Membrane Potentials & Neurotransmission (3)

September 9

Electrogenic

• transports charged particles

• transports a net charge, meaning it either transports a net positive or net negative across the membrane

Graded potential

• a temporary, local change in a cell's membrane potential that varies in size depending on the stimulus strength

• relatively small, doesn’t last very long, doesn’t go very far

• either depolarization (voltage difference less) or hyperpolarization (voltage difference more)

• Can be summed if they are close enough together in time and space

• There is a negative feedback mechanism which quickly brings them back to the RM

Action potential

a rapid, self-propagating electrical signal in excitable cells, such as neurons and muscle cells, that involves a sudden change in membrane voltage

What are potential changes generated by?

Opening of ion channels

Muscle and nerve cells.

Which types of cells are excitable and can exploit changes in membrane potential to encode messages?

Because rapid communication is necessary for the nervous system to function properly

Why must sensory input, CNS processing, and motor outputs occur quickly?

As changes in membrane potential.

In what format are electrically-coded messages transmitted in the nervous system?

Because cations enter

Why does potential decrease in ligand-gated ion channels?

Ligand gated ion channel

A membrane protein that opens an ion channel in response to a chemical messenger (ligand) binding

Neurotransmitters (e.g., acetylcholine, glutamate, GABA).

What usually acts as the ligand for ligand-gated ion channels in neurons?

How do ligand-gated ion channels contribute to changes in membrane potential?

By allowing ions (Na⁺, K⁺, Ca²⁺, or Cl⁻) to flow across the membrane, altering the charge distribution.

Are ligand-gated ion channels involved in graded or action potentials?

They initiate graded potentials, which can lead to action potentials if threshold is reached.

On the postsynaptic membrane at synapses.

Where are ligand-gated ion channels typically found in neurons?

Spatial summation

Graded potential that occurs when they are summed near each other

Temporal summation

Graded potential that occurs when they are summed close enough together in time

Resting membrane potential

the stable, voltage difference across a cell's membrane in its non-excited state, typically around -70 millivolts

Depolarization

when the membrane potential becomes less negative (more positive)

Hyperpolarization

when the membrane potential becomes more negative at a particular spot on the neuron’s membrane

What is the cause for different levels of pain?

more pain = more action potentials being sent

Depolarizing pain

pain caused by the depolarization (a change in the electrical charge) of pain-sensing neurons (nociceptors)

Absolute refractory period

no AP possible

Relative refractory period

APs are harder to start

Electromyography (EMG)

How the electrical activity in the muscles can be measured by

By nerves that react to the electrical stimulation

How are the muscles controlled?

Conduction velocity

the speed at which electrical impulses travel along a nerve, measured during a nerve conduction study (NCS)

Tetany

• A very long-lasting type of muscle cramp

• what skeletal muscle is stimulated by, using sufficient currents with frequencies of 40-110 Hz

Death grip

Occurs because the flexor muscles are stronger than the extensors

Which of the following contributes to the positive feedback mechanism of the action potential?

• Nerve conduction

• Neurotransmission

How do we make the action potential work for us?

Nerve conduction

• the movement of nerve impulses down

  neurons

• also called the propagation of nervous impulses

Neurotransmission

the transmission of nerve impulses across a synapse

Because the message is an action potential with a refractory period

Why can’t the axons send a second message until they have repolarized?

Purposes of the refractory period

• Assures that transmission is a one-way event

• Assures that the APs are separate events

• Allows the cell to recover to the resting state

Saltatory conduction

the process where electrical impulses "jump" along myelinated axons from one Node of Ranvier to the next

Electrical synapse

• when 2 cells are connected by ion channels

• No jumping so almost no delay

• Allows cells to stay synchronized in their actions

Gap junctions

Found between smooth muscle cells

• Heart muscle (myocardium)

• Uterine muscle (myometrium)

• Large parts of the brain

Where do we find electrical synapses?

Neurotransmitter

• a substance that transmits signals across the synapse

• released from axons (the presynaptic membrane) and infrequently from the postsynaptic membrane

• some __________ change the membrane voltage as in the generation of the EPSPs and IPSPs while others alter cell function in some other way 

• Axons (presynaptic membrane)

• infrequently from the postsynaptic membrane

Where are neurotransmitters released from?

Biogenic amines

• contain the amino functional group (R-NH2, where “R” is the rest of the molecule

  • Catecholamines

    • dopamine

    • norepinephrine (a.k.a. noradrenalin- released into the blood from the adrenals and also released at synpases)

    • epinephrine (a.k.a. adrenalin- sympathetic, postganglionic

      neurotransmitter

Acetylcholine

Looks very similar to the biogenic amines but is unrelated chemically

Histamine

• Particularly important in the CNS (mostly associated with allergic response in the rest of the body)

• Yet another biogenic amine

Amino acids

• Glutamate (excitatory)

• Aspartate (excitatory)

• GABA (gamma-aminobutyric acid ), an inhibitory neurotransmitter

• Glycine (inhibitory)

• Only found in the CNS

Peptides

• Substance P is the pain neurotransmitter

• Oxytocin- stimulates uterine contractions and the expression of milk

• Enkephalins- (e.g., leu-enkephalin) provide an opiate-like analgesia by inhibiting neurotransmission in ascending pain pathways

Postsynaptic potentials (PSPs) 

• EPSP increase in voltage (brings the cell closer to the threshold) 

• Often due to Na+ channels opening (Na+ enters the cell)

• IPSP a decrease in voltage (brings the cell further from the threshold) 

• Often due to K+ channels opening (K+ moves out of the cell) 

Serotonin

• Associated with sleep and dreaming, nausea and vomiting, affective tone (mood)

• A biogenic amine

Inactivation and reuptake

What does neurotransmitter removal involve?

Inactivation in neurotransmitter removal

• Usually involves some specific enzyme breaking down the neurotransmitter in synapse

  • Acetylcholine is broken down by acetylcholinesterase (AChase)

  • Epinephrine is broken down by monoamine oxidase (MAO)

Reuptake in neurotransmitter removal

• The dominant removal method for some neurotransmitters is reuptake into the presynaptic neuron

  • All of the catecholamines are taken back up and it is the major mechanism for removal

    • The reuptake is specific for the neurotransmitter (it is a type of active transport)

• The reuptake can be blocked

  • Selective serotonin reuptake inhibitors (SSRIs)

    • Fluoxetine (PROZAC) inhibits the reuptake of serotonin from the synapse and can be used as an antidepressant since increased serotonin is associated with increased affective tone

  • Norepinephrine/dopamine reuptake inhibitors (NDRIs)

    • Buproprion (WELLBUTRIN)

  • The uptake inhibitors should never be combined with MAOi drugs like tranylcypromine (a tricyclic antidepressant)