Notes – Big idea 3 Ch. 48 – Nervous system

Essential knowledge 3.E.2*the types of nervous systems, development of the human

nervous system, details of the various structures and features of the brain parts, and details of specific neurologic processes

are beyond the scope of the course of the AP Exam

Nervous systems consist of circuits of neurons and supporting cells

• All animals except sponges have a nervous system

• What distinguishes nervous systems of different animal groups is how neurons are organized into circuits

Nervous systems in animals• The simplest animals with

nervous systems, the cnidarians (jelly fish, sea anemone), have neurons arranged in nerve nets

• Sea stars have a nerve net in each arm connected by radial nerves to a central nerve ring

• Relatively simple cephalized (nervous tissue is concentrated on one end) animals, such as flatworms, have a central nervous system (CNS)

Animal nervous systems cont.

• Annelids (ringed worms) and arthropods have segmentally arranged clusters of neurons called ganglia

• These ganglia connect to the CNS and make up a peripheral nervous system (PNS)

Animal nervous systems cont.

• In vertebrates, the central nervous system consists of a brain and dorsal spinal cord

• The PNS connects to the CNS

Overview: Command and Control Center

• The human brain contains about 100 billion nerve cells, or neurons

• Each neuron may communicate with thousands of other neurons

• Brain imaging and other methods reveal that groups of neurons function in specialized circuits dedicated to different tasks

Optical illusions

• When you look at something, whatever your right eye sees goes to the left side of your brain

• Whatever your left eye sees, it goes to the right side of your brain

• http://michaelbach.de/ot/

Overview

• The brain – two different hemispheres (left and right (each controls different things)

• Corpus callosum connects the two sides of the brain and allows things to be shared back and forth

• If the brain goes haywire (electrical storm), you can get seizures– Will sever the corpus callosum to control them– Radical procedure

Neuron – the basic unit of the nervous system

• Two main parts:– Dendrites – branched extensions that

receive signals from other neurons– Axon – transmit signals to a synapse

• Other parts:– Synapse – space between neurons– Nucleus (it’s a regular cell, so it will

have– Soma (cell body) – where most of the

organelles are held– Myelin – fat material the axon is

wrapped in and acts as insulation, and speeds up message transmision

– Schwann cells – wrap their way around the axon and form the myelin sheath

– Nodes of Ranvier – gaps between the myelin

• A nerve is a bunch of neurons grouped together

• *action potential – the message sent down the neuron

Cells have voltage!• Cells live in a sea of charged

ions• Opposite charges on

opposite sides of cell membrane

• Anions (negative ions)– More concentrated within the

cell• Cations (positive ions)

– More concentrated in the extracellular fluid

*Stored energy (like a battery)

Neurons cont.• Think of a neuron as a “salty banana”• Salt is high in sodium (Na+)• Banana is high in potassium (K)• If you look at the axon portion of the

neuron, Na+ is on the outside, and K on the inside

• Channel proteins allow these ions to pass in and out (if they aren’t open they can’t pass)

• Every Na and K ion has a + charge• If there are more Na+ on the outside,

there will be more of a positive charge on the outside

• We can measure that (a typical neural cell has a voltage of -70 millivolts (mV) )

Neurons cont.• A stimulus will trigger the opening of the first sodium

channel, so the sodium will diffuse along it’s concentration gradient (which means it will enter through a protein channel from high to low concentration)

• This will change the voltage (from -70 to maybe -50 mV)

• If it reaches the point of -55mV (which is the same in all animals), that’s called the membrane threshold, and it’s going to have an action potential

• The channels are activated by changes in the voltage, so it will open up the next sodium channel down the way

• Potassium channels aren’t effected by this, so we get a domino cascade of sodium opening channels

• Now our charges a switched, which closes the sodium channels and OPENS our POTASSIUM channels

• Potassium will flow out when the charge inside is more positive than the charge outside

• The charge will become more negative and it will go back to an equal charge on either side

Neurons cont.

• Creates a plunging/falling phase (up and down with charges)

• Undershoot – resets itself with the sodium potassium pump

Neurons cont.• After firing a neuron has to

re-set itself• Now we have to reestablish

the gradient by using the Na+/K pump

• Na+ needs to move back out

• K+ needs to move back in• both are moving against

concentration gradients• need a pump!!

Information Processing

• Nervous systems process information in three stages: sensory input, integration, and motor output

LE 48-3

Sensor

Sensory input

Motor output

Integration

EffectorPeripheral nervous

system (PNS)Central nervous

system (CNS)

messages• Action potentials go to your brain, which then needs

to decide where it’s coming from and what it is• In order to get to your brain, the action potential

goes through many gaps (synapse is a gap between two different neurons)

• The action potential moves down the axon (through the sodium potassium pump) until it reaches the synapse

• It can’t just flow across the gap • You get an influx of calcium and neurotransmitters

as the action potential gets to the end of the synapse

• The calcium releases the neurotransmitters which diffuse across the gap (high to low concentration)

• they match up with another ion channel on the other side, and change its shape so it can take ions in

• Now we get sodium flowing in, potassium flowing out, and the action potential move across the synapse

Messages cont.• GABA – a chemical that

forms neurotransmittors• A negative neurotransmittor • Acts as an inhibitor• If GABA flows across the

synapse, it hits receptors on the other side that say “don’t send an action potential”

• Excitatory – tells action potentials to move across

More Neurotransmitters• Acetylcholine

– transmit signal to skeletal muscle• Epinephrine (adrenaline) &

norepinephrine– fight-or-flight response

• Dopamine– widespread in brain– affects sleep, mood, attention &

learning– lack of dopamine in brain associated

with Parkinson’s disease– excessive dopamine linked to

schizophrenia• Serotonin

– widespread in brain– affects sleep, mood, attention &

learning

Neurotransmitters cont.• Weak point of nervous system• any substance that affects

neurotransmitters or mimics them affects nerve function– gases: nitric oxide, carbon

monoxide– mood altering drugs:

• Stimulants– amphetamines, caffeine,

nicotine• Depressants• hallucinogenic drugs• Prozac

– poisons

Neurotransmitters cont.

• Acetylcholinesterase– Enzyme which breaks

down neurotransmitter acetylcholine

• inhibitors = neurotoxins• snake venom, sarin,

insecticides

active sitein red

neurotoxinin green

snake toxin blockingacetylcholinesterase active site

Inhibitory vs. excitatory• Think of these messages like a vote• If one neuron is headed to the brain, it will get a

number of different messages from many neurons (so the connections are very important)

• Some messages are inhibitory (telling it not to fire), and some will be excitatory (telling the connections to fire)

• Depending on the amount of exitatory and inhibitory we have, it’s either going to send the message or not

• It sits at threshold, and an action potential is going to depolarize, undershoot, and repolarize again and again

• Every time we get an inhibitory, that’s going to be pushing the voltage down

• Every time we get an excitatory, it’s going to be pushing it up until we hit the theshold of -55

• So all the neurons are voting – should we or shouldn’t we fire the message?

Transmission of a signal

• Start the signal– Channels are set up

• Propagate the signal – Once 1st is opened, the rest open in succession

• Re-set the system– Re-set the channels so neuron can react again

Importance of neurons

• Nerves are important in sending messages• The connections in your brain (excitatory and

inhibitory messages) help form memories• The memories are the connections

Simplest Nerve Circuit• Reflex, or automatic

response– Rapid response– automated– signal only goes to spinal cord

• Adaptive value– essential actions– don’t need to think or make

decisions about• blinking• Balance• pupil dilation• startle

Human Brain• All vertebrate brains develop

from three embryonic regions: forebrain, midbrain, and hindbrain

• Brainstem– The “lower brain” or hindbrain– medulla oblongata– pons– midbrain

• Functions– homeostasis– coordination of movement– conduction of impulses to higher

brain centers

LE 48-23

Embryonic brain regions

Forebrain

Midbrain

Telencephalon

Diencephalon

Mesencephalon

Hindbrain

Metencephalon

Myelencephalon

Diencephalon

MesencephalonMetencephalon

MyelencephalonMidbrain

Hindbrain

ForebrainTelencephalon

Spinal cord

Spinal cord

Embryo at one month Embryo at five weeks

Cerebral hemisphere

Medulla oblongata (part of brainstem)

Pituitarygland

Pineal gland(part of epithalamus)

Diencephalon:HypothalamusThalamus

Brainstem:MidbrainPonsMedullaoblongata

CerebellumCentral canal

Adult

Pons (part of brainstem), cerebellum

Midbrain (part of brainstem)

Diencephalon (thalamus, hypothalamus, epithalamus)

Cerebrum (cerebral hemispheres; includes cerebralcortex, white matter, basal nuclei)

Brain structures present in adult

Medulla oblongata & Pons• Controls autonomic

homeostatic functions– breathing– heart & blood vessel

activity– swallowing– vomiting– digestion

• Relays information to & from higher brain centers

Midbrain

• Involved in the integration of sensory information– regulation of visual

reflexes– regulation of auditory

reflexes

• As a human brain develops, the most profound change occurs in the forebrain, which gives rise to the cerebrum (not fully developed until adulthood)

Cerebrum• forebrain• Most highly evolved

structure of mammalian brain

• Cerebrum divided hemispheres– left = right side of body– right = left side of body

• Corpus callosum– major connection between

2 hemispheres

Lateralization of Cortical Function• During brain development, competing functions

segregate and displace each other in the cortex of the left and right cerebral hemispheres

• This process results in lateralization of functions• The left hemisphere is more adept at language,

math, logic, and processing of serial sequences• The right hemisphere is stronger at pattern

recognition, nonverbal thinking, and emotional processing

Lateralization of Brain Function• Left hemisphere

– language, math, logic operations, processing of serial sequences of information, visual & auditory details

– detailed activities required for motor control

• Right hemisphere– pattern recognition, spatial

relationships, non-verbal ideation (forming ideas), emotional processing, facial recognition

Cerebrum specialization

• Regions of the cerebrum are specialized for different functions– Lobes– frontal– temporal– occipital– parietal

Limbic system• The limbic system is a ring of

structures around the brainstem• These structures mediate primary

emotions and attach emotional “feelings” to survival-related functions– Mediates basic emotions (fear,

anger), involved in emotional bonding, establishes emotional memory

– Structures of the limbic system form in early development and provide a foundation for emotional memory, associating emotions with particular events or experiences

Neural Stem Cells• The adult human brain

contains stem cells that can differentiate into mature neurons

• Induction of stem cell differentiation and transplantation of cultured stem cells are potential methods for replacing neurons lost to trauma or disease

Alzheimer’s Disease• Alzheimer’s disease (AD) is a

mental deterioration characterized by confusion, memory loss, and other symptoms

• AD is caused by the formation of neurofibrillary tangles and senile plaques in the brain

• A successful treatment in humans may hinge on early detection of senile plaques

Senile plaque