NERVOUS SYSTEM

INTRODUCTIONNervous systems consist of circuits of neurons and supporting cellsNeurons are nerve cells that transfer information within the bodyNeurons use two types of signals to communicate: electrical signals (long-distance) and chemical signals (short-distance)

All animals except spongesHave some type of nervous system

What distinguishes the nervous systems of different animal groupsIs how the neurons are organized into circuits

Organization of Nervous SystemsAnimals are multicellular and most groups respond to stimuli using systems of neurons

The simplest animals with nervous systems, the cnidarians

Have neurons arranged in nerve netsControls the contraction & expansion ofthe gastrovascular cavity

More complex animals have nervesNerves are bundles that consist of the axons of multiple nerve cellsSea stars have a nerve net in each arm connected by radial nerves to a central nerve ring

In relatively simple cephalized animals, such as flatworms

A central nervous system (CNS) is evidentClustering of sensory neurons at the anterior (front) endOne or more nerve cords extending toward the posterior end connect these structures with nerves elsewhere in the body

Annelids and arthropodsHave segmentally arranged clusters of neurons called gangliaThese ganglia connect to the CNS and make up a peripheral nervous system (PNS)

In vertebratesThe central nervous system consists of a brain and dorsal spinal cordThe peripheral nervous system (PNS) connects to the CNSNerves and ganglia comprise the PNS

Nervous system organization usually correlates with lifestyleSessile molluscs (for example, clams and chitons) have simple systems, whereas more complex molluscs (for example, octopuses and squids) have more sophisticated systems

Information ProcessingNervous systems process information in three stagesSensory inputIntegrationMotor output

Many animals have a complex nervous system that consists of

A central nervous system (CNS) where integration takes place; this includes the brain and a nerve cordA peripheral nervous system (PNS), which carries information into and out of the CNS The neurons of the PNS, when bundled together, form nerves

Central nervous system (CNS)BrainSpinal cordPeripheral nervous system (PNS)Cranial nervesGanglia outside CNSSpinal nervesThe vertebrate nervous system.

Sensors detect external stimuli and internal conditions and transmit information along sensory neurons

Sensory information is sent to the brain or ganglia, where interneurons integrate the information

Motor output leaves the brain or ganglia via motor neurons, which trigger muscle or gland activity

The three stages of information processingAre illustrated in the knee-jerk reflex

Neuron Structure and FunctionMost of a neurons organelles are in the cell body

Most neurons have dendrites, highly branched extensions that receive signals from other neurons

The axon is typically a much longer extension that transmits signals to other cells at synapses

The cone-shaped base of an axon is called the axon hillock

NucleusDendritesStimulusAxon hillockCell bodyPresynaptic cellSignal directionAxonSynapseNeurotransmitterSynaptic terminalsPostsynaptic cellSynaptic terminals

The synaptic terminal of one axon passes information across the synapse in the form of chemical messengers called neurotransmitters

A synapse is a junction between an axon and another cell

Information is transmitted from a presynaptic cell (a neuron) to a postsynaptic cell (a neuron, muscle, or gland cell)

Most neurons are nourished or insulated by cells called glial cells or glia

DendritesAxonCell bodyPortion of axonSensory neuronInterneuronsMotor neuron

Figure 48.6Glia80 mCell bodies of neurons

Ion Pumps & Ion Channels Maintains the Resting Potential of a NeuronEvery cell has a voltage (difference in electrical charge) across its plasma membrane called a membrane potential

The resting potential is the membrane potential of a neuron not sending signals ( between -60 and -80 mV )

Changes in membrane potential act as signals, transmitting and processing information

Formation of the Resting PotentialIs the membrane potential of a neuron that is not transmitting signals

In all neurons, the resting potentialDepends on the ionic gradients that exist across the plasma membrane

In a mammalian neuron, the concentration of K+ is highest inside the cell, while the concentration of Na+ is highest outside the cell

Sodium-potassium pumps use the energy of ATP to maintain these K+ and Na+ gradients across the plasma membrane

These concentration gradients represent chemical potential energy

The opening of ion channels in the plasma membrane converts chemical potential to electrical potential

A neuron at resting potential contains many open K+ channels and fewer open Na+ channels; K+ diffuses out of the cell

The resulting buildup of negative charge within the neuron is the major source of membrane potential

The values shown represent the approximate concentrations in millimolesper liter (mM) for ions in the fluids within and surrounding a mammalian neuron.

The basis of the membrane potentialThe membrane potential begins primarily from the interaction between the membrane and the actions of two kinds of transmembrane proteins embedded in the plasma membrane. The membrane provides as both an insulator and a diffusion barrier to the movement of ions. The Ion transporter/pump proteins actively push ions across the membrane to set up concentration gradients across the membrane, and ion channels permit ions to move across the membrane down to those concentration gradients, a process termed as facilitated diffusion. In nearly fundamental illustration of this, the ion transporter Na+/K+-ATPase pumps sodium cations from inside to the outside, and potassium cations from outside to inside of the cell. This begins two concentration gradients: a gradient for sodium where its concentration is much higher at outside than inside the cell, and the gradient for potassium where its concentration is much higher at inside the cell than outside. In the case of K+, its diffusion down its concentration gradient generates transmembrane voltage which is negative relative to the outside of the cell, and usually 60 to 80 millivolts (mV) in amplitude.

The Action PotentialAction potentials are the nerve signals or nerve impulses conducted by axons

If a cell has gated ion channelsIts membrane potential may change in response to stimuli that open or close those channels, changing the membranes voltage

Gated Ion ChannelsGated ion channels open or closeIn response to a change in the membrane potentialAlters the membranes permeability to particular ions which in turn alters the membrane potential

Hyperpolarization and DepolarizationWhen gated K+ channels open, K+ diffuses out, making the inside of the cell more negative

This is hyperpolarization, an increase in magnitude of the membrane potential

In a resting neuron, hyperpolarization results from any stimulus that increases the outflow of positive ions or the inflow of negative ions

StimulusThresholdResting potentialHyperpolarizationsTime (msec)50050100102345Membrane potential (mV)

Opening other types of ion channels triggers a depolarization, a reduction in the magnitude of the membrane potential

For example, depolarization occurs if gated Na+ channels open and Na+ diffuses into the cell

StimulusThresholdResting potentialDepolarizationsTime (msec)50050100102345Membrane potential (mV)

HyperpolarizationDepolarizationGated K+ channels in a resting neuron are stimulated to openIncrease the membranes permeability to K+K+ diffuses out of the neuronIncrease the magnitude of the membrane potentialMaking the inside of the membrane more negativeGated Na+ channels in a resting neuron are stimulated to openIncrease the membranes permeability to Na+ diffuses into the cellReduce the magnitude of the membrane potentialMaking the inside of the membrane less negative

Graded Potentials and Action PotentialsAn action potentialIs an all-or-none law the neurons transmit an impulse in a similar way no matter how weak/strong the impulse isThe neuron either transmits an action potential (all) or does not (none)

Both voltage-gated Na+ channels and voltage-gated K+ channelsAre involved in the production of an action potential

Generation of Action PotentialsAn action potential can be considered as a series of stagesOccur whenever a depolarization increases the membrane voltage to a particular value, called the threshold (mammalian neurons, -55 mV)

At resting potentialMost voltage-gated sodium (Na+) channels are closed; most of the voltage-gated potassium (K+) channels are also closed

OUTSIDE OF CELLINSIDE OF CELLInactivation loopSodium channelPotassium channelThresholdResting potentialTimeMembrane potential (mV)50100500NaKKeyResting stateFigure 48.11-1

When an action potential is generatedVoltage-gated Na+ channels open first and Na+ flows into the cellDuring the rising phase, the threshold is crossed, and the membrane potential increases During the falling phase, voltage-gated Na+ channels become inactivated; voltage-gated K+ channels open, and K+ flows out of the cell

OUTSIDE OF CELLINSIDE OF CELLInactivation loopSodium channelPotassium channelThresholdResting potentialTimeMembrane potential (mV)50100500NaKKeyResting stateDepolarizationFigure 48.11-2

OUTSIDE OF CELLINSIDE OF CELLInactivation loopSodium channelPotassium channelAction potentialThresholdResting potentialTimeMembrane potential (mV)50100500NaKKeyResting stateDepolarizationRising phase of the action potentialFigure 48.11-3

OUTSIDE OF CELLINSIDE OF CELLInactivation loopSodium channelPotassium channelAction potentialThresholdResting potentialTimeMembrane potential (mV)50100500NaKKeyResting stateDepolarizationRising phase of the action potentialFalling phase of the action potentialFigure 48.11-4

5.During the undershoot, membrane permeability to K+ is at first higher than at rest, then voltage-gated K+ channels close and resting potential is restored

OUTSIDE OF CELLINSIDE OF CELLInactivation loopSodium channelPotassium channelAction potentialThresholdResting potentialTimeMembrane potential (mV)50100500NaKKeyResting stateUndershootDepolarizationRising phase of the action potentialFalling phase of the action potentialFigure 48.11-5

During the refractory period after an action potential, a second action potential cannot be initiated

The refractory period is a result of a temporary inactivation of the Na+ channels

Animation: Action Potential

Conduction of Action PotentialsAt the site where the action potential is generated, usually the axon hillock, an electrical current depolarizes the neighboring region of the axon membrane

Action potentials travel in only one direction: toward the synaptic terminals

Inactivated Na+ channels behind the zone of depolarization prevent the action potential from traveling backwards

Conduction of an Action Potential

Conduction SpeedThe speed of an action potentialIncreases with the diameter of an axon

In vertebrates, axons is electrical insulation The electrical insulation that surrounds axons is called a myelin sheathAlso causing the speed of an action potential to increase

Action potentials in myelinated axonsJump between the nodes of Ranvier in a process called saltatory conduction

Saltatory conduction. In a myelinated axon, depolarizing current during anaction potential at one node of Ranvier spreads along the interior of the axonto next node (blue arrow), where it reinitiates itself. Thus the action potentialjumps from node to node as it travels along the axon (red arrows).

Neurons communicate with other cells at sypnapsesIn an electrical synapseElectrical current flows directly from one cell to another via a gap junction

The vast majority of synapses are chemical synapses

In a chemical synapse, a presynaptic neuron Releases chemical neurotransmitters, which are stored in the synaptic terminal

When an action potential reaches a terminal

The final result is the release of neurotransmitters into the synaptic cleft

Synaptic cleft : narrow gap that separates the presynaptic neuron from the postsynaptic cell

A chemical synapse. (1)When an action potential depolarizes the plasma membrane of the synaptic terminal, it (2) opens voltage-gated calcium channels in the membrane, triggering an influx of Ca+ . (3) The elevated Ca+ concentration in the terminal causes synaptic vesicles to fuse with the presynaptic membrane. (4) The vesicles release neurotransmitter into synaptic cleft. (5) The neurotransmitter binds to the receptor portion of ligand-gated ion channels in the postsynaptic membrane, opening the channels. In the synapse illustrated here, both Na+ and K+ can diffuse through the channels. (6) The neurotransmitter is released from the receptors, andthe channel close. Synaptic transmission ends when the neurotransmitter diffuses out of the synaptic cleft.

Generation of Postsynaptic PotentialsThe receptor protein that binds and responds to neurotransmitters is a ligand-gated ion channel

Direct synaptic transmission involves binding of neurotransmitters to ligand-gated ion channels in the postsynaptic cell

Neurotransmitter binding causes ion channels to open, allows specific ions to diffuse, generating a postsynaptic potential, a graded potential in the postsynaptic cell

Postsynaptic potentials fall into two categories

Excitatory postsynaptic potentials (EPSPs) are depolarizations that bring the membrane potential toward thresholdInhibitory postsynaptic potentials (IPSPs) are hyperpolarizations that move the membrane potential farther from threshold

Excitatory Postsynaptic Potentials (EPSP)

- Depolarize the postsynaptic neuron-The binding of neurotransmitter to postsynaptic receptors opens gated channels that allow Na+ and K+ diffuse out of the cell-The inside of the cell becomes more positive, hence causing a local depolarization - If enough depolarization occurs an action potential is generated

Inhibitory Postsynaptic Potentials (IPSP)

-decrease the cells ability to develop action potentials-channels are permeable to Cl- and K+ where Cl- moves into the cell and K+ moves out of the cell -The inside of the cell thus becomes more negative, hence causing a local hyperpolarization make it more difficult for the cell membrane potential to reach threshold less likely that an action potential will be generated

Summation of Postsynaptic PotentialsThe magnitude of the postsynaptic potential at any one synapse varies with a number of factors

Unlike action potentialsPostsynaptic potentials are graded and do not regenerate themselves

Synaptic terminals on the cell body of a postsynaptic neuron (colorized SEM)

Since most neurons have many synapses on their dendrites and cell bodyA single EPSP is usually too small to trigger an action potential in a postsynaptic neuron

If two EPSPs are produced in rapid successionAn effect called temporal summation occurs

EPSPs produced nearly simultaneously by different synapses on the same postsynaptic neuron add togetherAn effect called spatial summation occurs

Through summationAn IPSP can counter the effect of an EPSP

Major neurotransmitters

DID YOU KNOW:

It is very quick, and has the ability of transmitting impulses at the speed of 100 meters per second. The speed of message transmission to the brain can be as high as 180 miles per hour.

*Figure 49.4 The vertebrate nervous system.**Figure 48.4 Neuron structure and organization.**Figure 48.5 Structural diversity of neurons.**Figure 48.6 Glia in the mammalian brain.**Figure 48.7 The basis of the membrane potential.**Figure 48.10 Graded potentials and an action potential in a neuron.**Figure 48.10 Graded potentials and an action potential in a neuron.**Figure 48.11 The role of voltage-gated ion channels in the generation of an action potential.**Figure 48.11 The role of voltage-gated ion channels in the generation of an action potential.**Figure 48.11 The role of voltage-gated ion channels in the generation of an action potential.**Figure 48.11 The role of voltage-gated ion channels in the generation of an action potential.**Figure 48.11 The role of voltage-gated ion channels in the generation of an action potential.