MCB Ch 13 Cell Communication

If a cell receives no signal…

death by apoptosis

Requirements for cell-cell communication:

1. signal

2. receptor

3. signaling proteins

4. modification of target proteins 

Gap junctions

the cytoplasms of adjacent cells are connected through connexons

• occur in animals

Connexons

hollow transmembrane complexes of connexin proteins 

Plasmodesmata

• occur in plants

• no transmembrane protein systems

• plasma membranes of adjacent cells are fused through holes in the cell walls 

Juxtacrine 

direct cell-cell contact

signal and receptor are both surface molecules

Endocrine signaling

hormones

the secreted signals enter the circulatory system

the signals are stable and can reach distant target cells 

Autocrine signaling

the secreted signal can trigger responses in the same cell that secretes it 

Paracrine Secretion

secreted signals can only reach neighboring cells

signals do not enter circulatory system

Synaptic signaling

special type of paracrine signaling: the target is the postsynaptic cell, in a permanent interaction

• signals are neurotransmitters (stored in synaptic vesicles at the axon terminal of the neuron) that are released and reach the target cell over a small gap : chemical synapse

The propagation of electric signals is caused by ____ through the axon membrane by the alternating opening, inactivation, and closing of _______

ion currents 

voltage-gated ion channels 

Presynaptic membrane =

axon terminal

Postsynaptic membrane =

target cell

Ions that flow through the postsynaptic membrane after a neurotransmitter opens ion channels may have an ___ or ___ effect 

Excitatory ;  inhibitory 

Excitatory neurotransmitters

cause the depolarization of the postsynaptic membrane

• open Na+ channels and cause inflow of ion

• can trigger action potentials 

Membrane Polarization = EPSP

Inhibitory neurotransmitters

cause the hyperpolarization of the postsynaptic membrane

• they open Cl- channels on the membrane and cause inflow of ion

• prevents action potentials 

Membrane Depolarization = IPSP

Acetylcholine (ACh)

neurotransmitter that crosses the synapse b/w motor neuron and skeletal muscle

excitatory neurotransmitter in neuromuscular junctions

• opens ACh - gated Na+ channels 

• depolarizes muscle cells (EPSP)

• triggers action potentials and the contraction of the muscle cell

Which enzyme degrades ACh to terminate the signal and allow the muscle to relax?

Acetylcholinesterase

Glutamate

Excitatory neurotransmitter in CNS

• glutamate receptors are Na+ and Ca2+ channels 

• modulated by reuptake into neuron

• excessive stimulation by glutamate results in neurodegeneration (Huntington’s disease)

GABA (gamma-aminobutyric acid)

inhibitory neurotransmitter in CNS

• receptors are gated Cl- channels 

• important for neural control of body movements and brain functions

• modulated by reuptake into neuron

• Valium and Xanax enhance binding of GABA to receptor 

Dopamine

excitatory biogenic amine in the CNS

• causes EPSP  (depolarization)

• controller of body movements and pleasure sensations

• activity is controlled by reabsorption

• if dopamine-releasing neurons degenerate, tremors occur (Parkinson’s disease)

• molecules removed from synapse by transporters

Dopamine is active in what pleasure pathways in the brain?

amygdalae, part of the limbic system

Reabsorption of dopamine can be blocked by…

cocaine

Down-regulation

response by post-synaptic cell, decreasing number of surface dopamine receptors due to overstimulation by cocaine 

Serotonin

• EPSP/IPSP depending on the receptor 

• regulator of sleep and emotional stress

• activity is controlled by reuptake into neuron

• insufficient serotonin activity may result in clinical depression

Which drug is a selective serotonin reuptake inhibitor?

Prozac

→ it blocks reabsorption of serotonin so more is available for the stimulation of post synaptic cells 

Ecstasy causes..

massive release of serotonin

LSD

psychoactive drug that acts on some excitatory serotonin receptors

Neuropeptides

short chains of amino acids

Substance P

released at synapses in the CNS by sensory neurons and are activated by painful stimuli (P = pain)

excitatory(EPSP): results in pain sensations

Intensity of pain is modulated by …

endogenous opiates

Endorphins

are endogenous opiates. produced by brain to block pain perception

Morphine & Heroin

are exogenous opioids

they have analgesic (pain reducing) effect because they can bind to endorphin receptors

Synthesis and diffusion of NO (nitric oxide) results in:

1. relaxation of the smooth muscles surrounding blood vessels

2. dilatation of blood vessels and increased blood flow

Synthesis of NO (nitric oxide) is stimulated by 

acetylcholine