3 nerve ppt. 20.09.12.ans

Nerve physiologyNerve physiology

Physiology of NervesPhysiology of Nerves

There are two major regulatory There are two major regulatory systems in the body, the nervous systems in the body, the nervous system and the endocrine system.system and the endocrine system.

The endocrine system regulates The endocrine system regulates relatively slow, long-lived responsesrelatively slow, long-lived responses

The nervous system regulates fast, The nervous system regulates fast, short-term responsesshort-term responses

Neuron structureNeuron structure Neurons all have same basic Neurons all have same basic

structure, a cell body with a number structure, a cell body with a number of dendrites and one long axon.of dendrites and one long axon.

Types of neuronsTypes of neurons

Divisions of the nervous Divisions of the nervous systemsystem

Non-excitable cells of the Non-excitable cells of the nervous systemnervous system

Structure of gray matterStructure of gray matter

Signal transmission in Signal transmission in neuronsneurons

Resting potentialResting potential

Ionic basis of EIonic basis of Emm NaK-ATPase NaK-ATPase

pumps 3Napumps 3Na+ +

out for 2 Kout for 2 K++ pumped in.pumped in.

Some of the Some of the KK++ leaks leaks back out, back out, making the making the interior of interior of the cell the cell negativenegative

Electrochemical Electrochemical GradientsGradients

Figure 12.12

Ion channelsIon channels Remember Remember

Ohm’s Law: Ohm’s Law: I=E/RI=E/R

When a channel When a channel opens, it has a opens, it has a fixed resistance.fixed resistance.

Thus, each Thus, each channel has a channel has a fixed current.fixed current.

Using the patch-Using the patch-clamp technique, clamp technique, we can measure we can measure the current the current through through individual individual channelschannels

Gated channels: ligand-Gated channels: ligand-gatedgated

Gated channels: voltage-Gated channels: voltage-gatedgated

Gated channels: Gated channels: mechanically-gatedmechanically-gated

A change in potential that decreases A change in potential that decreases with distance with distance Localized depolarization or Localized depolarization or

hyperpolarizationhyperpolarization

Graded potentialGraded potential

Graded PotentialsGraded Potentials

Graded PotentialsGraded Potentials

Appears when region of excitable Appears when region of excitable membrane depolarizes to thresholdmembrane depolarizes to threshold

Steps involvedSteps involved Membrane depolarization and sodium Membrane depolarization and sodium

channel activationchannel activation Sodium channel inactivationSodium channel inactivation Potassium channel activationPotassium channel activation Return to normal permeabilityReturn to normal permeability

Action PotentialAction Potential

Figure 2.16.1

The Generation of an The Generation of an Action PotentialAction Potential

GradedGraded potentials potentials vsvs

Action Action PotentialPotential

Generation of action potential follows all-or-none Generation of action potential follows all-or-none principleprinciple

Refractory period lasts from time action potential Refractory period lasts from time action potential begins until normal resting potential returnsbegins until normal resting potential returns

Continuous propagation Continuous propagation spread of action potential across entire membrane in spread of action potential across entire membrane in

series of small stepsseries of small steps salutatory propagationsalutatory propagation

action potential spreads from node to node, skipping action potential spreads from node to node, skipping internodal membraneinternodal membrane

Characteristics of action Characteristics of action potentialspotentials

The Generation of an Action The Generation of an Action PotentialPotential

Induction of an action Induction of an action potential Ipotential I

Induction of an action Induction of an action potential IIpotential II

Voltage-gated NaVoltage-gated Na++ channelschannels

These channels These channels have two voltage have two voltage sensitive gates.sensitive gates.

At resting EAt resting Emm, one , one gate is closed and gate is closed and the other is open.the other is open.

When the When the membrane membrane becomes becomes depolarized depolarized enough, the enough, the second gate will second gate will open.open.

After a short time, After a short time, the second gate the second gate will then shut.will then shut.

Voltage-gated KVoltage-gated K++ channelschannels

Voltage-gated K+ Voltage-gated K+ channels have channels have only one gate.only one gate.

This gate is also This gate is also activated by activated by depolarization.depolarization.

However, this However, this gate is much gate is much slower to slower to respond to the respond to the depolarization.depolarization.

Cycling of V-G channelsCycling of V-G channels

Action potential Action potential propagationpropagation

When the V-G Na+ When the V-G Na+ channels open, channels open, they cause a they cause a depolarization of depolarization of the neighboring the neighboring membrane.membrane.

This causes the This causes the Na+ and K+ Na+ and K+ channels in that channels in that piece of membrane piece of membrane to be activatedto be activated

AP propagation AP propagation cont.cont.

The V_G chanels in The V_G chanels in the neighboring the neighboring membrane then membrane then open, causing that open, causing that membrane to membrane to depolarize.depolarize.

That depolarizes That depolarizes the next piece of the next piece of membrane, etc.membrane, etc.

It takes a while for It takes a while for the Na+ channels the Na+ channels to return to their to return to their voltage-sensitive voltage-sensitive state. Until then, state. Until then, they won’t respond they won’t respond to a second to a second depolarization. depolarization.

Propagation of an Action Propagation of an Action Potential along an Potential along an Unmyelinated AxonUnmyelinated Axon

Saltatory Propagation along Saltatory Propagation along a Myelinated Axona Myelinated Axon

Saltatory Propagation Saltatory Propagation along a Myelinated Axonalong a Myelinated Axon

Schwann cells Schwann cells cont.cont.

In unmyelinated In unmyelinated nerves, each nerves, each Schwann cell can Schwann cell can associate with associate with several axons.several axons.

These axons become These axons become embedded in the embedded in the Schwann cell, which Schwann cell, which provides structural provides structural support and support and nutrients.nutrients.

Synaptic transmissionSynaptic transmission

Aminobutyric AcidAminobutyric Acid

Also know as GABAAlso know as GABA

Two know receptors for GABATwo know receptors for GABA

Both initiate hyperpolarization in the Both initiate hyperpolarization in the post-synaptic membranepost-synaptic membrane GABAGABAAA receptor allows an influx of Cl receptor allows an influx of Cl-- ions ions

GABAGABABB receptors allow an efflux of K receptors allow an efflux of K++ ions ions

Transmitter effects on ETransmitter effects on Emm

Most chemical stimuli result in an influx of cationsMost chemical stimuli result in an influx of cations This causes a depolarization of the membrane potentialThis causes a depolarization of the membrane potential

At least one transmitter opens an anion influxAt least one transmitter opens an anion influx This results in a hyperpolarization.This results in a hyperpolarization.

EPSPs and IPSPsEPSPs and IPSPs

If the transmitter opens a cation influx, If the transmitter opens a cation influx, the resulting depolarization is called an the resulting depolarization is called an Excitatory Post Synaptic PotentialExcitatory Post Synaptic Potential (EPSP). (EPSP).

These individual potentials are sub-These individual potentials are sub-threshold.threshold.

If the transmitter opens an anion influx, If the transmitter opens an anion influx, the resulting hyperpolarization is called the resulting hyperpolarization is called an an Inhibitory Post Synaptic PotentialInhibitory Post Synaptic Potential (IPSP(IPSP

All these potentials are additive.All these potentials are additive.

Post-synaptic integrationPost-synaptic integration

Signal integrationSignal integration

Signal integration cont.Signal integration cont.

Presynaptic inhibitionPresynaptic inhibition

Presynaptic facillitationPresynaptic facillitation

Neural circuits INeural circuits I

Neural circuits IINeural circuits II

Myelination IMyelination I

In the central In the central nervous system, nervous system, myelin is formed by myelin is formed by the the oligodendrocytes.oligodendrocytes.

One oligodendrocyte One oligodendrocyte can contribute to can contribute to the myelin sheath of the myelin sheath of several axons.several axons.

Myelination IIMyelination II

In the peripheral In the peripheral nervous system, nervous system, myelin is formed by myelin is formed by Schwann cells.Schwann cells.

Each Schwann cell Each Schwann cell associates with only associates with only one axon, when one axon, when forming a forming a myelinated myelinated internode.internode.

White and gray matter in White and gray matter in the nervous systemthe nervous system

Structure of the spinal Structure of the spinal cord Icord I

The CNS is The CNS is made up not made up not only of the only of the brain, but also brain, but also the spinal cord.the spinal cord.

The spinal cord The spinal cord is a thick, hollow is a thick, hollow tube of nerves tube of nerves that runs down that runs down the back, the back, through the through the spine.spine.

Structure of the spinal Structure of the spinal cord IIcord II

Structure of the spinal Structure of the spinal cord IIIcord III

Structure of the spinal Structure of the spinal cord IVcord IV