:

1. Leakage (nongated) channels —always open

2. Gated channels (three types): Chemically gated (ligand-

gated) channels—open with binding of a specific neurotransmitter

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

Mechanically gated channels —open and close in response to physical deformation of receptors

Receptor

Na+

K+

K+

Na+

Neurotransmitter chemicalattached to receptor

Chemicalbinds

Closed Open

Na+Na+

Closed Open

Membranevoltagechanges

Figure 11.7

Voltmeter

Microelectrodeinside cell

Plasmamembrane

Ground electrodeoutside cell

Neuron

Axon

Potential difference across the membrane of a resting cell (= Resting Potential)

Electrochemical gradient or potential difference is established by the powered pumping of more positive ions in ECF than ICF.

Intracellular fluid (ICF)

Extracellular fluid (ECF)

Resting Membrane Potential Differences in ionic makeup

• ICF has lower concentration of Na+ and Cl– than ECF

• ICF has higher concentration of K+ and negatively charged proteins (A–) than ECF

Differential permeability of membrane

• Impermeable to A–

• Slightly permeable to Na+ (through leakage channels)

• 75 times more permeable to K+ (more leakage channels)

• Freely permeable to Cl–

Negative interior of the cell is due to much greater diffusion of K+ out of the cell than Na+ diffusion into the cell

Sodium-potassium pump stabilizes the resting membrane potential by maintaining the concentration gradients for Na+ and K+

Na+Na+

Na+

Na+

Na+Na+Na+Cl–

Cl–

Cl–

K+K+

K+K+

K+K+

K+ Na+ Na+

Na+Na+Na+ Na+

A-

A-A-

A-K+

Na+

ICF

ECF

ICF

ECFCl–

Cl–

ICF

ECF ++

++ +

+++

++

+++

+ + + K+

Figure 11.8

Finally, let’s add a pump to compensate for leaking ions.Na+-K+ ATPases (pumps) maintain the concentration gradients, resulting in the resting membrane potential.

Suppose a cell has only K+ channels...K+ loss through abundant leakagechannels establishes a negativemembrane potential.

Now, let’s add some Na+ channels to our cell...Na+ entry through leakage channels reducesthe negative membrane potential slightly.

The permeabilities of Na+ and K+ across the membrane are different.

The concentrations of Na+ and K+ on each side of the membrane are different.

Na+

(140 mM )K+

(5 mM )

K+ leakage channels

Cell interior–90 mV



K+

Na+

Na+-K+ pump

K+

K+K+

K+

Na+

K+

K+K

Na+

K+K+ Na+

K+K+

Outside cell

Inside cell Na+-K+ ATPases (pumps) maintain the concentration gradients of Na+ and K+

across the membrane.

The Na+ concentration is higher outside the cell.

The K+ concentration is higher inside the cell.

K+

(140 mM )Na+

(15 mM )

To Recap….

Membrane Potentials That Act as Signals Membrane potential changes when:

• Concentrations of ions across the membrane change

• Permeability of membrane to ions changes

Changes in membrane potential are signals used to receive, integrate and send information

Membrane Potentials That Act as Signals Two types of signals

• Graded potentials o Incoming short-distance signals

o Short-lived, localized changes in membrane potential

o Depolarizations or hyperpolarizations

o Graded potential spreads as local currents change the membrane potential of adjacent regions

o Act to enhance or limit chances of an action potential but does not send an electrical “message”

• Action potentials

o Long-distance signals of axons

Graded potentials

Figure 11.9a

Depolarizing stimulus

Time (ms)

Insidepositive

Insidenegative

Restingpotential

Depolarization

(a) Depolarization: The membrane potentialmoves toward 0 mV, the inside becoming less negative (more positive). Increases the probability of producing a nerve impulse.

Graded Potential: Depolarization

Stimulus causes gated ion channels to open

• E.g., receptor potentials, generator potentials, postsynaptic potentials

Magnitude varies directly (graded) with stimulus strength

Decrease in magnitude with distance as ions flow and diffuse through leakage channels

Figure 11.9b

Hyperpolarizing stimulus

Time (ms)

Restingpotential

Hyper-polarization

(b) Hyperpolarization: The membranepotential increases, the inside becomingmore negative. Decreases the probability of producing a nerve impulse.

Graded Potential: Hyperpolarization

Figure 11.10c

Distance (a few mm)

–70Resting potential

Active area(site of initialdepolarization)

(c) Decay of membrane potential with distance: Because current is lost through the “leaky” plasma membrane, the voltage declines with distance from the stimulus (the voltage is decremental ). Consequently, graded potentials are short-distance signals.

Mem

bran

e po

tent

ial (

mV)

Membrane Potentials That Act as Signals Two types of signals

• Graded potentials

o Incoming short-distance signals

o Short-lived, localized changes in membrane potential

o Depolarizations or hyperpolarizations

o Graded potential spreads as local currents change the membrane potential of adjacent regions

• Action potentials

o Long-distance signals of axons

Action Potential (AP) Brief reversal of membrane potential with a

total amplitude of ~100 mV

Occurs in muscle cells and axons of neurons

Does not decrease in magnitude over distance

Principal means of long-distance neural communication

Actionpotential

1 2 3

4

Resting state Depolarization Repolarization

Hyperpolarization

1 1

2

3

4

Time (ms)

ThresholdMem

bran

e po

tent

ial (

mV)

Figure 11.11 (1 of 5)

• Only leakage channels for Na+ and K+ are open

• All gated Na+ and K+ channels are closed

Depolarizing local currents open voltage-gated Na+ channels

Na+ influx causes more depolarization

•Na+ channel slow inactivation gates close

•Membrane permeability to Na+ declines to resting levels

•Slow voltage-sensitive K+ gates open

•K+ exits the cell and internal negativity is restored

•Some K+ channels remain open, allowing excessive K+ efflux

•This causes after-hyperpolarization of the membrane (undershoot)

Anatomy of an Action Potential

Actionpotential

1

2

3

4

Na+ permeability

K+ permeability

Rela

tive

mem

bran

e pe

rmea

bilit

y

Channel gating (online animation)

Figure 11.12a

Voltageat 0 ms

Recordingelectrode

(a) Time = 0 ms. Action potential has not yet reached the recording electrode.

Resting potentialPeak of action potentialHyperpolarization

Voltage Change at Point in Neuron as Action Potential Passes By

Figure 11.12b

Voltageat 2 ms

(b) Time = 2 ms. Action potential peak is at the recording electrode.


Figure 11.12c

Voltageat 4 ms

(c) Time = 4 ms. Action potential peak is past the recording electrode. Membrane at the recording electrode is still hyperpolarized.


Action potential online

Threshold Stimulus

Subthreshold stimulus—weak local depolarization that does not reach threshold

Threshold stimulus—strong enough to push the membrane potential toward and beyond threshold (Membrane is depolarized by 15 to 20 mV)

AP is an all-or-none phenomenon—action potentials either happen completely, or not at all

All action potentials are alike and are independent of stimulus intensity

Figure 11.13

Threshold

Actionpotentials

Stimulus

Time (ms)

Stimulus

Absolute refractoryperiod

Relative refractoryperiod

Time (ms)

Depolarization(Na+ enters)

Repolarization(K+ leaves)After-hyperpolarization

ARP

Time from the opening of the Na+ channels until the resetting of the channels

Ensures that each AP is an all-or-none event

Enforces one-way transmission of nerve impulses

RRP

•Most Na+ channels have returned to their resting state

•Some K+ channels are still open

•Repolarization is occurringThreshold for AP generation is elevatedExceptionally strong stimulus may generate an AP

Figure 11.14

Refractory Periods

Figure 11.15

Size of voltage

Voltage-gatedion channel

StimulusMyelinsheath

Stimulus

Stimulus

Node of Ranvier

Myelin sheath

(a) In a bare plasma membrane (without voltage-gatedchannels), as on a dendrite, voltage decays becausecurrent leaks across the membrane.

(b) In an unmyelinated axon, voltage-gated Na+ and K+

channels regenerate the action potential at each pointalong the axon, so voltage does not decay. Conduction is slow because movements of ions and of the gatesof channel proteins take time and must occur beforevoltage regeneration occurs.

(c) In a myelinated axon, myelin keeps current in axons(voltage doesn’t decay much). APs are generated onlyin the nodes of Ranvier and appear to jump rapidlyfrom node to node, about 30 times faster than a bare axon.

1 mm

AP Velocity a Function of Axon Diameter and Myelination

Saltatoryconduction

Continuous conduction

Multiple Sclerosis (MS) Nature

• An autoimmune disease that mainly affects young adults

• Symptoms: visual disturbances, weakness, loss of muscular control, speech disturbances, and urinary incontinence

• Myelin sheaths in the CNS become nonfunctional scleroses

• Shunting and short-circuiting of nerve impulses occurs

• Impulse conduction slows and eventually ceases

Treatment

• Some immune system–modifying drugs, including interferons and Copazone:

o Hold symptoms at bay

o Reduce complications

o Reduce disability

Nervous Tissue and Function Function of the Nervous System Organization (Structural and Functional) Supporting Cells of the Nervous System Anatomy of a Neuron Classification of Neurons by Function Graded and Action Potentials Myleination and MS Reflexes Synapses

EPSPs and IPSPs Neurotransmitters

The Reflex Arc

Types of Reflexes and Regulation Autonomic reflexes

• Smooth muscle regulation

• Heart and blood pressure regulation

• Regulation of glands

• Digestive system regulation

Somatic reflexes

• Activation of skeletal muscles

Two Kinds of Synapses

Electrical Synapses

• Less common than chemical synapses

• Neurons are electrically coupled (joined by gap junctions)

• Communication is very rapid, and may be unidirectional or bidirectional

• Are important in:o Embryonic nervous tissue

o Some brain regions

Chemical Synapses

• Specialized for the release and reception of neurotransmitters

• Typically composed of two parts

o Axon terminal of the presynaptic neuron, which contains synaptic vesicles

o Receptor region on the postsynaptic neuron

How Neurons Communicate at Synapses

Irritability – ability to respond to stimuli

Conductivity – ability to transmit an impulse`

SodiumPotassium pump (online animation)

Events at the Synapse (online animation)

Narrated synapse (online)

Figure 11.17, step 1

Action potentialarrives at axon terminal.

Chemical synapsestransmit signals fromone neuron to anotherusing neurotransmitters.

Ca2+

Synapticvesicles

Axonterminal

Mitochondrion

Postsynapticneuron

Presynapticneuron

Presynapticneuron

Synapticcleft

Ca2+

Ca2+

Ca2+

Postsynapticneuron

1



Voltage-gated Ca2+

channels open and Ca2+

enters the axon terminal.


Ca2+

Synapticvesicles

Axonterminal

Mitochondrion

Postsynapticneuron

Presynapticneuron

Presynapticneuron

Synapticcleft

Ca2+

Ca2+

Ca2+

Postsynapticneuron

1

2



Voltage-gated Ca2+



Ca2+ entry causesneurotransmitter-containing synapticvesicles to release theircontents by exocytosis.


Ca2+

Synapticvesicles

Axonterminal

Mitochondrion

Postsynapticneuron

Presynapticneuron

Presynapticneuron

Synapticcleft

Ca2+

Ca2+

Ca2+

Postsynapticneuron

1

2

3



Voltage-gated Ca2+





Ca2+

Synapticvesicles

Axonterminal

Mitochondrion

Postsynapticneuron

Presynapticneuron

Presynapticneuron

Synapticcleft

Ca2+

Ca2+

Ca2+

Neurotransmitterdiffuses across the synapticcleft and binds to specificreceptors on thepostsynaptic membrane.

Postsynapticneuron

1

2

3

4


Ion movementGraded potential

Binding of neurotransmitteropens ion channels, resulting ingraded potentials.

5


Reuptake

Enzymaticdegradation

Diffusion awayfrom synapse

Neurotransmitter effects are terminatedby reuptake through transport proteins,enzymatic degradation, or diffusion awayfrom the synapse.

6

Figure 11.17


Voltage-gated Ca2+





Ca2+

Synapticvesicles

Axonterminal

Mitochondrion

Postsynapticneuron

Presynapticneuron

Presynapticneuron

Synapticcleft

Ca2+

Ca2+

Ca2+

Neurotransmitterdiffuses across the synapticcleft and binds to specificreceptors on thepostsynaptic membrane.

Binding of neurotransmitteropens ion channels, resulting ingraded potentials.

Neurotransmitter effects areterminated by reuptake throughtransport proteins, enzymaticdegradation, or diffusion awayfrom the synapse.

Ion movementGraded potential

Reuptake

Enzymaticdegradation

Diffusion awayfrom synapse

Postsynapticneuron

1

2

3

4

5

6

SodiumPotassium pump (online animation)

Events at the Synapse (online animation)

Narrated synapse (online)

Excitatory Synapses and EPSPs Neurotransmitter binds to and opens chemically gated channels that allow simultaneous flow of Na+ and

K+ in opposite directions Na+ influx is greater that K+ efflux, causing a net depolarization Excitatory postsynaptic potential (EPSP) helps trigger AP at axon hillock if EPSP is of threshold

strength and opens the voltage-gated channels

Figure 11.18a

An EPSP is a localdepolarization of the postsynaptic membranethat brings the neuroncloser to AP threshold. Neurotransmitter binding opens chemically gated ion channels, allowing the simultaneous pas-sage of Na+ and K+.

Time (ms)

Threshold

Stimulus

Mem

bran

e po

tent

ial (

mV)

Inhibitory Synapses and Inhibitory Postsynaptic Potential (IPSPs)

Neurotransmitter binds to and opens channels for K+ or Cl–

Causes a hyperpolarization (the inner surface of membrane becomes more negative) Reduces the postsynaptic neuron’s ability to produce an action potential

Figure 11.18b

An IPSP is a localhyperpolarization of the postsynaptic membraneand drives the neuron away from AP threshold. Neurotransmitter binding opens K+ or Cl– channels.

Time (ms)

Threshold

Stimulus

Mem

bran

e po

tent

ial (

mV)

Chemical Classes of Neurotransmitters Acetylcholine (Ach) (Mostly excitory in CNS, PNS if prolonged prod. tetanus (with nerve

gases), receoptors destroyed in myasthenia gravis)

• Released at neuromuscular junctions and some ANS neurons

• Synthesized by enzyme choline acetyltransferase

• Degraded by the enzyme acetylcholinesterase (AChE)

Biogenic amineso Catecholamines

Dopamine (“Feeling good” CNS neurotransmitter, uptake blocked by cocaine, excitory or inhibitory) , norepinephrine (NE), and epinephrine

o Indolamines Serotonin (Roles in sleep, appetite, nausea, mood; blocked by seritonin-specific reuptake inhibitors (SSRIs) like Prozac

and LSD, enhanced by ecstasy (3,4-Methylenedioxymethamphetamine)), histamine

• Broadly distributed in the brain; play roles in emotional behaviors and the biological clock Amino acids include:

o GABA—Gamma ()-aminobutyric acid (inhibitory brain NT augmented by alcohol, benzodiazepine-valium)

o Glutamate (excitory in CNS, causes stroke when overreleased, overstimulation of neurons) , Glycine, Aspartate

Peptides (neuropeptides) include:o Substance P, endorphins, somatostatin, cholecystokinin

Purines such as ATP Gases and lipids

• NO, CO, Endocannabinoids

Nervous Tissue and Function

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