there's a threshold to begin a nerve impulse. it is an all or nothing response.

there's an
​electrical synapse

2. a shift from this resting potential towards a less negative potential occurs = depolarization

​
Myelin is interrupted at gaps called the nodes of Ranvier, where action potentials jump from one node to the next (saltatory conduction). 
This jumping mechanism allows for even faster signal transmission compared to continuous conduction along unmyelinated axons. 

how does a nerve impulse work?

the nerve impulse
​is called an
action potential. it travels along the axon of a nerve cell. 

some background first...

neurotransmitter imbalances

vs 
presynaptic facilitation by serotonin 

a chemical synapse is a gap between cells where a nerve impulse is relayed 
chemically by neurotransmitters. 
chemical synapses are found at:
a. junctions between two neurons
b. between neurons and muscles (neuromuscular junction)
c. between neurons and glands (neuroglandular junction) 

review question:

explain the process by which a chemical synapse relays an action potential to an adjacent cell. 

5. the cell regains its
polarized state =|resting potential

adrenergic receptors (adrenoceptors) are metabotropic. they bind catecholamines such as
norepinephrine/noradrenaline and
epinephrine/adrenaline. 

excitatory neurotransmitters initiate a post synaptic action potential by causing depolarization of the post synaptic cell producing an
EPSP = excitatory postsynaptic potential.
inhibitory neurotransmitters cause the post synaptic cell to hyperpolarize therefore suppressing an action potential producing an
IPSP = inhibitory post synaptic potential.
remember...
it is NOT the neurotransmitter that determines how the post synaptic cell will respond but rather the post synaptic receptor of that neurotransmitter. 

 nicotinic vs muscarinic cholinergic receptors 

 the adrenal medulla is the core of the adrenal gland. it participates in the flight or fight response by secreting epinephrine (adrenaline) and norepinephrine (noradrenaline)

types of
​propagation:

1. continuous via unmyelinated axons 
2. saltatory via myelinated axons

review question:

do electrical synapses employ neurotransmitters? explain. 

temporal vs spatial summation
a single excitatory post synaptic potential (ESPS)  (0.5mv)won't provide enough stimulation to reach threshold generating an action potential but individual ESPSs can combine to reach threshold. This can occur 2 ways...

spatial summation = multiple presynaptic neurons release enough neurotransmitter to reach threshold
temporal summation = one presynaptic neuron releases neurotransmitter many times over a period of time to reach threshold

review question:

explain how the membrane potential of a cell is altered.

other neurotransmitters and their functions 

resting membrane potential =
cell is inactive at the moment

producing a wave of depolarization...

​all nicotinic receptors are ionotropic (ligand-gated ion channels)
nicotinic receptors respond to drugs such as nicotine, an agonist. 

​ 1. a stimulus initiates a rapid change in voltage. 

an ionotropic receptor is a membrane protein that responds to ligand binding by allowing ions to move across the membrane through an open ion channel.

​  2. depolarization is caused by a the opening of sodium           channels causing an influx of Na+ ions.
      threshold is reached.

review question:

explain how an action potential is propagated. 

the rest of this subpage is cool stuff but not required: extra credit...

review question:

how does a stronger stimulus differ from a weaker stimulus?

now let's factor in the ions.
their movement shifts the polarity...the charge

3. restoring the normal resting potential after depolarization is called ​repolarization

   biogenic amines  

 nicotinic                         muscarinic  

review question:

chemical synapses can transmit action potentials between nerves axons and 3 different types of post synaptic cells.

List them.

​​​  4. hyperpolarization is caused by the efflux of K+ ions and closing          of the K+ channels.

let's look at the synapse

saltatory 
​propagation along a myelinated axon travels 18-120 m/sec
(40-270 mph)

review question:

explain the relationship between myelin and saltatory propagation. 

                     adrenergic receptors                   

muscarinic receptors are metabotropic
(g-protein coupled receptors)
which trigger a series of intracellular events using second messengers. 

a nerve impulse travels down a cell membrane
just like dominoes...

its all about the Na+ & K+ ​ions crossing the membrane...

neurotransmitter function: direct vs indirect effects

Myelin increases the speed of nerve impulses:
faster movement, reflexes, and cognitive functions.

in a
​nutshell...

​ we start with a polarized (resting) cell

polarized = resting

propagation of an action potential

​​​ 5. the resting polarized state is re-established when the membrane       potential returns to the voltage before the stimulus. 

this is done by moving the ions (via a carrier protein) back to where they started... K+ inside and Na+ outside the cell. 

muscarinic receptors are found at cholinergic synapses in the CNS and at postganglionic parasympathetic nerve terminals. 

inside cell

review question:

will ANY stimulus trigger an action potential? explain.

outside cell

ions move across the membrane via
voltage gated channels

 current = ​a measurement of ions crossing the membrane 

nicotinic receptors are found at cholinergic synapses in the CNS, at parasympathetic and sympathetic autonomic ganglia, and at the neuromuscular junction.

and there's a  chemical synapse

the nerve  impulse

nodes are 1-2 mm apart 

review question:

explain the difference between saltatory and continuous propagation. 

neuropeptides

1. ECF = extracellular fluid
    ICF = intracellular fluid
     ECF and ICF have different composition of ions:
              ECF has high Na+ and Cl-
              ICF has high K+ and (-) charged proteins

2. plasma membranes are selectively permeable...
             ions must move across via ion channels.
             proteins can't cross the membrane 

​3. the difference in charge inside vs outside the membrane 
    is considered potential energy. 

​4. voltage is the measurement of this potential energy.
    we use millivolts (mV) to quantify this membrane's                      potential energy

5. a membrane potential is the difference in                  voltage between the inside and outside of a cell's membrane.

neurotransmitters
relaying an action potential

Myelin acts like the plastic coating around electrical wires.

 amino acids 

1. a cell at rest has a resting potential of -70mv and is said to be polarized.
a stimulus triggers action potential

electrical synapses are electrical and mechanical connections between two neurons ​found at a gap junction. 
An electrical synapse has channel proteins between pre/postsynaptic neurons connecting the two neurons so the electrical signal can travel straight over the synapse.
Found in the retina, the brain, the olfactory bulb and other areas of the CNS. 

4. an increase in the negativity of the resting potential = hyperpolarization

(kind of like an overcompensation) 

what's myelin?

review question:

what is meant by the membrane potential of a cell? how is membrane potential quantified? 

continu0us
​propagation along
an unmyelinated axon travels
​1 meter/sec (2 mph)

so what starts an action potential?

speaking of neuromodulation...
presynaptic inhibition by GABA

​  3. repolarization results from Na+ channel closing and a large efflux         of potassium (K+) ions from activated K+ channels.