the nerve  impulse

types of
​propagation:

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

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

axon diameter vs propagation speed

other neurotransmitters and their functions 

​​  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)

                     adrenergic receptors                   

nodes are 1-2 mm apart 

neuropeptides

speeds:
type A - 130 m/sec
type B - 15 m/sec
type C - 2 m/sec  

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. 

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

and there's a  chemical synapse

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

cholinergic synapses release acelylcholine

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

axon diameter affects
action potential speed.
 larger diameter =
lower resistance =
faster propagation velocity 

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

its
all
about
the
Na+
and
K+
​ions...

neurotransmitter function: direct vs indirect effects

it all starts with the cell membrane...

    1. the ECF (extracellular fluid) and ICF (intracellular fluid)
             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.
             some channels are more "accessible" than others
                    outgoing K+ channels are "leakier" than incoming Na+ channels
                    proteins can't cross the membrane which contributes to cell's (-) charge  

voltage is the measurement of potential energy generated by separate charges
​measured in (mv)

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

neurotransmitter imbalances

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.

current = a measurement of ions crossing the membrane. resistance of the membrane slows down ion exchange =
weakens current

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) 

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

in a
​nutshell...

​ we start with a polarized (resting) cell

propagation of an action potential

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.

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

what's myelin?

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

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

the rest of this subpage is cool stuff but FYI...

now let's factor in the ions...

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

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

so what starts an action potential?

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

speaking of neuromodulation...
presynaptic inhibition by GABA

   biogenic amines  

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 

 current = ​a measurement of ions crossing the membrane 

Millivolts (mV) are a unit of measurement used to quantify the membrane potential, which is the electrical charge difference across a cell membrane. 

the potential difference
is due to the distribution of ions  across the membrane.

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

 nicotinic                         muscarinic  

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

 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)