phys: neural communication

How do neurons communicate?

 A combination of:

• Electrical signals (action potentials): Within neurons, transmission along the axon

• Chemical signals (neurotransmitters): Between neurons, transmission at the synaps

 What is selective membrane permeability?

The cell membrane allows some ions to pass through but not others, via ion channels (leak channels, gated channels).

What happens to potassium (K⁺) at rest?

 K⁺ leaves the cell through leak channels, following its concentration gradient.

What happens to sodium (Na⁺) at rest?

There is less Na⁺ inside the cell, so little Na⁺ moves in or out at rest.

 How does the negative charge inside the cell develop?

As K⁺ leaves, the inside of the cell becomes negatively charged.

What maintains the resting membrane potential?

 The negative charge attracts K⁺ back into the cell, opposing further K⁺ efflux. The balance of the K⁺ concentration gradient and electrical gradient maintains the resting membrane potential (~−70 mV in neurons).

What is the Nernst potential?

 The membrane potential at which there is no net flow of a specific ion across the membrane. The voltage that exactly balances the electrochemical gradient for that ion.

Formula: Eₓ = (26.8/z) × ln([X]₀/[X]ᵢ)

Note: Applies ONLY when the ion can move across the membrane.

What is the sodium-potassium pump (Na⁺/K⁺-ATPase), and what does it do?

An active transport mechanism that maintains the resting membrane potential by pumping 3 Na⁺ out and 2 K⁺ in against their concentration gradients, using ATP.

 What are the four types of synapses based on structure?

1. Axo-dendritic

2. Axo-somatic

3. Axo-axonic

4. Dendro-dendriti

 What is an electrical synapse?

 Neurons interconnected by gap junctions (connexin/innexin proteins). Permits direct passage of ions between cells — enables electrical current flow without neurotransmitter release.

 What is an EPSP?

 Depolarization of the postsynaptic neuron caused by the presynaptic neuron. Brings the membrane potential closer to threshold.

What is an IPSP?

 Hyperpolarization of the postsynaptic neuron caused by the presynaptic neuron. Moves the membrane potential further from threshold.

 What is a graded potential?

 The result of the summation of EPSPs and IPSPs. It is a change in membrane potential that varies in magnitude (graded) and can be excitatory or inhibitory.

What is temporal summation?

The combined effect of multiple stimuli arriving at the same location on a neuron over a short period of time.

 What is spatial summation?

The combined effect of multiple stimuli arriving at different locations on a neuron at approximately the same time.

 Where does summation occur, and what is the result?

: Summation occurs at the axon hillock. If the summed membrane potential reaches threshold, an action potential is generated.

 What happens during depolarization of a neuron?

: Voltage-gated Na⁺ channels open → Na⁺ enters the cell → membrane potential becomes more positive → action potential is generated.

 What happens during repolarization of a neuron?

 Na⁺ channels close and K⁺ channels open → K⁺ exits the cell → negative charge restored.

 What happens during hyperpolarization?

 Further efflux of potassium → membrane potential becomes more negative than resting → redistribution of ions and resting potential restored.

What is the refractory period, and what is its function?

 A time interval after an action potential in which a neuron is temporarily unable to generate another action potential. It ensures action potentials propagate in one direction and prevents them from reverberating back and forth along the axon.

 What is the absolute refractory period?

: The period during which Na⁺ channels are inactivated and NO action potential can be generated, regardless of stimulus strength.

What is the relative refractory period?

The period during which K⁺ channels are open and the membrane is hyperpolarized. An action potential can be generated but requires a stronger than normal stimulus.

 How does an action potential propagate along an axon?

1. Depolarization spreads down the axon

2. The first part of the membrane repolarizes (Na⁺ channels inactivated, K⁺ channels opened)

3. The action potential continues to travel down the axon

What is saltatory conduction?

" The process where nerve impulses ""jump"" between the nodes of Ranvier along a myelinated axon, allowing for significantly faster and more efficient electrical signal transmission compared to continuous conduction in unmyelinated axons."

What is the function of the myelin sheath?

 An insulating layer around the axon that causes the signal to skip over the insulated sections, only regenerating at the uninsulated nodes of Ranvier.

 Compare continuous conduction and saltatory conduction.

• Continuous conduction: Occurs in non-myelinated axons; slower

• Saltatory conduction: Occurs in myelinated axons; faster and more efficient

What is lidocaine, and how does it affect action potentials?

 A local anesthetic that blocks Na⁺ channels → loss of sensation.

How do anti-arrhythmic drugs affect action potentials?

 They alter Na⁺/K⁺ channel activity → stabilize cardiac rhythm

: What is tetrodotoxin, and how does it affect action potentials?

 A toxin from puffer fish that blocks Na⁺ channels → paralysis.

 What is botulinum toxin, and how does it affect action potentials?

A toxin that blocks ACh release → flaccid paralysis.

What is tetanus toxin, and how does it affect action potentials?

A toxin that blocks inhibitory signals → rigid paralysis.

What is demyelination, and what are its clinical signs?

 Loss of myelin → slow/blocked conduction. Clinical signs: weakness, ataxia, sensory loss. Examples: Canine distemper, EPM (horses).

What is canine distemper, and how does it affect the nervous system?

 A viral disease that causes demyelination → slow/blocked conduction → weakness, ataxia, sensory loss.

 What is EPM, and how does it affect the nervous system?

: A disease in horses that causes demyelination → slow/blocked conduction → weakness, ataxia, sensory loss.

 What two things do neurons do?

"

1. Conduct ""electrical"" signals (action potentials)
2. Release ""chemical"" signals (neurotransmitters)

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