BE202-Chapter4_Review Notes

Polarization

• separation of changes across a membrane creates potential difference

• at rest the inside of a cell is negative compared to the outside

Depolarization

membrane becomes less polarized, inside of cell becomes less negative

Hyperpolarization

membrane becomes more polarized inside of the cell becomes more negative

Repolarization

membrane returns to resting potential after having been depolarized

Resting membrane potential

potential of cell , when not active, usually around -70 mV

Threshold potential

specific membrane potential that when reached there is a positive feedback mechanism controlling the opening of Na⁺ channels resulting in the triggering of an action potential.

Action Potential

brief rapid and large (100mV) all or none change in membrane potential

Refractory Period

time period during which a region of membrane that has just undergone an action potential cannot have another action potential when stimulated normally

All-or-non Law

an excited membrane either responds to stimulus by having an action potential or it does note respond with an action potential at all

Describe permeability changes and ion fluxes during an action potential

Rising phase

• increase in Na⁺ permeability

• Na⁺ enters the cell

Falling Phase

• K⁺ permeability increased

• Na⁺ permeability decreased

• K⁺ leaves the cell

(the Na⁺/K⁺ ATPase pump continues to function throughout both phase)

Compare the events at excitatory and inhibitory synapses

Excitatory Synapses

• binding of neurotransmitter to receptor opens both Na⁺ and K⁺ channels

• the electrochemical gradient favors the movement of Na⁺ into cell resulting in small depolarization .

Inhibitory Synapses

• the binding of neurotransmitter to a receptor opens both K⁺ and Cl^- channels

• Cl^- enters the cell and K⁺ leaves the cell resulting hyperpolarization

Compare contiguous conduction and saltatory conduction

Contiguous conduction

• spread of action potential along the entire axon and occurs in unmyelinated fibers.

Saltatory conduction

• occurs in myelinated fibers as action potential moves from node along axon, with currents driving subsequent nodes to threshold

Four kinds of gated channels that opens or closes them

1. Voltage-gated channels

• response to changes in membrane voltage

2. Chemically gated channels

• change conformation in response to the binding of an extracellular chemical messenger to a surface receptor

3. Mechanically gated channels

• respond to stretching/mechanical deformations

4. Thermally gated receptors

• respond to changes in temperature

Neurotransmitter

• small molecule

• produces rapid and brief response changing the membrane potential of the post synaptic cell

(e.g. acetylcholine)

Neuromodulator

a chemical that does not cause the formation of EPSPs or IPSPs but instead acts slowly to bring about long-term changes that subtly modulate the action of the synapse (e.g. dopamine, serotonin)

Neurohormone

a hormone released into the blood by neurosecretory neurons (e.g. vasopressin)

Discuss the possible outcomes of the grand postsynaptic potential (GPSP) brought about by interactions between EPSPs (Excitatory Postsynaptic Potential) and IPSPs (Inhibitory Postsynaptic Potential)

• EPSPs and IPSPs are graded potential can interact at the postsynaptic cell to influence its membrane potential

• if IPSP dominate,

  • the postsynaptic cell does not move toward threshold

• if EPSP dominate

  • the cell is driven toward threshold and if threshold is reached will generate an action potential

GPSP

• interaction of EPSP and IPSP overtime and the space produce total potential changes in the postsynaptic cell

Distinguish between presynaptic inhibition and an inhibitory postsynaptic potential

Presynaptic inhibition

• one neuron inhibits the release of neurotransmitter from another neuron

Inhibitory postsynaptic potential (IPSP)

• associated with changes to the membrane potential of the postsynaptic cell

The binding of neurotransmitter to a receptor opens both K⁺

and Cl^- channels on the postsynaptic cell. Cl^- enters the cell, K⁺ leaves the cell and the cell becomes hyperpolarized.

Types of Intercellular Communication

Direct intercellular communication, Indirect intercellular communication

Direct intercellular communication

involves physical contact between cells via gap junctions or through direct linkups of surface markers

Indirect intercellular communication

occurs when extracellular chemical messengers released by one cell interact with receptors on the target cell

Signal transduction

Process by which incoming signals are conveyed into target cells, where they tranfsorm into a specific cellular response

Compare the tyrosine kinase and JAK/STAT pathways

Tyrosine Kinase pathway

• receptor itself functions as an enzyme

JAK(Janus family Tyrosine Kinase) pathway

• the tyrosine kinase activity resides in a family of separate cytosolic enzymes

STAT pathway

• Activated JAKs phosphorylate signal transducers and activators of transcription (STAT) in the cytosol

Distinguish between first and second messengers

First messenger

• extracellular signal molecule that binds to the surface receptor on the target cell

Second messenger

• an intracellular molecule formed inside the cell as. aresult of the first messenger binding to receptors on the target cells

Compare cytokines and hormones

Cytokines

• a collection of protein signaling molecules secreted by cells of the immune system and other cell types that largely act locally to regulate immune responses

Hormones

• either hydrophilic or lipophilic and either peptides, amines, or steroids

• endocrine glands to make chemical adjustments to all body cells

Describe how arachidonic acid is converted into prostaglandins, thromboxane, and leukotrienes

• Arachidonic acid split from the plasma membrane by a membrane-bound enzyme, phospholipase

Convert to 3 main classes of eicosanoids

• prostaglandins

• thromboxane

• leukotrienes

Enzyme COX(cyclooxygenase)

• initiates a pathway leading to formation of prostaglandins and thromboxane

Enzyme LOX(lipooxygenase)

• initiates another pathway that results in generation of leukotrienes

Describe the sequence of events in the cAMP second-messenger pathway

• one first messenger to a surface receptor can activate multiple adenylyl cyclases, each of which can produce multiple cAMP molecules

• the original signal has been amplified significantly

• amplification can occur in other step s so that the original signal can be amplified millions of times

Sequence of events in the Ca²⁺second messenger pathway

• the first messenger to the surface receptor leads by means of G proteins to activation of the enzyme phospholipase C.

• the enzyme breaks down phosphatidylinositol bisphosphate (PIP₂)

• PIP2 breakdown are diacylglycerol (DAG) and inositol trisphosphate (IP₃)

• Liquid soluble DAG remains in the lipid bilayer of the plasma membrane, but water-soluble IP3 diffuses into cytosol

• IP3 mobilizes intracellular Ca2+ stored in the ER to increase cytosolic Ca2+ by binding with IP3-gated receptor-channels in the ER membrane.

• Ca2+ then takes on the role of second messenger, ultimately bringing about the response commanded by the first messenger

Explain how the cascading effect of hormonal pathways amplifies the response.

• With one event triggering the next in the sequence, there is a tremendous amplification of the initial signal.

• This allows very low concentrations of hormones and other chemical messengers to trigger pronounced cell responses

Compare the nervous and endocrine systems.

The nervous system

• releases neurotransmitter into synapses that act over a short distance.

• produces rapid responses (milliseconds) that last for a brief period of time (milliseconds).

The endocrine system

• it releases hormones into the blood that act over long distances.

• The response is slow (minutes to hours) and lasts for an extended period of time (minutes to days).

• hormones control activity that requires long-duration responses.