NERVOUS SYSTEMCH 48

NERVOUS SYSTEM

Central Nervous system – Brain & spinal cord

Peripheral nervous system- nerves that communicate motor & sensory signals through the body

NEURONS

Sensory neuron – input from external stimuli

Interneuron – integration: analyze & interprets input

Motor neuron– signal sent to muscle or gland cells

Nucleus

Dendrites

Stimulus

Axon hillock

Cellbody

Presynapticcell

Signaldirection

Axon

Synapse

Neurotransmitter

Synaptic terminals

Postsynaptic cell

Synapticterminals

Parts of a neuron

Axon Myelin sheath

Schwanncell

Nodes ofRanvier

Node of Ranvier

Layers of myelin

Axon

SchwanncellNucleus ofSchwann cell

NERVE SIGNALS

Membrane potential - the electrical charge difference across a membrane

• Due to different concentrations of ions in & out of cell

Resting potential – the membrane potential of an unstimulated neuron

• About -70 mV (more negative inside)

KeyNa

K

Sodium-potassiumpump

Potassiumchannel

Sodiumchannel

OUTSIDEOF CELL

MAINTAINING RESTING POTENTIAL

To keep sodium & potassium in the right gradients, the sodium-potassium pump uses ATP to maintain gradients

The sodium-potassium pump pumps 2K+ in and 3Na+ out each time.

TYPES OF ION CHANNELS:

Ungated ion channels – always open

Gated ion channels – open or close in response to stimuli

• Ligand gated ion channels (chemically gated)–in response to binding of chemical messenger (i.e. neurotransmitter)

• Voltage gated ion channels – in response to change in membrane potential

• Stretch gated ion channels – in response to mechanical deformation of plasma membrane

HYPERPOLARIZATION

When gated K+ channels open, K+ diffuses out, making the inside of the cell more negative

Stimulus

Threshold

Restingpotential

Hyperpolarizations

50

0

50M

emb

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ote

nti

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mV

)

DEPOLARIZATION

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

Stimulus

Threshold

Restingpotential

Depolarizations

50

0

50

10010 2 3 4 5

Mem

bra

ne

po

ten

tial

(m

V)

ACTION POTENTIALS

Signals conducted by axons, transmitted over long distances

Occur as the result of gated ion channels that open or close in response to stimuli

- “All or nothing”

ACTION POTENTIAL

Steps:

1) resting state

2) threshold

3) depolarization phase

4) repolarization phase

5) undershoot Threshold

Restingpotential

50

0

50

10010 2 3 4 5

Mem

bra

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po

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tial

(m

V)

6

Actionpotential

OUTSIDE OF CELL

INSIDE OF CELLInactivation loop

Sodiumchannel

Potassiumchannel

Actionpotential

Threshold

Resting potential

TimeM

emb

ran

e p

ote

nti

al(m

V)

50

100

50

0

Na

K

Key

2

1

34

5

1

2

3

4

5 1

Resting state Undershoot

Depolarization

Rising phase of the action potentialFalling phase of the action potential

ACTION POTENTIAL

https://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter14/animation__the_nerve_impulse.html

HOW DO ACTION POTENTIALS “TRAVEL” ALONG A NEURON?

Where action potential is generated (usually axon hillock), the electrical current depolarizes the neighboring region of membrane

Action potentials travel in one direction – towards synaptic terminals

K

K

K

Na

Na

Na

Actionpotential

Axon

Plasma membrane

Cytosol

Actionpotential

Actionpotential

2

1

3

Why doesn’t it travel backwards?

The refractory period is due to inactivated Na+ channels, so the the depolarization can only occur in the forward direction.

SPEED OF ACTION POTENTIALS

Speed is proportional to diameter of axon, the larger the diameter, the faster the speed

Several cm/sec – thin axons

100 m/sec in giant axons of invertebrates such as squid and lobsters

Ganglia

Brain

Arm

NerveEye Mantle

Nerveswith giant axons

http://www.youtube.com/watch?v=omXS1bjYLMI

SPEEDING UP ACTION POTENTIAL IN VERTEBRATESMyelination (insulating layers of membranes) around axon

Myelin is deposited by Schwann cells or oligodendrocytes.

Cell body

Schwann cell

Depolarized region(node of Ranvier)

Axon

Action potentials are formed only at nodes of Ranvier, gaps in the myelin sheath where voltage-gated Na+ channels are found

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

SYNAPSES

Neurons communicate with other cells at synapses

Electrical synapse-

• Direct communication from pre to post synaptic cell

• Gap junctions connect cells and ion currents flow between cells

CHEMICAL SYNAPSE

Much more common in vertebrates & most invertebrates

1) Action potential reaches synaptic terminal

2) This depolarization causes Ca+ to rush into neuron through voltage gated calcium channels

3) Synaptic vesicles fuse with presynaptic membrane and release neurotransmitters.

4) Neurotransmitter diffuses across synaptic cleft and binds to ligand gated ion channels in second neuron.

5) Ligand gated ion channels open, generating a post-synaptic potential

6) Neurotransmitter is removed quickly – by enzymes or by surrounding cells uptake

Presynapticcell Postsynaptic cell

Axon

Presynapticmembrane

Synaptic vesiclecontainingneurotransmitter

Postsynapticmembrane

Synapticcleft

Voltage-gatedCa2 channel

Ligand-gatedion channels

Ca2

Na

K

2

1

3

4

EXCITATORY SYNAPSES

Some synapses are excitatory – they increase the likelihood that the axon of the postsynaptic neuron will generate an action potential

Opens channel for both Na+ & K+ - allows Na+ to enter & K+ to leave cell, so this depolarizes the membrane

EPSP – excitatory postsynaptic potential

INHIBITORY SYNAPSES

Some synapses are inhibitory – they make it more difficult for the postsynaptic neuron to generate an action potential

Opens channel that is permeable for only K+ or Cl-, so this hyperpolarizes the membrane

IPSP – inhibitory postsynaptic potential

SUMMATION OF POSTSYNAPTIC RESPONSES

A single EPSP is usually not enough to produce an action potential

Summation = the additive effect of postsynaptic potentials

The axon hillock is the neuron’s integrating center

• Temporal summation• Spatial summation

NEUROTRANSMITTERS

Many different types – 5 main groups:

Acetylcholine

biogenic amines

amino acids

Neuropeptides

gases

One neurotransmitter can have more than a dozen different receptors

ACETYLCHOLINE

- One of the most common neurotransmitters in vertebrates and invertebrates

- Can be inhibitory or excitatory

- Released at neuromuscular junctions, activates muscles

- inhibits cardiac muscle contraction

-also involved in memory formation, and learning

BIOGENIC AMINES

Biogenic amines are derived from amino acids

They include

• Norepinephrine – excitatory neurotransmitter in the autonomic nervous system

• Dopamine – rewards increase dopamine levels

• Serotonin - helps regulate mood, sleep, appetite, learning and memory

They are active in the CNS and PNS

ENDORPHINS

- Decrease our perception of pain

- Inhibitory neurotransmitters

- produced during times of physical or emotional stress – i.e. childbirth, exercise

Opiates (i.e. morphine & heroin) bind to the same receptors as endorphins and can be used as painkillers

VERTEBRATE BRAIN SPECIALIZATION

Cerebrum – 2 hemispheres, higher brain functions such as thought & action

Brain Hemispheres

VERTEBRATE BRAIN SPECIALIZATION

Cerebellum – helps coordinate movement, posture, balance

VERTEBRATE BRAIN SPECIALIZATION

Brainstem – controls homeostatic functions such as breathing rate, heart rate, blood pressure. Conducts sensory & motor signals between spinal cord & higher brain centers

Allan Jones: A map of the brain

http://www.ted.com/talks/allan_jones_a_map_of_the_brain.html

The mysterious workings of the adolescent brain

http://www.ted.com/talks/sarah_jayne_blakemore_the_mysterious_workings_of_the_adolescent_brain.html