physiology pt 2

absolute refractory period

• second stimulus will not elicit a new AP

• Na+ channels inactivated

• occurs during depolarization and repolarization

relative refractory period

• second AP produced only if stimulus is considerably greater than normal

• K+ channels open, K+ leaving cell

• occurs during hyperpolarization

what is the purpose of the refractory period

• limits number of action potentials

• ensures APs only travel unidirectionally

where are voltage gates concentrated?

at nodes of ranvier

what happens when a action potential reaches the nerve terminal at the end of the axon?

it innervates another neuron, muscle cell, or gland

synapse

tiny spatial gap between nerve ending and target

what events occur at the synapse?

• arriving AP triggers Ca 2+ voltage gated ion channels in pre-synaptic knob to open

• Ca+ ions rush into cell from ECF

• synaptic vesicles containing neurotransmitters fuse with plasma membrane

• neurotransmitters diffuse across synaptic junction and bind with receptor on postsynaptic membrane

• chemical binding activates chemically gated ion channels

EPSP

• excitatory post-synaptic potential

• small depolarization resulting from binding of neurotransmitter ex. ACh) which opens Na+ and K+ channels

• brings cell closer to threshold

IPSP

• inhibitory post-synaptic potential

• small hyperpolarization resulting from binding of neurotransmitter (ex. GABA) which opens K+ or Cl- channels

• makes cell less likely to reach threshold

what type of potentials are EPSP’s and IPSP’s?

graded potentials

synaptic summation

sum of all EPSP’s and IPSP’s determines whether postsynaptic neuron reaches threshold

temporal summation

EPSP’s or IPSP’s from a single, repetitively firing presynaptic input occur so rapidly that they add together

what happens if excitatory inputs dominate in synaptic summation?

cell is brought closer to threshold

what happens if inhibitory inputs dominate in synaptic summation?

cell is taken farther from threshold

what happens if excitatory and inhibitory activity is balanced?

membrane potential remains close to resting potential

what occurs at postsynaptic membrane if positive ion gates open?

membrane becomes depolarized resulting in EPSP

what occurs if the sum of EPSP’s reaches threshold potential at the axon hillock?

an action potential is generated and will travel down the axon

what happens if K+ or Cl- gates open at postsynaptic membrane?

membrane becomes more polarized resulting in IPSP

what happens if IPSPs dominate at postsynaptic membrane?

hillock region moves away from threshold and neuron is less likely to generate action potential

grand post synaptic potential (GPSP)

sum of signals from all EPSPs and IPSPs

convergance

when a single cell is influenced by thousands of presynaptic cells

divergence

branching axon terminals of one neuron affect thousands of postsynaptic cells

EPSPs and IPSPs can sum in____ and____

time and space

what is the result of summation of EPSPs and IPSPs at the dendrite?

synaptic silence or grand post-synaptic potential

synaptic silence

• inputs do not reach threshold when summed

• no action potential occurs

what happens when a neurotransmitter molecule arrives at the postsynaptic membrane?

it binds with a receptor which initiates a response in the postsynaptic cell that is specific to the neurotransmitter

what is another way of signaling in the body?

use of hormones circulating through bloodstream and interacting with target tissues

are neurotransmitters and hormones extracellular or intracellular chemical messengers?

extracellular messengers that depend on receptors

actions of chemical messengers

• opening receptor-channels

• activating receptor-enzymes

• activating an intracellular second messenger via G-protein-coupled receptors

tyrosine kinase receptor

• most abundant receptor

• enzyme

• binding leads to phosphorylation of tyrosine residues on intracellular side of receptor

• ex. insulin receptor

G-protein coupled receptors

• have 7 transmembrane protein receptors

• mechanism by which water soluble hormones work

• generate intracellular second messengers

water soluble hormones

• cannot diffuse across membrane and enter into cell

• use signaling mechanism to transduce message into cell signal

intracellular second messengers

• cAMP

• IP3 and DAG (increases calcium)

what determines which second messenger is generated?

depends on type of subunits contained in G protein connected to receptor

3 monomers of G protein

• ⍺- alpha

• β- beta

• Ɣ- gamma

forms of ⍺ subunit

• ⍺_s

• ⍺_q/11

⍺_s subunit

leads to adenylyl activation and cAMP production

⍺_q subunit

leads to phospholipase C activation and IP3 and DAG production

what does cAMP activate?

PKA

what does PKA activate?

• activates other proteins into cytoplasm that will culminate in whatever cellular response was indicated by binding of signal molecule at cell surface

• turns transcription on

DAG and IP3

• DAG stays membrane associated and activates protein kinase C

• IP3 binds to receptor causing calcium to flow out of ER

PKC

phosphorylates other cytosolic proteins, activating them and leading to cellular response