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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
ran
e p
ote
nti
al (
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
ne
po
ten
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