Physiology Unit 1 Part 2

lecture 2, 3, 4

Membrane Potentials

• the potential current flow across the cell if barrier of membrane is removed

• present in all cells

Nernst equation

• predicts equilibium potential

  • when chemical and electrical gradients are opposite and equal→ no net mvmt

• for each ion

Goldman Equation

• calculates voltage of membrane potential

• only looks at 1 ion at equilibrium unlike the nerst

Why are membrane potentials reversal (Why does the electrical charge change?)

• sodium ions follow electrochemical gradient in

• potassium ions follow chemical gradient out

How can membrane potential change if only positive ions move past each other?

• each ion has its own gate

• the gates don’t open or close at the same time

Glial Cells a.k.a Neuroglia

• regulating extracellular fluid

• preventing potassium accumulation

• sodium depletion

• support and maintain nervous cells

• prevent crosstalk btwn adjacent fibers

• make up majority of of brain tx

Eukaryotic Potassium Channel

• open due to electrical voltage changes

• large and complex

Bacterial Potassium Channel

• open in response to pH changes or chemical binds

• small and minimalist

Selectivity Filter in Potassium Channels

• in humans and bacteria

• potassium ions are wrapped in water molecules

• to enter gate, potassium must take of water coat

• oxygen fits right on top of potassium now, so potassium can go through

• sodium is too small and cant reach oxygen because its too much energy

Domain I Sodium Channels

• 6 segments

• when an electrical trigger happens, the sensor pulls the tunnel open so sodium can rush into the cell

Domain II Sodium Channels

• 6 segments

• when an electrical trigger happens, the sensor pulls the tunnel open so sodium can rush into the cell

Domain III Sodium Channels

• 6 segments

• when an electrical trigger happens, the sensor pulls the tunnel open so sodium can rush into the cell

Domain IV Sodium Channels

• 6 segments

• triggers the inactivation gate to shut down channel

Resting Potential

• -70mV

• sodium and potassium gates shut

Depolarization

• positive feedback

• depolarization causes a few sodium gates to open so sodium can enter, causing more depolarization, so more gates open

• potassium gates are opening much more slowly

All or none Response

• if enough sodium gates opened, all sodium and potassium gates are opened

  • all APs on axon are identical and show same wave form

• if too few sodium gates open, then not enough sodium before gates shut and no AP

• gates like springs

How are the first Na+ gates opened?

• the soma collects incoming signals

• signals travel to axon hillock (base of axon)

• electrical signal opens up sodium gates

• positive sodium charges rush into first tiny part of axon membrane, then next part of axon membrane

Hyperpolarization

• end of AP

• prevents depolarization from acheiveing threshold

What direction do action potentials travel in?

one direction

Velocity of AP

• more resistance less of it

• increase diameter of fiber for axon → reduce resistance

• decrease diameter of fiber for axon → increase resistance

How do you keep action potentials moving fast?

gaps btwn Schwann cells, known as nodes of Ranvier allow action potentials to skip from point to point

Saltatory Conduction

• schwann cells wrap around axon to form myelin sheath

• the sheath acts as an electrical insulator, preventing ions from leaking

• the nodes of Ranvier have lots of sodium gates

• electrical current flows quickly through axon and regenerates action potentials only in nodes

Dietary fatty acids

• n-6 and n-3 fatty acids are good for neural development

• human breast milk is 40-55% fat in first 6 months of feeding

Synapse

• fluid-filled gap btwn two cells

Pre-synaptic Cell

• the action potential travels down axon of this neuron

• releases neurotransmitters to chemical synapse or ions through gap junction

Cleft

• physical space between pre and post synaptic cells

Post-synaptic Cell

• neurotransmitters land on specific receptor proteins in chemical synapse

• binding opens ion channels in chemical synapse

• receive ions from presynaptic cell

Gap Junction- Typse of Synpase

• direct transmission of AP

  • no neurotransmitters or cleft

  • ions just flow into postsynaptic cell

• tight junction or electrical junctions

• smooth, cardiac muscle, retina, CNS

Chemical Synapse- Type of Synpase

• action potential arrives at pre-neuron ending

• depolarization opens Ca++ gates on pre-

• Ca++ activates kinase

• release of neurotransmitter into cleft

• common

Kinase

causes vesicles containing neurotransmitter to fuse with pre-membrane, emptying contents into cleft in chemical synapse

What happens after neurotransmitters are released into cleft?

• NT diffuse across cleft

• NT binds to receptors sites on post-membrane

• binding causes ion gates to open

• post- is depolarized or hyperpolarized

• enzymes on post-break down NT, so gates close

Excitatory Post-Synaptic Potential (EPSP)

• if depolarized

• only in chemical synapse

Inhibitory Post-Synaptic Potential (IPSP)

• if hyperpolarized

• only in chemical synapse

Fast Chemical Transmission

• miliseconds

• neurotransmitters opens a gate directly

• briefly changes the cell’s electrical charge to spark or stop an action

• controls rapid actions like reflexes, movement, and quick thoughts

Slow Chemical Transmission

• seconds to minutes

• neurotransmitter starts a chain reaction

• alters the cell’s internal chemistry

• regulates longer-lasting states like mood, sleep, hunger, and focus

Different categories of neurotransmitters

1. amines

2. catecholmines

3. amino acids

4. polypeptides

5. hormones

Types of amine neurotransmitters

• acetylcholine

• serotonin

Types of catecholamines

• nor-epinephrine

• dopamine

Cholinergic Fibers

• releases acetylcholine (Ach) as its chemical messenger

• can be excitatory or inhibitory

• same NT but different gates on post-

Two Types of cholinergic fibers

• nicotinic

• muscarinic

Nicotinic

• ACh binds to non-specific ion gate

  • only open and close with presence or absence of Ach

  • 2 AcH required to open, and when open is unstable

• more sodium enters than potassium leaves

• excitatory

• fast chemical synapse

Muscarinic

1. Excitatory Effect

   1. ACh binds to G-protein complex, which subunit closes some potassium channels so potassium can’t leak

   2. cell keeps becoming positive because Na-K pump still pumping potassium and sodium inside

   3. causes slow depolarization (makes it so that when a neurotransmitter or ions come by, it triggers a very fast action potential)

   4. makes cell more sensitive

2. Inhibitory Effect

   1. ACh binds to other G-protein complex, which subunit of it opens the potassium channels or stay open longer

   2. the potassium leaves cell, making cell more negative

   3. hyperpolarization happening

Acetylcholine esterase or AChe

• breaks ACh down into acetate and choline

• acetate and choline is taken up by pre-fiber and recycled into ACh

• found in cleft

Anti-cholinesterases

• causes continual depolarization

• blocks synaptic transmittion

Adrenergic synpases

• use nor-epinephrine as NT

• can cause EPSP or IPSP

What are 4 types of adrenergic synapses?

1. a1

2. a2

3. B1

4. B2