neuropeptides

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. 

resting membrane potential =   -70mv
cell is inactive at the moment (at rest)

producing a wave of depolarization...

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

how does a nerve impulse work?

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

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

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

ecf

some background first...

neurotransmitter imbalances

icf

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

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. junctions between neurons and muscles (neuromuscular junction)

c. junctions between neurons and glands (neuroglandular junction) 

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

in a
​nutshell...

​                            to recap...
​we start with a polarized (resting) cell

remember:
​polarized = resting

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 the opening of sodium           channels causing a rush of Na+ ions into the cell 
(as long as 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

(kind of like an overcompensation) 

what's myelin?

​​​ 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+ is pumped back inside the cell and Na+ is pumped back outside the cell. 

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

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

inside 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. 

outside cell

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

ions move across the membrane via
voltage gated
channel proteins

   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 

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+ channels closing and a large rush of potassium (K+) ions out of the cell via 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)

and there's a  chemical synapse

action potential = 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

other neurotransmitters and their functions 

​​​​  4. hyperpolarization is temporarily caused by more K+ ions leaving the cell than Na+ ions entering the cell as the K+ channels close 

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 

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