Decathlon Science (Neuro) Section I

study of the nervous system

what is neuroscience?

molecular bio, cell bio, physiology, chem, physics, psychology, cognitive science, comp sci, and math.

What other sciences are in neuroscience?

neuron

• nerve cell

• in almost all animals

• most important

• can’t be made easily

• however connections can change

• many subtypes

nucleus

• DNA is stored

• found in cell body/soma

• contains cytosol

• separated by neuronal membrane

dendrite

• brings input to the cell

• comes from greek word for tree like

• can have many branches

• like an antenna

• size and shape of spines = ability to receive input there

• can have multiple

axon

• typically have one only

• serves as wire to transmit output from a neuron

• uniform thickness and can be several feet long

• no organelles inside it

• can be covered in myelin to help send signals

• begin at soma

  • at a part of the axon called the axon hillock

• end at slightly larger sections called axon terminals

• structure is made of microtubules

myelin

• fatty substance from the glial cells

• insulates axons

• like insulation on a wire

• looks white even without stains

• fatty

Nodes of Ranvier

• small gaps in the Myelin coat of an axon that allow the axon to take in fluids

Axon Hillock

part of the axon that’s connected to the soma

Axon terminal

• slightly wider end of axon

• where the neuron will release the neurotransmitter

  • also through electric impulse

  • usually to another neuron’s dendrite spine

Synapse

• point of communication between neurons

• not connected

synaptic cleft

gap between two neurons at a synapse

presynaptic neuron

• the transmitting neuron

• the axon terminal is usually filled with spherical packets (vesicles)

  • neurotransmitters

postsynaptic neuron

the receiving neuron

neurotransmitters

• produced in soma

• sent down axon to terminal

• bind to receptors that receive the the signal on postsynaptic neuron

• packaged into vesicles

morphollogy

shape of a neuron

dendritic arbor

• size and shape of the branching dendrites

• related to the region of the body that the neuron receives from and how it can send signals to other neurons

examples of how structure = important to function

• motor neurons

  • soma at one end with a small set of dendrites and a big axon to connect to muscle

• sensory neurons

  • some have dendrites at both ends connected by a long axon with he cell body somewhere in the middle

• interneurons

  • often quite small without long axons or dendrites

interneuron

work to connect other neurons within the nervous system

grey matter and white matter

what are the two types of tissue in the nervous system?

grey matter

• made of cell bodies and dendritic arbors of neurons

• mostly on the outside of the brain

• inside for spinal cord

white matter

• made of myelinated axons of neurons

• tucked inside brain

• outside on spinal chord

Glia/Glial cells

• non neuronal cells found in the nervous system

• variety including

  • strocytes,

  • oligodendrocytes,

  • Schwann cells

  • microglia

• produced throughout the life of an organism

• once thought to be passive

Schwann Cells

• form the myelin sheath around axons in the PERIPHERAL nervous system

Oligodendrocytes

• form the myelin sheath around axons in the CENTRAL nervous system

Astrocytes

• most common

• star-like appearance

• regulate chemical makeup of extracellular fluid between cells in the nervous system

  • remove excess signaling molecules and maintain proper balance of ions

• also react to tissue damage

• send out long extensions that wrap around blood vessels in brain to create blood brain barrier

• once thought to be passive

• can alter signals that are sent and received

• helps keep neurons nourished

blood brain barrier

• keeps stuff in the blood out of the brain

• keeps the brain from getting infections from damaged neurons

• makes it harder for nutrients to get into the brain

• also hard to get drugs in there to help stop disease

Microglia

• smaller than other glial cells

• immune cells of the brain

  • clear away pathogens and damaged/dead neurons

  • it does so by eating them

  • similar to macrophages in our normal immune system

• hyperactive ones can actually end up damaging the brain

  • can play a role in neurodegenerative diseases eg. Alzheimer’s

Central nervous system

• two parts encased in bone

  • brain

  • spinal cord

• nervous system doesn’t have contact with bones

  • three layers of meninges surround the system to protect it

dura mater, arachnoid membrane, and pia mater

What are the three layers of meninges protecting the central nervous system called?

dura mater

• latin for tough mother

• outer most layer

  • thick and leathery

  • spall space between it and skull

    • filled with fats to absorb shock

    • called epidural space

      • useful for injections

arachnoid membrane

• long stringy components that look like spider webs.

• usually very little subdural space (space between arachnoid and dura mater)

  • brain trauma can put blood there

• subarachnoid space

• space below arachnoid membrane

  • filled with cerebrospinal fluid

  • cushions/protects brain allowing it to float

• allows blood vessels room to go to brain

  • allows it to get oxygen and nutrients

pia mater

• latin for gentle mother

• very thin

• fits Brian very closely

• keeps cerebrospinal fluid out of brain but allowed blood to pass through

Peripheral Nervous system

• consists of nerves/axons and some ganglia that are apart from the central nervous syste

• basically the relay between central nervous system and the outside world

• more easily damaged (bc no bones) and reparable

• no BBB means easier to get infected

• has different sections

Ganglia

clusters of cell bodies

Somatic and Autonomic

What are some parts of the peripheral nervous system?

Somatic Nervous system

• composed of all axons leaving/entering spinal cord that brings info to and from the tissues of the body

• includes the axons above the spinal chord that brings motor commands or sensory input of the head and neck

• voluntary control

Autonomic nervous system

• involuntary responses

• regulates functions of internal organs, smooth muscle (heart) and glands

• name comes from greek word Autonomia

• two divisions

Sympathetic and parasympathetic

What are the two divisions of the automatic nervous system?

Sympathetic nervous system

• fight or flight

  • comes from adrenal glands from stress

• circuits of this division start with a neuron in CNS (usually in thoracic or lumbar parts of SC)

  • this neuron synapses to a neuron on the sympathetic chain

  • axon from this new neuron goes to the desired organ

  • chain design = good for general message to general audience

• MEANT FOR SHORT TERM ONLY

  • long term use = chronic stress and damage

sympathetic chain

specialized chain of ganglia next to the spine

heart (increase BP), lungs (increase breath rate), digestive system (Bile production) secretory glands (sweat, tear, and saliva production)

What are the targets of the sympathetic nervous system?

Parasympathetic nervous system

• rest and digest

• connected to Brain stem and Sacral Spinal chord

• ganglia involved are scattered everywhere

  • usually close to the organ that the second neuron in the sequence is targetting

• this organization isn’t good for speedy general messages

  • its good for super specialized instruction

• the role of this is balance

things that activate the parasympathetic nervous system

• digestion

• cell growth/devision

• immune responses

• energy storage

• homeostasis

same as sympathetic but opposite affects.

What are the targets of the parasympathetic nervous system?

membrane potential

way of describing the electrical charge a cell has in comparison to the charge outside (measured in millivolts mV)

resting and action

what are the two main potentials?

cytosol, extracellular fluid and neuronal membrane (separates the two)

What are the three things that dictate the potentials?

Resting membrane potential

• electric charge inside the neuron compared to outside when neuron is at rest

• similar to the eq potential for K+ because of the leak channels

• usually negative

• about -60mV or -80mV

neuronal membrane

• allows the electric charge of inside of the cell to be different than outside.

• wall preventing/inhibiting changes in [ion]

  • goes through channels

• phospholipid bilayer

• passive transport

  • natural force (no req energy)

  • done by [] and electric potential.

• Active transport

  • requires energy

  • pumps

How do ions cross the membrane

ion channel

• proteins that form specialized openings for ions

• usually specific for ions

  • eg sodium channel

• can also be gated

  • must be triggered open

  • some are triggered by changes in environment

voltage-gated

If the channel opens when the membrane potential is at a particular voltage it is?

• this type also usually opens when voltage changes from rest

Ligand gated

• open when a particular molecule binds to a specific location

  • this changes the conformation and opens the channel

• intensity of flow comes from natural forces

NA/K pump

• crucial in neurons for resting potential.

• breaks down ATP and changes its conformation

• takes two K (from outside) in phos state and 3 Na in dephos state

• exchanges Na for K.

• increases [] gradient

• always in background

• helps restore to normal concentration gradient after the falling phase

Potassium leak channels

• open and ungated

• allow positive potassium to just leave

• causes negative rest potential

Electrical Driving Force

when the electric potential outside the cell becomes strong enough so that the K+ are repelled and sent back into the cell through the leak channels, what driving force is exhibited?

• size of this force is proportional to the electrical difference inside and outside the cell

Chemical driving force

the result of the [] gradient. It causes the initial diffusion. size of the force is proportional to the []

when Chem and Electric Driving force are equal

When is the cell at eq?

eq potential

the membrane potential at which a particular ion reaches eq. (each ion will have a different one)

Nernst Equation

• gives the value of eq potential for an ion.

• takes in the following factors:

  • temp of cell

  • ratio of external internal [] in ion

Goldman Equation

• Starts from Nernst Equation but factors in the permeability for multiple ions (like Na, K, and Cl)

Active membrane potential

• the electric potential across the membrane when the cell is active and firing

• brief drastic change usually only for a few milliseconds

• AKA spikes, nerve impulses or APs

rising phase, peak/overshoot, falling phase

What are the phases of Action potential?

rising phase

• first phase of the action potential.

• membrane experiences depolarization

depolarization

when the membrane potential becomes more and more positive

peak/overshoot

• when the charge inside the cell becomes more positive than the outside of the cell

• usually goes up to +30mV or +40 mV

Falling phase

• final phase before reset

• becomes more negative due to loss of + ions

  • this is known as repolarization (returning to polarized)

  • or hyper polarization (becoming more polarized)

• However it continues and the membrane becomes extra negative stopping at around -80mV

  • this is called undershoot/afterhyperpolarization

• light or darkness in the retina

• inner ear neurons respond to sound waves

• neurons in skin respond to heat, cold, pressure, pain

Examples of triggers for action potential in neurons

raise the charge of the membrane potential to smt more positive, usually between -65mV and -50mV

What do the stimuli have in common?

All or none principle

• the idea of either firing or not

• cell either reaches threshold and fires or doesn’t

• reaches the requirements to open the sodium gates

• this allows for a rush of Na because of the high [] gradient and high electrical driving force

  • this rapidly depolarizes the cell

(those channels get blocked again once the cell reaches it’s peak phase)

Why is this threshold so important?

Absolute refractory period

• the one millisecond that the sodium gates are inactivated after an action potential

• another action potential can’t happen at this time

• is also the reason that signals only go one way

Voltage gated K gates

• open when the potential = approx 0mV

• slow to open and close

  • starts opening at peak

• use the large [] gradient of k to push it out of the cell

• also electric potential is present

• causes hyper polarization

• also causes afterhyperpolarization bc it is slow to close

effluxes

flows out of the cell

Relative refractory period

• the time period where more the potential is more negative due to the afterhyperpolarization

• more stimulus needed to reach threshold at this point

propagates

to be carried down the axon

when sodium enters the cell, it dissuades through the axon which triggers the next set of channels which leads to a cycle of triggering throughout the axon

Why does’t the signal degrade or decrease over the length of the axon?

by insulating the axon. However, the signal also degrades a little because it can only be regenerated at each Node of Ranvier.

How does the Myelin sheath speed up action potential?

Saltatory Conduction

• conduction down a myelinated neuron

• comes from the word saltare meaning jump

  • bc message jumps from node to node

synaptic transmission

the process in which two neruons communicate

presynaptic cell axon terminal

• contains mitochondria to help produce ATP to power transmission

• the microtubule of the axon cease at the terminals

electrical and chemical

What are the two main types of synapses?

electrical synapse

• the two communicating cells are so close that a single set of channels (gap junctions) connect the membranes

• because of this, the positive ions of the Action Potential go straight into the next cell

• super fast but no changes

  • changing the signal is how the Brain encodes info

  • therefore this synapse is a bit limiting

Chemical Synapse

• much more complex of the two synapses

• the two cells have two vastly more complicated jobs

Chemical synapse: presynaptic cell

• has to translate electric signal to chem signal

• axon terminal

  • crucial voltage gates = calcium

  • calcium floods in

• vesicle

  • once vesicle with transmitter reaches axon terminal it docks facing the synaptic cleft (active zone) bc of SNARE proteins

  • this is done before the action potential arrives for quick transmission

• Then exocytosis happens

SNARE proteins

special proteins that are membrane bound ad forma. complex that locks vesicles into place

Exocytosis

• in fluxing Ca binds to a SNARE protein called synaptotagmin.

  • this one changes the SNARE complex and brings the vesicle outward so the two membranes can fuse together

  • creates a pore

  • allows neurotransmitter to diffuse across synaptic cleft towards the

ionotropic/ligand receptors

• work very quickly to change the postsynaptic membrane potential through neurotransmitters

• uses endocytosis and stuff

• use ion channels

Metabotropic receptors

• AKA GPCR

• much slower and complex responses

• activated in steps

  • binding of neurotransmitter

  • activates G proteins

• although it may seem similar to the other one, the main difference is that it can activate many G proteins leading to a wider effect

• some do more than just activating channel

  • they can make use of proteins “downstream” of the G protein and alter the cells metabolism

effector

When the G protein is activated, it splits apart into subunits which each go on to activate its own set of signal/ _________ proteins

effector proteins

• many different kinds

  • for example: in some cells, G proteins bind to a special ligand channel. These are special because the gate is towards the cytosol and it’s keyed to only the G protein. In this case the ion channel IS the _________

  • also ion channels that are activated by G subunits

• one pathway involves activation of PKC (Protein kinase C

  • responsible for many important molecules by phosphorylating them

• The G protein binds to the effector protein phospholipase C (PLC)

  • then splits into two molecules

  • inositol-1,4,5-triphosphate (IP₃)

  • diacylglycerol (DAG)

  • this is the branching point bv each molecule acts as a second messenger

• IP binds to some ligand gated calcium channels in the ER,

  • allows internal Ca into the cytosol which binds to signal proteins which have effects

• DAG stays on the membrane and activates PKC which phosphorylates a lot of target proteins

  == signal cascades

How do metaprobic receptors change metabolism?

phosphorylating

to add a phosphate too

second messenger

when the molecules is produced by the effector protein and diffuses away in the cytosol to impact other parts of the cell it is a _________.

signal cascades

take longer than regular inotropic paths but has a stronger signal

Prozac and Zoloft

• Which drugs inhibit reuptake transporter found in synapses that release serotonin.

• AKA Selective reuptake inhibitors (SSRIs)

• they allow serotonin to have more time to get across the cleft

agonist, or antagonist

What are the two usual ways that drugs impact receptors?

receptor agonist

• drug type that helps enhance the effect of the transmitter

• usually mimic the action of the endogenous neurotransmitter the receptor binds

• competitive agonists compete against the endogenous NT

  • must be much more [] than the endogenous NT

• noncompetetive agonists make the system more efficient and don’t compete

• partial agonists compete for the same spot but don’t have as much of an effect

• drugs that increase NT synthesis