4- Neural conduction and synaptic transmission

Membrane potential

difference in electrical charge between the inside and the outside of a cell.

Microelectrodes

The intracellular electrodes

are called -; their tips are less than onethousandth

of a millimeter in diameter—much too small

to be seen by the naked eye

-70 mV

resting membrane potential is about -

Negative

The inside of the neuron is - with respect to the outside

Polarized

In its resting

state, with the "70 mV charge built up across its membrane,

a neuron is said to be -

Ions

Like all salts in solution, the salts in neural tissue separate

into positively and negatively charged particles

called

Na+; K+

In resting neurons, there are more - ions outside

the cell than inside and more - ions inside than outside

ion channels

These unequal distributions of Na+ and K+ ions are maintained

even though there are specialized pores, called -, in neural membranes through which ions can

pass.

Electrostatic pressure

Because opposite charges attract, the -70 mV charge attracts. the positively charged Na+ ions into resting neurons

Random motion

for Na+ ions to move down their concentration gradient

Alan Hodgkin and Andrew Huxley

became interested in the stability of the resting membrane

potential

Sodium-potassium pump

Such ion transport is performed by mechanisms in the cell

membrane that continually exchange three Na+ ions inside

the neuron for two K+ ions outside. These transporters are

commonly referred to as

transporters

mechanisms in the membrane

of a cell that actively transport ions or molecules

across the membrane

1. Na+

2. Cl-

3. K+

4. Negatively charged proteins (A-)

ions contributing to resting potential

neurotransmitters

When neurons fire, they release from their terminal buttons

chemicals called -, which diffuse

across the synaptic clefts and interact with specialized

receptor molecules on the receptive membranes of the

next neurons in the circuit.

Depolarizations

making the membrane potential less negative

Hyperpolarization

making the membrane potential more negative

excitatory postsynaptic potentials (EPSPs)

Postsynaptic depolarizations

inhibitory postsynaptic potentials (IPSPs)

Postsynaptic hyperpolarizations

EPSPs and IPSPs

travel passively from their sites of generation

at synapses, usually on the dendrites or cell body, in

much the same way that electrical signals travel through a

cable.

Deceremental

EPSPs and IPSPs decrease in amplitude

as they travel through the neuron

axon initial segment

action potentials were generated at the -,

threshold of excitation

If the sum of

the depolarizations and hyperpolarizations reaching the

axon initial segment at any time is sufficient to depolarize

the membrane to a level referred to as its

-65 mV

the threshold of excitation is usually about -

Action potential

massive but momentary—

lasting for 1 millisecond—reversal of the membrane

potential

All-or-none responses

they either occur to their full extent or do not occur at all.

Integration

Adding or combining a number of individual signals

into one overall signal is called

Spatial summation

integration of events happening at different places

Temporal summation

integration of events happening at different times

Voltage-activated ion channels

ion channels that open or close in response

to changes in the level of the membrane potential

Absolute refractory period

brief period of about 1 to 2 milliseconds after the

initiation of an action potential during which it is impossible

to elicit a second one.

Relative refractory period

the period during which

it is possible to fire the neuron again but only by applying

higher-than- normal levels of stimulation

Antidromic conduction

If electrical stimulation of sufficient

intensity is applied to the terminal

end of an axon, an action potential

will be generated and will travel along

the axon back to the cell body

Orthodromic conduction

Axonal

conduction in the natural direction—

from cell body to terminal buttons—

Nodes of ranvier

In myelinated axons, ions can pass

through the axonal membrane only

at the — the gaps

between adjacent myelin segments

Saltatory conduction

transmission of action potentials in

myelinated axons is called

Hodgkin-Huxley model

The preceding account of neural conduction is based heavily

on the

Hodgkin-Huxley model

model based on squid motor neurons

axodendritic synapses

synapses of axon terminal buttons on dendrites

dendritic spines

many axodendritic

synapses terminate on -, nodules of

various shapes that are located on the surfaces of many

dendrites

Axosomatic synapses

synapses of axon terminal buttons on somas

dendrodendritic synapses

often capable of transmission in either direction

axoaxonic synapses

they can mediate

presynaptic facilitation and inhibition.

Directed snyapse

synapses at which the site of

neurotransmitter release and the site of neurotransmitter

reception are in close proximity.

Nondirected synapses

synapses at which the site of release is at some

distance from the site of reception.

Neuropeptides

short amino

acid chains composed of between

3 and 36 amino acids

Golgi complex

Small-molecule neurotransmitters

are typically synthesized in the

cytoplasm of the terminal button

and packaged in synaptic vesicles

by the button’s -.

Small neurotransmitter molecules

synthesized in termal button, packaged in synaptic vesicles

Large neurotransmitter molecules

assembled in cell body, packaged in vesicles, transported to axon terminal

Co-existence

many neurons contain

two neurotransmitters

Exocytosis

process of neurotransmitter release

voltage-activated calcium channels

When a neuron

is at rest, synaptic vesicles that contain small-molecule

neurotransmitters tend to congregate near sections of the

presynaptic membrane that are particularly rich in -

Ca2+

The entry of the -

ions causes synaptic vesicles to fuse with the presynaptic

membrane and empty their contents into the synaptic cleft

receptors

Once released, neurotransmitter molecules produce signals

in postsynaptic neurons by binding to - in

the postsynaptic membrane.

Ligand

Any molecule that binds to

another is referred to as its -, and a neurotransmitter

is thus said to be a - of its receptor

receptor subtypes

The different types of receptors

to which a particular neurotransmitter

can bind are called the -

Ionotropic receptors

associated

with ligand-activated ion channels

Metabotropic receptors

are associated with

signal proteins and G proteins

Ionotropic receptors

NT binds and an associated ion channel opens or closes, causing a PSP

Metabotropic receptors

effects are slowe, longer-lasting, more diffuse, and more varied

Reuptake

scoop up and recycle NT

Enzymatic degradation

other neurotransmitters are degraded

(broken apart) in the synapse by the action of enzymes

Acetylcholinesterase

acetylcholine,

one of the few neurotransmitters for which enzymatic

degradation is the main mechanism of synaptic deactivation,

is broken down by the enzyme -

Gap junctions

narrow spaces between adjacent cells that are bridged by

fine, tubular, cytoplasm-filled protein channels

Connexins

fine, tubular, cytoplasm-filled protein channels

Astrocytes

appear to communicate and to modulate neuronal activity

tripartite synapse

The hypothesis that synaptic

transmission depends on communication among three cells

1. AMINO ACIDS

2. MONOAMINES

3. ACETYLCHOLINE

classes of conventional small molecule neurotransmitters

unconventional neurotransmitters

fourth group

of various small-molecule neurotransmitters, which are

often referred to as -because

their mechanisms of action are unusual.

Neuropeptides

class of large molecule neurotransmitters

• Glutamate

• Aspartate

• Glycine

• Gamma-aminobutyric acid (GABA)

amino acid neurotransmitters

Glutamate

most prevalent excitatory neurotransmitter in the mammalian

central nervous system

Glutamate

participates in relay of sensory information and learning

GABA

most prevalent

inhibitory neurotransmitter

Monoamine neurotransmitters

are slightly larger than amino acid neurotransmitters,

and their effects tend to be more diffuse.

• Dopamine

• Epinephrine

• Norepinephrine

• Serotonin

monoamine neurotransmitters

• Dopamine

• Norepinephrine

• Epinephrine

catecholamines

Tyrosine

Each catecholamine is synthesized from the amino acid -

Serotonin

single indolamine

Tryptophan

Indolamines are synthesized from the amino acid -

Noredrenergic

neurons that release norepinephrine

Adrenergic

Neurons that release epinephrine

Acetylcholine

neurotransmitter at neuromuscular

junctions, at many of the synapses in the autonomic

nervous system, and at synapses in several parts of

the central nervous system

Acetylcholine

muscle contraction in the PNS, cortical arousal in the CNS

Cholinergic

neurons that release acetylcholine are said to be -

Anandamide

Pain reduction, increase in appetite

Norepinephrine

Brain arousal and other functions like mood, hunger, and sleep

Serotonin

Mood and temperature regulation, aggression, and sleep cycles

Dopamine

motor function and reward

Soluble gas neurotransmitters

Endocannabinoids

classes of unconventional

neurotransmitters

• Nitric oxide

• Carbon monoxide

soluble-gas neurotransmitters

soluble-gas neurotransmitters

neurotransmitters are produced in the neural cytoplasm

and immediately diffuse through

the cell membrane into the extracellular

fluid and then into nearby

cells.

Retrograde transmission

At some

synapses, they transmit feedback

signals from the postsynaptic neuron

back to the presynaptic neuron.

Endocannabinoids

neurotransmitters that are similar

to delta-9-tetrahydrocannabinol

delta-9-tetrahydrocannabinol

the main psychoactive (producing psychological effects)

constituent of marijuana

Anandamide

most widely studied endocannabinoid

Pituitary peptides

contains neuropeptides that were first identified

as hormones released by the pituitary

Hypothalamic peptides

contains neuropeptides that were

first identified as hormones released by the hypothalamus

brain-gut peptides

contains neuropeptides

that were first discovered in the gut

opioid peptides

contains neuropeptides that are

similar in structure to the active ingredients of opium