Nerves transmit electrical signals through action potentials along axons. Action potentials are generated when a threshold level of depolarization is reached due to voltage-gated sodium channels opening. They then propagate along axons through continued depolarization and repolarization of membrane potentials. At synapses, neurotransmitters are released by presynaptic neurons and bind to receptors on postsynaptic neurons, generating excitatory or inhibitory postsynaptic potentials which can summate to influence firing. Neurons communicate through complex circuits using these synaptic potentials for parallel and serial processing of information.

1. Nerves

2. Basic principles of electricity
Voltage is a measure of the potential
difference between separated electrical
charges of opposite sign.

3. Current refers to the flow of electrons from
one point to another.
Resistance is hindrance to charge flow.
Ohm’s law: current (I) = voltage (V)
resistance (R)

6. Chemically gated channels are ion channels
that open when the appropriate
neurotransmitter binds.
Voltage gated channels are ion channels
that open and close in response to changes
in the voltage.

7. The resting potential
Resting membrane potential is the voltage
that exists across the plasma membrane
during the resting state of an excitable cell.
When a membrane is in its resting potential,
it is said to be polarized.

9. Membrane potentials
Depolarization refers to the loss of a state of
polarity.
Hyperpolarization occurs when the
membrane potential or voltage increases,
becoming more negative than the resting
potential.

11. Graded potentials
A graded potential is local change in
membrane potential that varies directly with
the strength of the stimulus.

15. Action potentials
An action potential is a self-propagating
wave of depolarization.
A nerve impulse is another name for an
action potential.

16. Generation of an action potential
Resting state:active channels closed.
Depolarization phase: increase in sodium
permeability and reversal of the membrane
potential; threshold.
Depolarizing phase: decrease in sodium
permeability.

18. Repolarizing phase: Increase in potassium
permeability; repolarization.
Undershoot: Potassium permeability
continues; undershoot.

19. An action potential must be propagated, or
transmitted, along the axon’s entire length if
it is to serve as the neuron’s signaling
device.

23. The all-or-none phenomenon refers to the
fact that an action potential either happens
completely or it doesn’t happen at all.

25. The absolute refractory period is the period
following stimulation during which no
additional action potential can be evoked.
The relative refractory period, which
follows the ARP is the interval when a
threshold stimulus is unable to trigger an
action potential.

27. The larger the axon’s diameter, the faster it
conducts impulses.
The myelin sheath greatly increases the rate
of impulse propagation.
Saltatory conduction refers to the triggering
of action potentials at nodes, such that it
jumps from node to node along the axon.

29. The synapse
A synapse is a unique junction that
mediates the transfer of information from
one neuron to another neuron, or to an
effector cell.
Synapses that occur between the axonal
endings of one neuron and the dendrites or
cell bodies of other neurons, are called
axodendritic or axosomatic, respectively.

31. Neurons conducting impulses toward the
synapse are called presynaptic neurons, and
neurons that transmit the electrical signal
away from the synapse are called
postsynaptic neurons.
Neurotransmitters are chemicals that act to
open or close ion channels that influence
membrane permeability.

32. Electrical synapses are bridged junctions
that correspond to the gap junctions found
between certain other body cells.
Chemical synapses are specialized for
release and reception of neurotransmitters.

35. A synaptic vesicle is a small membranous
sac containing the neurotransmitter
acetylcholine.
A synaptic cleft is the fluid-filled space at a
synapse.

36. Information transfer across
chemical synapses
Calcium gates open in the presynaptic
axonal terminal.
Neurotransmitter is released by exocytosis.
Neurotransmitter binds to postsynaptic
receptors.
Ion channels open in the postsynaptic
membrane.

37. Synaptic delay is the time required for an
impulse to cross a synapse between two
neurons.

38. Postsynaptic potentials and
synaptic integration
An excitatory postsynaptic potential is a
local depolarization of the postsynaptic
membrane that brings the neurons closer to
threshold for actin potential generation.
An inhibitory postsynaptic potential results
in hyperpolarization of the postsynaptic
neuron and drives the neuron away from the
threshold for firing.

39. EPSPs can add to together to influence the
activity of a postsynaptic neuron.
Temporal summation occurs when one or
more presynaptic neurons transmit impulses
in rapid-fire order, which results in waves
of neurotransmitter released in quick
succession.

40. Spatial summation occurs when the
postsynaptic neuron is stimulated by a large
number of terminals from other neurons at
the same time.
When partially depolarized neurons are
more easily excited by successive
depolarization events, the neurons are said
to be facilitated.

42. Synaptic potentiation refers to the
enhancement of the neuron’s ability to
excite the postsynaptic neuron, due to
repeated or continuous use of a synapse.

43. Presynaptic inhibition occurs when the
release of an excitatory neurotransmitter by
one neuron is inhibited by the activity of
another neuron.
Neuromodulation occurs when chemicals
other than neurotransmitters modify
neuronal activity.

44. Neurotransmitters and their
receptors
Acetylcholine is a neurotransmitter released
by some nerve endings, and at
neuromuscular junctions.
Acetylcholinesterase (AChE) is an enzyme
that degrades acetylcholine, so that it
transmit an action potential across a
synapse.

45. Biogenic amines
Catecholamines include dopamine,
norepinephrine, and epinephrine.
Indolamines include serotonin and
histamine.

46. Amino acids
Amino acid neurotransmitters include
gamma-aminobutyric acid (GABA),
glycine, aspartate, and glutamate.

47. Peptides
The neuropeptides include substance P,
endorphins, and enkephalins.

48. Novel messengers
Adenosine triphosphate (ATP), nitric oxide
(NO), and carbon monoxide (CO), have
recently been shown to be
neurotransmitters.

49. Channel-linked receptors mediate direct
transmitter action, and are activated in
response to ligand binding.
The effects of G protein-linked receptors
are mediated by second messengers, such as
cyclic AMP.

51. Organization of neurons
Neuronal pools are functional groups of
neurons that process and integrate
information.

53. Types of circuits
The patterns of synaptic connections in
neuronal pools are called circuits.
When one incoming fiber triggers responses
in ever-increasing numbers of neurons
along the circuit, it is said to be a diverging
circuit.

55. Converging circuits occur when the pool
receives inputs from several presynaptic
neurons, and the circuit has a concentrating
effect.
Reverberating circuits occur when the
incoming signal travels through a chain of
neurons with collateral synapses.

57. Parallel after-discharge circuits occur when
the incoming fiber stimulates several
neurons arranged in parallel arrays that
eventually stimulate a common output cell.

59. Patterns of neural processing
When one neuron stimulates the next in
sequence, eventually causing a specific
response, it is known as serial processing.
A reflex is an automatic reaction to stimuli.
A reflex arc is a neural pathway over which
reflexes occur.

61. Inputs may be segregated into many
different pathways, and simultaneously
analyzed by neural circuitry in parallel
processing.