a complex network that coordinates our body's voluntary and involuntary actions

and transmits signals between different parts of the body it is divided into two

anatomical divisions the central nervous system CNS which consists of the brain

and the spinal cord and the peripheral nervous system PNS which consists of all

the nerves and neurons outside of the central nervous system now if we were to

look at the cross-section of the spinal cord we would find there three types

of neurons that are of importance to us number one afferent neurons which carry

signals to the CNS from sensory receptors in peripheral tissues number

two efferent neurons that carry signals from the CNS to the effector organs such

as muscle and number three interneurons which are located in between afferent

and efferent neurons and integrate information that flows between the two

so for example if you touch a hot object sensory receptors located in your

fingers would immediately initiate a signal that would travel through afferent

neurons to the spinal cord and then from there a response signal would be

initiated and would travel from the spinal cord through efferent neurons to

the muscles in your hand which would result in reflexive withdrawal of the

hand and all this would happen in just a fraction of a millisecond now let's take

a closer look at the functions of sympathetic and parasympathetic system

so I'm going to draw sympathetic division on the left side and

parasympathetic division on the right side but first let's quickly talk about

the types of neurons that we find in both of these divisions first we have

cholinergic neurons which use neurotransmitter acetylcholine

to send messages and I'm going to use the red color to draw them and the

second we have adrenergic neurons which use neurotransmitters norepinephrine and

epinephrine to send messages and I'm going to use color blue to draw them now

let's focus on sympathetic division first so sympathetic neurons arise from

thoracic and lumbar regions of the spinal cord and first we have

preganglionic neurons which originate directly from the spinal cord they are

short and they release acetylcholine so they are cholinergic and they synapse

with postganglionic neurons which are long and release norepinephrine so they

are the adrenergic neurons and for simplicity and to conserve some space

I'm going to skip drawing sympathetic ganglion but just keep in mind that

there is this chain of synaptic connections of sympathetic neurons that

lies alongside the spinal cord so now let's talk about how sympathetic neurons

affect each part or organ of our bodies

so sympathetic nervous system is responsible for fight-or-flight response

and the activation of this system leads to the following dilation of pupils

inhibition of salivation relaxation of airways acceleration of the heartbeat

inhibition of digestion stimulation of glucose release from the liver

inhibition of gall bladder inhibition of intestines activity stimulation of

adrenal medulla which results in secretion of epinephrine and

norepinephrine relaxation of urinary bladder and stimulation of ejaculation

or vaginal contraction and just to clarify the adrenal medulla is the only

autonomic neuroeffector organ which is

innervated by one long sympathetic cholinergic neuron on the other side we

have parasympathetic nervous system which is responsible for rest-and-digest

response parasympathetic neurons arise from cranial nerves 3 7 9 and 10 and the

sacral region s2 to s4 in contrast to sympathetic division parasympathetic

preganglionic neurons are long and postganglionic neurons are short and

they actually both release neurotransmitter acetylcholine so the activity of

parasympathetic division would have the opposite effect of sympathetic

stimulation and would result in the following

constriction of pupil stimulation of salivation constriction of airways

slowing of the heartbeat stimulation of digestion stimulation of glucose uptake

stimulation of intestines activity stimulation of urinary bladder

contraction and stimulation of erection now I want to briefly talk about

receptors involved in this whole system so let's zoom in on this synapse from

sympathetic system and also let's zoom in on this synapse right here from

parasympathetic system so the receptors on postganglionic neurons within

sympathetic and parasympathetic system are all nicotinic receptors moreover the

receptors in skeletal muscle are also nicotinic so nicotinic receptors are

also known as ionotropic receptors and this is because they simply work as an

ion channel so for example when neurotransmitter such as acetylcholine

binds to such channel it causes it to open up allowing passage of ions such as

sodium which eventually leads to an action potential

on the other hand all the receptors on the effector organs that are receiving

signal from sympathetic neurons are called adrenergic receptors and the ones

that are receiving signal from parasympathetic neurons are called

muscarinic receptors so for example if we zoom in at this area right here where

the very ends of these postganglionic neurons meet the smooth muscle cell of

the eye the sympathetic adrenergic neuron would stimulate

adrenergic receptors leading to pupil dilation and the parasympathetic

cholinergic neuron would stimulate muscarinic receptors leading to pupil

constriction now adrenergic and muscarinic receptors are of type known

as metabotropic receptors and unlike the ionotropic receptors the metabotropic

ones are all linked to G protein that acts via intracellular second messengers

and this is discussed in more details in pharmacodynamics video so please check

it out finally to put all of this in perspective remember that most organs

are innervated by both sympathetic and parasympathetic neurons however usually

only one system predominates in controlling any given organ overall

autonomic nervous system is very dynamic constantly shifting from the dominance

of one system to another depending on the situation and with that I wanted to

thank you for watching and I hope you enjoyed this video