in this video I want to talk about nervous system so our nervous system is
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