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Membrane potential and excitation
Somatic reflex arc:
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Pain receptor
2 – 3 (at least) neural connections
Sensory neurons
Motor neurons
Unipolar sensory neurons have one long axon which
connects receptors to the spinal cord or brain
(A unipolar neuron has only one neurite extending from
the cell body)
Multipolar inter- and motoneurons have many
dendrites and one axon

Bipolar neurons have one dendrite and one axon

Cell resting membrane potential
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High K+ concentration inside
High Na+ concentration outside
Cl- ions outside help to balance positive
charge
 Negatively charged proteins help balance
charge from positive ions inside the cell –
everything’s electrically neutral
 Resting conditions: potential difference
across membrane of -70mv: inside relatively more negative than outside
 Membrane potential is very specific to the concentration of the ions: can alter it
by changing ion concentration
 Charge moves across the cell by charged ions through transport channels,
creating a voltage difference:
- K+ that leaks out is actively transported back in
- Na+ is actively transported out
- 2 K+ in, 3 Na+ out (Na+/K+ ATPase)
K+ has a resting potential of -90mv
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Together these forces are known as the
electrochemical gradient
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Na+ and Cl- on
the outside balance each other out
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Since the chemical and electrical forces on K+
are equal and opposite, there will be no net
movement of K+ across the membrane
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We can calculate the equilibrium potential of K+ using the Nernst equation - gives resting
membrane potenital/ equilibrium potential for a single ion:

(Simplified equation)

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The electrochemical gradient drives Na+ into the cell
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The Goldman equation gives the equilibrium potential for multiple ions – takes into account
K+ and Na+ concentrations:

Hodgkin and Huxley
Looked at squid axons – recorded membrane potential
Using very fine electrode to pierce neuron, made sure it formed a seal so you wouldn’t get
much leakage
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Triggered action
potentials by opening Na+ channels
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Neurons, action potentials and synapses
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Dendrites: receive signals
Cell body: contains nucleus, mitochondria etc
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neurotransmitters
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Retrograde
transport – going from nerve terminals back to cell body
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Neurotransmitters bind to
these
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Conductive segment:- Voltage-gated K+ and Na+ channels
Transmission segment (synaptic knob):- Voltage-gated Ca2+ channels
Na+/K+ pump in every region
(2K+ in, 3Na+ out)

Closed inactive voltage-gated
channels ensure that the action
potential moves in one direction

All or none principle
A stimulus which exceeds the threshold will first produce large local depolarization, creating
an action potential
Generator potential: the degree of stimulus will depend on the degree in change of the
membrane potential, both in amplitude and duration
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Membrane potential becomes
less negative- positive (+30mv)
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1
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If
stimulus is great enough, threshold is reached,
generating an action potential
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2
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Activation of Na+
voltage-gated channels: Positive feedback loop
(cascade) causes depolarisation
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Inactivation of Na+ voltage-gated channels
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Potassium moves out due to
electrochemical gradient
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4
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Slow
voltage-gated K+ channels begin to close at -70mv
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Voltage-gated K+
channels close, Na+/K+ pumps restore levels of ions to the resting potential
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Relative refractory period: Neuron can generate an action potential but only if it is
depolarised to a value more positive than threshold
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The speed of conduction depends on:
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The diameter of a fibre: bigger – AP travels faster
Temperature: AP quicker if hotter
Myelinated or not

Saltatory conduction: Myelination makes Action Potential jump between Nodes of Ranvier
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A fibres: large myelinated axons that conduct the action potential at 15-120 m/s (fast
pain)
B fibres: medium diameter myelinated axons that conduct the action potential at 315 m/s
C fibres: unmyelinated axons that conduct the action potential at <3 m/s (slow pain)

Targets of neurons:
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Post-synaptic neurons
Skeletal muscle (neuromuscular junction)
Secretory gland (endocrine/hormone system)

The synapse
Electrical synapses:
 Direct contact between cells (gap junctions) – movement of ions
 Extremely rare
 Located in CNS, PNS
Chemical synapses:
 Cells are not directly coupled
 Involves release of neurotransmitters
 Most abundant
Neurotransmitters contained in vesicles
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These then open, so Ca2+ floods in and triggers vesicles to bind to the
membrane and release their content
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Vesicle SNARES and
membrane SNARES
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Once Ca2+ is present and detected by
the protein that picks up that Ca2+
level change, they becomes more
relaxed and intertwine with each
other, bringing the vesicle closer to
the plasma membrane
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Neurotransmitters
Can be both excitatory and inhibitory
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This can generate an action potential
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This supresses the generation of an action potential
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Cocaine inhibits the
removal of dopamine from synapses in the brain, resulting in feelings of euphoria
 Inhibitory: precise control of movement – inhibits random muscle contraction
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There can be a combination

of positive depolarising EPSPs and hyperpolarising IPSPs, giving a net potential
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Temporal summation: number of signals over a period of time at a particular site
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An action
potential is propagated due to multiple signals combining to reach threshold

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