The Nervous SystemThe Nervous SystemNervous Tissues and the Nervous Tissues and the

SynapseSynapseChapter 11Chapter 11

Nervous TissuesNervous TissuesTwo types of cells:Two types of cells:

Neuroglia (glial cells)Neuroglia (glial cells)Support, insulate, help young neurons Support, insulate, help young neurons

grow grow (page 390)(page 390)Ex: Schwann cellsEx: Schwann cells

Neurons (nerve cells)Neurons (nerve cells)Conduct electrical impulsesConduct electrical impulsesDo not divide (amitotic)Do not divide (amitotic)High metabolic ratesHigh metabolic rates

Glial Cell ExamplesGlial Cell Examples

Schwann Cell Schwann Cell FormationFormation

Myelin SheathMyelin Sheath Nucleus

neurilemmaNode of Ranvier

axon

Key Neuron PhysiologyKey Neuron Physiology DendritesDendrites

receive incoming nerve signal (action potential)receive incoming nerve signal (action potential) AxonAxon

transmits AP down length of neurontransmits AP down length of neuron Axon TerminalsAxon Terminals

pass AP to next neuron (via pass AP to next neuron (via synapsesynapse)) Myelin SheathMyelin Sheath

insulates axon, increases speed of AP as it moves down insulates axon, increases speed of AP as it moves down axonaxon

Schwann cellSchwann cell wraps around axon, forms myelin (example of a glial cell)wraps around axon, forms myelin (example of a glial cell)

NeurilemmaNeurilemma plasma membrane of neuronplasma membrane of neuron

Nodes of RanvierNodes of Ranvier gaps in myelin sheath, allows diffusion across gaps in myelin sheath, allows diffusion across

neurilemma (ions*, wastes, nutrients)neurilemma (ions*, wastes, nutrients)

Structural Classification of Structural Classification of NeuronsNeurons

Classified by # Classified by # of processes of processes extending from extending from cell body cell body (3 (3 types)types)

1.1.MultipolarMultipolar3+ processes3+ processesMost common Most common

neuron type neuron type (99%)(99%)

2. Bipolar2. Bipolar

2 processes 2 processes extend from cell extend from cell bodybodyAxon, dendriteAxon, dendrite

More rare, found More rare, found in CNS and some in CNS and some special sense special sense organsorgans

3. Unipolar3. Unipolar

Short Short extension of extension of cell body split cell body split into “T”into “T”

Sensory Sensory neurons in neurons in PNSPNS

Functional Classification of Functional Classification of NeuronsNeurons

Sensory (afferent) neuronsSensory (afferent) neurons Transmit impulses from receptors in body Transmit impulses from receptors in body

(PNS) to CNS(PNS) to CNS (Usually) Unipolar (…sometimes bipolar…)(Usually) Unipolar (…sometimes bipolar…)

Motor (efferent) neuronsMotor (efferent) neurons Transmit impulses from CNS towards Transmit impulses from CNS towards

effector organs (muscles, glands)effector organs (muscles, glands) MultipolarMultipolar

InterneuronsInterneurons (aka association neurons) (aka association neurons) Transmit signals through CNSTransmit signals through CNS IntegrationIntegration BipolarBipolar

The Action PotentialThe Action PotentialNerve impulses travel as an Nerve impulses travel as an

electrical signalelectrical signalDendrites -> cell body -> axon -> Dendrites -> cell body -> axon ->

axon terminalsaxon terminalsDue to a sudden change in voltage Due to a sudden change in voltage

(amplitude) as (amplitude) as ionsions travel in and travel in and out of the cell through protein out of the cell through protein channelschannelsIons create electrical charge!Ions create electrical charge!

The SynapseThe Synapse

axon terminal axon terminal of of presynaptic presynaptic neuronneuron meets meets the the dendrite dendrite of of postsynaptic postsynaptic neuronneuron..

At the synapse…At the synapse…Nerve impulses are passed Nerve impulses are passed

from one neuron to the nextfrom one neuron to the nextMay be:May be:

Electrical synapses (less Electrical synapses (less common)common)

Chemical synapsesChemical synapses

Electrical Electrical SynapseSynapse

Use protein channelsUse protein channelsIons flow directly from one Ions flow directly from one

neuron to the nextneuron to the nextVery rapid signal transmissionVery rapid signal transmission

Sleep arousal, primitive Sleep arousal, primitive emotions, some memory (all in emotions, some memory (all in brain)brain)

Early embryonic nervous tissueEarly embryonic nervous tissue

Chemical SynapseChemical Synapse

Neurotransmitters (NTs) released Neurotransmitters (NTs) released from pre-synaptic axon terminalfrom pre-synaptic axon terminal

NTs bind to NTs bind to receptors receptors on post-on post-synaptic neuron dendritessynaptic neuron dendritesConvert electrical signal --> Convert electrical signal -->

chemical signal --> electrical chemical signal --> electrical signal signal

Chemical Synapse Animation

NeurotransmittersNeurotransmitters= = Chemicals Chemicals used used

to relay signals to relay signals from axon from axon terminal of pre-terminal of pre-synaptic neuron synaptic neuron to dendrites of to dendrites of the post-synaptic the post-synaptic neuron (across neuron (across the the synaptic synaptic cleft / gapcleft / gap))

*In regards to this synapse

PRE-synaptic neuron

POST-synaptic neuron

At the chemical At the chemical synapse…synapse…

At rest = Neurotransmitters (NTs) are At rest = Neurotransmitters (NTs) are stored in synaptic stored in synaptic vesicles vesicles until until released in response to influx of released in response to influx of CaCa2+2+ in in axon terminalaxon terminal

1.1. Electrical nerve impulse (AP) opens Electrical nerve impulse (AP) opens voltage-gated calcium channels on on pre-synaptic axon terminal pre-synaptic axon terminal neurilemmaneurilemma

2.2. CaCa2+2+ floods in to floods in to pre-syn. neuronpre-syn. neuron3.3. NTs released from vesicles, float NTs released from vesicles, float

across the across the synaptic cleft synaptic cleft toward post-toward post-synaptic dendritesynaptic dendrite

Chemical Synapse Chemical Synapse cont…cont…

4.4. NTs bind to NTs bind to receptors receptors on post-syn. on post-syn. neuronneuron

Ion channels open Ion channels open (in post-syn. (in post-syn. neurilemma)neurilemma)

5.5. Ions Ions flow through channels into flow through channels into post-post-synaptic neuron’s dendritesynaptic neuron’s dendrite

Membrane potential (voltage) changes, Membrane potential (voltage) changes, passing on electrical signalpassing on electrical signalMay be May be excitatory excitatory or or inhibitory inhibitory (based (based

on NT released)on NT released)

Chemical Synapse Chemical Synapse Cont…Cont…

Leftover NTs in synapse:Leftover NTs in synapse:*Taken back up by pre-synaptic *Taken back up by pre-synaptic

neuron to be reused or…neuron to be reused or…Degraded by enzymes or…Degraded by enzymes or…Diffuse awayDiffuse away

Animation one more time

Just like with muscles… Just like with muscles… Summation!Summation!

SummationSummation - increase duration or - increase duration or intensity of nerve impulsesintensity of nerve impulsesTemporalTemporal - post-synaptic neuron - post-synaptic neuron

stimulated by 1 pre-synaptic neuron stimulated by 1 pre-synaptic neuron in rapid fire (similar to wave in rapid fire (similar to wave summation)summation)

SpatialSpatial - post-synaptic neuron - post-synaptic neuron stimulated by many axon terminals at stimulated by many axon terminals at same time (similar to recruitment)same time (similar to recruitment)

Some Specific Some Specific NeurotransmittersNeurotransmitters

Excitatory:Excitatory: GlutamateGlutamate

learning and memorylearning and memoryInhibitory:Inhibitory: GABAGABA

main inhibitory neurotransmitter in brainmain inhibitory neurotransmitter in brain effects exaggerated by alcoholeffects exaggerated by alcohol

SerotoninSerotonin ““feel good” neurotransmitterfeel good” neurotransmitter involved in sleep, appetite, mood regulationinvolved in sleep, appetite, mood regulation Prozac blocks its uptake, keeps serotonin in Prozac blocks its uptake, keeps serotonin in

synapse, relieves anxiety/depressionsynapse, relieves anxiety/depression

More More NeurotransmittersNeurotransmitters

Can be either excitatory or inhibitory:Can be either excitatory or inhibitory: DopamineDopamine

excitatory or inhibitory depending on receptor excitatory or inhibitory depending on receptor ““feel good” neurotransmitter (reward pathway)feel good” neurotransmitter (reward pathway)

reuptake blocked by cocainereuptake blocked by cocaine Plays role in motor controlPlays role in motor control

deficient in Parkinson’s patientsdeficient in Parkinson’s patients AcetylcholineAcetylcholine

excitatory to skeletal muscles, may be either to excitatory to skeletal muscles, may be either to visceral organsvisceral organs

major neuromuscular junction neurotransmittermajor neuromuscular junction neurotransmitter

Excitatory/InhibitoryExcitatory/Inhibitory

Excitatory Neurotransmitters…Excitatory Neurotransmitters…increase activity of postsynaptic increase activity of postsynaptic

neuronneuronincrease amount of NT releasedincrease amount of NT releasedExample: glutamateExample: glutamate

Inhibitory NeurotransmittersInhibitory Neurotransmittersdecrease activity of postsynaptic decrease activity of postsynaptic

neuronneurondecrease amount of NT releaseddecrease amount of NT releasedExample: GABAExample: GABA