Excitable Tissues: Neurons

What are the communication methods in neurons

1. Electrical signals (dendrites, cell body, axon)

2. Chemical signals (synapses)

Neuron Features

• Dendrite

  • Receive chemical signals

• Cell body

  • Passively conducts electrical signals

• Axon hillock

  • initiates action potential

• axon

  • Propagates action potential

• Axon terminal

  • release chemical signal

What is the RMP

• Cytosol has a potential that is 50 to 70mV lower than extracellular

• Only neurons and muscles can respond with a transient change of the potential

What generates RMP

• A separation of charge-more negative charges inside the cell than outside

• Unequal concentration of ions

• Unequal permeability of the cell membrane of ions

What is the concentration of K and Na ions inside and outside neurons

Outside: K=5mM Na=150mM

Inside: K=100nM Na=15mM

How are the concentration gradients actively maintained for K and Na

Na/Katpase pump

At a ratio of

3 Na out

2 K in

What are the 2 channels in neurons

1. Non gated (leak) channels

   1. Open at rest, allows for diffusion

2. Gated channels

   1. Closed at rest

There are more leak K channels than Na

What is the membrane permeability of neurons

Pk/PNa=40/1

What are intracellular potentials measured today

1. Microelectrode recording

2. Patch clamp

What is the equation for Equilibrium Potential

Nernst Equation

What is the equation for RMP

Goldman Equation

What is hyperpolarisation and depolarisation

Hyperpolarisation = membrane potential gets more negative

Depolarisation = membrane potential gets more positive

What is an action potential

• A fluctuation in membrane potential caused by a transient opening of voltage-gated ion channels

• Occur after membrane potential reaches a threshold -55mV

What are the three stages of action potential

1. Fast depolarisation

• MP reaches threshold and there is an opening of Na channels Pk/PNa is 1:20

2. Repolarisation

• Closing of Na channels and opening of K channels

3. After-hyperpolarisation

• Pk/PNa becomes 100:1

What are the stages of action potentials periods

1+2=absolute refractory period

3=relative refractory period

What is the voltage gated channel in resting state

Activation gate is closed

Inactivation gate is open

What are the stages of voltage gated channels

1. RMP - closed

2. Once threshold is reached, activation gate is opened

3. After a fraction of a millisecond, inactivation gate closes

4. Activation gate closes, inactivation gate opens

What are the 2 ways, AP can be evoked

Externally- electrical stimulation via a battery

Internally- post-synaptic build up

How does external AP evoking occur

When a current flows through the cell membrane from outside to inside=local hyperpolarisation

When current flows from inside to outside = local depolarisation

How are AP generated internally

1. APs are first generated at the axon hillock, which has the lowest threshold

2. Depolarisation to threshold is evoked by excitatory postsynaptic potentials which spread passivly from dendrites

3. AP are transmitted actively along the axon

What can modulate Voltage gated ion channels

Batrachotoxin

What are the effects of ion channels being indefinitely unable to close

paralysis - flaccid or spastic

What are the 2 types of axons

Unmyelinated axons

• small

• slow, continuous transmission of AP

Myelinated axons

• large,

• fast, saltatory transmission of AP

What are the 2 stages of AP transmission

1. Passive spread

2. Generation of action potentials

What are the stages of passive spread of AP

1. Depolarisation at one region of the axolemma

2. Passive current flow inside and outside the axon

3. Depolarisation of adjacent parts of the membrane

How far does passive depolarisation spread

less than 1mm as the current dissipates through leaky channels and resistance

What is the AP transmission in unmyelinated axons

1. Local depolarisation occurs

2. Passive current flow

3. Depolarisation of adjacent parts of membrane to threshold

4. Voltage gated Na channels in adjacent membrane open

5. New full size AP generated in adjacent membrane

What is the speed of AP transmission in unmyelinated axons, why

1m/s as AP must be regenerated at every point on the membrane which takes time

What is the speed of AP transmission in myelinated axons, why

20-100m/s as it increases efficiency of passive spread because AP are only regenerated at nodes of ranvier

Saltatory conduction

What is the structure of myelinated axons

Myelin sheath formed by:

• Oligodendrocytes in CNS

• Schwann in PNS

Myelin interrupted at nodes of ranvier

What are the benefits of myelination

1. Less passive current loss (leakage)

2. Less time required for regeneration

3. Less energy to maintain gradient

Why does AP conduct only in one direction in physiological conditions

As AP are triggered at Axon hillock at the top of the axon, AP moves downwards, the previously opened Na channels are in the refractory period

What does the PNS contains

1. Axons and cell bodies of sensory neurons

2. axons of motoneurons

3. neurons forming autonomic nervous system

How are AP generated in sensory neurons

1. Muscle spindle stretches and mechanically gated Na channels open to evoke a graded depolarisattion (receptor potential)

2. Receptor potential spreads passively to trigger zone (hillock), where AP are generated

3. AP spreads along the axon to CNS proprioception or motoneurons (reflex circuit)

What is a nerve

a bundle of axons

What happens if a bacteria breaks down myelin

blindness, paralysis, numbness as AP cannot occur quick enough

Why are not all axons myelinated

Myelinated nerves are too big to fit in the body