Comprehensive Overview of the Nervous System: Central and Peripheral Parts

the topic of this lecture will be parts of the nervous system so the nervous system consists of two

general Parts the Central and peripheral nervous systems so the central nervous system we can see over here consists of

the brain and the spinal cord the peripheral nervous system is going to consist of essentially everything else

which we'll we'll spend some time talking about the central nervous system brain and spinal cord that we'll talk

about the peripheral nervous system in turn so when we think about the organization

of the nervous system again we've got the nervous system it's kind of this overarching structure we then have the

central nervous system the brain and the spinal cord which we'll spend some time talking about in just a moment we then

have the peripheral nervous system which can then be further broken down into the autonomic and somatic systems the

autonomic system controls self-regulated actions of internal organs and glands so for example the sympathetic system the

arousing some in the system the parasympic parasympathetic system which is our calming system and then we'll

also mention the somatic system which is going to be responsible is going to be connected with things like our sensory

input as well as motor output like controlling skeletal movements so let's start with the central nervous

system so the central nervous system again consists of the brain and the spinal cord so let's start with the

brain okay the brain is comprised of billions of interconnected neurons and glia which we've talked about previously

the brain is bilateral meaning it has two sides two hemispheres the brain can further be separated into distinct lobes

there'll be four lobes that we'll talk about the frontal lobe parietal lobe occipital lobe and temporal lobe and

although these sections of the brain can be separated into lobes all areas are still interacting with all other areas

so if we take a quick glance up here we can see the frontal lobe which is going to be the part of our brain directly

behind our forehead the parietal lobe up here occipital lobe the part of the brain back here at the back of the head

and then the temporal lobe which is here behind the temple the spinal cord is going to be the other

part of the central nervous system what it's doing is it's providing information from the brain to the rest of the body

and also providing information from the rest of the body to the brain so it's going to be kind of like an information

Highway if you will between the brain and the rest of the body the spinal cord has its own system of

reflexes which we'll talk about as we move through the content in this chapter but essentially for example as pain

information is coming in so if you touch something that's really hot as that information is Flowing along the

neurons down the length of your arm and eventually hitting the spinal cord what happens is that there's a reflex like

action so as that information is continuing to travel up to the brain where it can be processed so your brain

can realize that you're experiencing pain when that pain signal hits the spinal cord it also starts to send a

reflex back to the finger that's touching something hot to jerk away from whatever that hot stimulus is right over

that painful stimulus when we're talking about these sorts of reflexes that's what we're talking about

and you've experienced this before if you've ever done anything like you you've touched something hot or cut your

finger on something sharp you'll recognize that you were already moving away from whatever that painful stimulus

was by the time you realize what was going on like for example I remember touching something hot once while

working on my car and my hand had already jerked back before I had even really processed that my hand hurt right

and that that's not uncommon because of the reflex-like behaviors that are stimulated by the spinal cord

the top of the spinal cord merges with the brain stem and the bottom ends uh bottom end of the spinal cord is just

below our ribs functionally the spinal cord is uh separated in 30 segments each of those

connected to a specific part of the body through the peripheral nervous system our sensory nerves bring messages in and

up to the brain the motor nerves send messages out to the muscles and the organs

in moments of survival automatic reflexes allow motor commands to be initiated without sending signals from

sensory nerves to the brain first which allows for very quick reactions as we were just talking about if we're

experiencing something really painful for example our our body will respond very quickly to prevent further damage

from taking place now let's shift to talking about the peripheral nervous system

so the peripheral nervous system is made up of two different parts the somatic nervous system which relays

sensory and motor information to and from the central nervous system the other part is going to be the

autonomic nervous system this is going to be responsible for controlling our internal organs and glands and can be

divided into the sympathetic and parasympathetic nervous systems the sympathetic nervous system is

involved in stress-related activities and functions basically this gets us ready for fight or flight I'm sure all

of you or at least most of you have heard of the flight or flight or fight response fight or flight response may

get that correct um what this has to do with it's the activation of the sympathetic division

it basically kind of ramps us up for activities what it allows us to do is to access our bodies energy reserves and it

increases our sensory capacity so that we can fight off a given threat or run away to safety right the reason why it's

called fight or fight is it either basically is preparing us to fight off some sort of a Potential Threat or run

away from that Potential Threat the parasympathetic nervous system is going to act as a complement to the

sympathetic system right so the sympathetic system kind of Gears us up for Action the parasympathetic system

basically uh helps calm us back down so the parasympathetic system is associated with routine day-to-day

operations of the body under relaxed conditions this is going to be the sort of rest and restore response basically

it's going to relax our body after some sort of fight-or-flight episode this parasympathetic response is also

sometimes referred to as the rest and digest response the reason for this is that when we're in fight or flight right

we're certainly not very restful and one of the things that happens during fight or flight is our digestion stops right

basically during fight or flight all of our bodies resources are targeted toward those uh toward those systems that are

necessary for our our survival in the moment so things like digestion are basically halted so that we can redirect

that energy to other parts of the body that it needs it more in the moment so then the parasympathetic system is

helping us relax and calm down and get back to things like digesting food the sympathetic and parasympathetic

systems uh complement each other as we were just talking about in order to maintain homeostasis basically it's kind

of state of equilibrium within the body part of why this is so important is the sympathetic nervous system we can't

exist for a prolonged or extended periods in fight or flight right there are that sort of uh that sort of level

of excitation just does not go well for us if it is if it exists over a long period of time so we need the

parasympathetic system to kind of kick in and kind of reduce our arousal level right so that's why there's there's so

important these systems complement each other here's an illustration of the way that

the sympathetic and parasympathetic divisions of our autonomic autonomic nervous system

are having opposite effects on various systems of the body so for example our pupils are going to be impacted by both

the sympathetic and parasympathetic systems right the sympathetic system is mobilizing resources right for fight or

flight so if something happens that startles this or frightens us are pupils dilate basically the black area of our

eye our pupils get larger allowing more light in so we can see a bit better for a moment

so we'll just go we'll go through the the sympathetic uh responses then we'll talk about the parasympathetic uh

complementary responses in a moment so during fight or flight our pupils dilate uh salivation is inhibited basically our

mouths feel dry if you've ever been driving along in your car and had near-miss automobile

accident right if you're really frightened and startled uh by the the near-miss accident right you may

recognize afterward that your mouth is really dry the reason is because salivation has been inhibited basically

you stop salivating because salivation isn't terribly important when your life is on the line uh your bronchial

passages uh basically dilate respiration increases heart rate increases all of these things are basically to increase

the flow of oxygenated blood right throughout the body basically start breathing heavier your heart rate speeds

up digestion is inhibited again we're trying to redirect our bodily resources toward those systems that are necessary

for our survival digestion is a lovely process but if it's if it's uh if it's a matter of survival in the moment your

your stomach can wait to digest the what you ate for breakfast right so digestion is inhibited it and basically those

bodily resources are directed elsewhere there's secretion of the adrenal hormones which we'll talk about later

when we get to the endocrine system we see increased secretion by the sweat glands so we start sweating more hair

follicles Rays so for example you can't see my arm terribly well but I have a lot of hair on my arm if I were really

frightened or angry what would happen is that the hair follicles would raise causing the hair on my arm to stand up a

little bit with goosebumps for example now in humans we we don't have compared to many other species we don't have a

great deal of bodily hair and so for example if if I were really frightened it wouldn't it wouldn't really change

much about my appearance if the hair on my arm stood up a little bit it just wouldn't be terribly noticeable

however in some other species like for example I have my wife and I have a bunch of cats if they're frightened uh

what happens is their hair will stand up and it can actually increase it makes them look larger right which is a very

nice survival response because it may make Predators be a little leerier about attacking them if they look a little bit

bigger in other species like for example and uh other non-human primates like

chimpanzees when they get angry they have enough bodily here that when it stands up it increases it makes them

look larger than they actually are right so that's part of where that comes from in humans it just doesn't it doesn't do

terribly much when our bodily hair stands on end also our bladder May relax causing us to

urinate ourselves in some cases so if you've ever seen someone who is frightened so badly that they urinated

themselves that this is the reason it's because it's because of the sympathetic division of the autonomic nervous system

the parasympathetic division here is going to be conserving energy so it's going to complement and basically try to

undo the things that were done by the sympathetic system following fight or flight right so your pupils constrict so

again if we uh let's go with the idea of if you were sitting in your apartment this evening

and suddenly a tiger burst into your apartment I don't know why there's a tiger loose in your apartment building

but let's go with that example your pupils would dilate salivation would be inhibited your heart would begin to

raise your you start breathing more heavily you begin uh sweating your hair follicles would would stand on in making

your arm hair go up a little bit after the tiger had left right and enough time had gone by that you really thought you

were safe so maybe you walked the front door again after you undid whatever damage the tiger caused by bursting into

your apartment to begin with your pupils may eventually start to constrict salivation may start again

your bronchial passages would start to constrict respiration rate would eventually start to come back down heart

rate would eventually start to come back down digestion would eventually start again and you would regain control over

your bladder right after after the tiger episode okay so hopefully this illustrates how

the autonomic nervous system is working but the sympathetic and the parasympathetic divisions