Chapter 11 slides lecture 2 narrated

Neurons (Nerve cells)

• primary cell in the nervous system

• amitotic- few exceptions

• live a long time (100 years or more)

• carry out the most basic functions of the nervous system
  - receive information through receptors
  - transmit and integrate that information
  - decide on an action plan then an output

• highly oxygen dependent

• high metabolic rate

• can not use fats only glucose
  - without enough glucose, you will feel fatigue and lethargic

Cell Body (Perikaryon or Soma)

• where the nucleus is

• rough er (Nissl body)

• well developed golgi apparatus

• vesicles that store NTM
  - the vesicles migrate down the axon to the axon terminals
  - requires energy
  - mitochondria supply the energy for this process

• can receive information

Cytoskeleton

• give the neuron shape and if it gets disoreinted it puts it back into the original shape

• neurofibrils and neurofilaments

Axon Hillock

• structure unique to neurons

• cone shaped structure on one end of the soma that connects the soma to the axon
  - axon hillock can be viewed as the end of the cell body or the beginning of the axon

• action potential originates in neuron here

Clusters of cell bodies are called:

• nuclei in the CNS

• ganglia in the PNS

Processes

• extensions from the cell body

• two types:
  1. dendrites
  - collection of axons in the PNS is a nerve
  2. axons
  - collections of axons in CNS are called tracts

Dendrites

• where neuron receives incoming information
  - receptive region of a neuron

• relatively short, they taper and branch to increase surface area
  - this large surface area allows them to communicate with thousands of different neurons

• when dendrites receive a stimulus from another neuron, it is referred to as a graded potential
  - that graded potential is carried towards the cell body and the axon hillock

The Axon/ Axon Terminals

• extension off soma

• axon is a region of plasma membrane

• axons transmit action potentials
  - action potentials are generated in the axon hillock and transmitted down the length of the axon
  - axons carry information away from the soma

• usually one axon per axon hillock and only one axon hillock per neuron

• terminal branches are at end of axon

• at the end of each terminal branch is the axon terminals
  - axon terminals are the secretory region
  - axon terminals are knob structures, referred to as buttons (“bootons”)

Myelin Sheath in PNS

• not all neurons in the body are myelinated
  - the unmyelinated parts have pores and channels which allow ions to move across the membrane

• cell type that myelinate in PNS
  1. Schwann Cells

• produce bead like structure on axon, hiding part of the axon
  - in between the wrapping of the cell is called nodes of ranvier

• physically protects axons

• insulates axons

• speeds up rate which action potential travels down the length of the axon
  - large increase in speed

• where myelin wraps around the axon, there are no channels, pores are anything since the myelin acts as an insulator
  - this part of membrane is impermeable meaning that no gases or liquids can pass

Myelin Sheaths in the CNS

• Oligodendrocytes
  - cell type that myelinate in CNS
  - does wrap around multiple axons simotsansly but nodes of ranvier are still present
  - vital to how the action potential is transmitted down the length of the axon

• not all axons are myelinated

White Matter and Gray Matter

• White matter
  - Dense collections of myelinated fibers

• Gray matter
  - Mostly neuron cell bodies and unmyelinated fibers

Neuron Functions

• You can disrupt membrane potential in two primary ways:
  - receptive area where there is a greater potential
  - if the stimulus generating a greater potential is adequate it will create an action potential
  - vary in duration and size
  - axon where there is action potential
  - axon potential are always the same size and roughly the same duration

Principles of Electricity

• every cell has a resting membrane potential and that membrane potential is the sum of charges outside of the cell and inside the cell
  - resting membrane potential allows us to generate an energy potential difference across the membrane, i.e: the inside in negative relative to the outside
  - more positive outside
  - mostly ions make up these charges
  - more negative inside
  - ions, phosphates or proteins

• the molecules that cross the membrane:
  1. sodium
  - most sodium outside of the cell, making a gradient
  2. chloride
  - most chloride is inside of the cell, making another gradient
  3. potassium
  - most potassium on inside of cell, making another gradient

• takes energy to create gradient in the form of active transport
  - convert chemical energy in the form of ATP to a potential energy in the form of gradients

• When a cell is depolarized (or excited) some of the charges flow down their concentration gradient and this changes the membrane potential which causes other things to happen

• voltage (V): measure of potential energy generated by separated charge

• Potential difference: voltage measured between two points

• Resistance (R): opposes to charge flow (provided by the plasma membrane)
  - plasma membrane opposes the flow of ions across the membrane

• Insulator: substance with high electrical resistance (Myelin)
  - prevent the flow of charges across the membrane
  - Schwann cells and oligodendrocytes

• Conductor: Substance with low electrical resistance (water with electrolytes)

Role of membrane ion channels

1. Leakage (non-gated) channels:
   - leaks allowing ions to flow from high to low concentration
   - no regulation, they are always open
   - important to establishing resting membrane potential of a cell

2. Gated Channels (three types)
   1. chemical gated (ligand gated) channels
   - found on the membranes associated with the dendrites and the soma (the receptive areas)
   - normally closed
   - may be called receptor but same thing
   - BUT when a neurotransmitter binds to the receptor, the ligand gated channel opens up and allows for the movement of ions across the membrane
   - when its open selected ions can flow across the membrane
   - NTM, hormone etc is the ligand and when the ligand binds to the channel, the channel changes shape to open and allows ions to move across membrane
   - temporary bond:
   - when the ligand bonds to the ligand gated channel the channel changes shape which causes the ligand to fall off and then the channel can close again
   2. Voltage-gated channels
   - nothing has to bind to it to cause it to change shape
   - instead it is sensitive to the membrane potential
   - normal membrane potential is -70 mV but if a stimulus comes along and changes the membrane potential and that mebrane potential rises high enough it can change the shape of the protein
   - at rest the channel is closed, when membrane potential changes, protein changes shape and opens up, when membrane potential goes back to normal protein goes back to regular shape
   - regulated channel
   - seen on axons of neurons
   3. mechanically gated channels
   - require a physical manipulation to change the shape and open up the channels

Resting Membrane Potential (Vr)

• when a cell is at homeostasis sodium is outside, chloride is outside, and potasium is inside

• resting membrane potential is not a static membrane

• How do we establish resting membrane potential?
  1. sodium potassium pump
  - primary active transport protein
  - moves sodium from inside the cell to the outside of the cell up concentration gradient (3 Na out)
  - moves potassium from outside of the cell to inside the cell also up a concentration gradient (2 K in)
  - during this process, it consumes 1 ATP
  2. Potassium Leakage Channels
  - specific to potassium
  - when potassium flows down its concentration gradient from the interior of the cell to the extracellular fluid you lose positive ions, inside of cell becomes more negative (-90 mV)
  3. Sodium Leakage Channels
  - because of sodium leakage channels, this causes positive ion flowing into the cell which raises the membrane potential inside the cell (brings membrane potential of cell up to -70)

• the sum of sodium leakage channels, potassium leakage channels and sodium potassium pump determines resting membrane potential
  -effect of this” maintain a stable resting membrane potential at around -70 mV

• Resting membrane potential is normally -70 mV, interior is negative relative to the outside

Membrane Potentials That Act as Signals

• Two types of signals
  1. Graded potentials
  - incoming short-distance signals
  - receptors pick up this information, information travels from the dendrites, and the cell body towards the cell body and eventually reaches the axon hillock
  - Associated with dendrites and the cell bodies
  -size in membrane potential varies
  - can be either depolarizing or hyper polarizing
  - when graded potential happens there is a small change in the membrane potential localized to the area of the stimulus
  - only tells you there is a stimulus that caused a change in membrane potential does not tell you anything about direction or about the size in the change of membrane potential
  - the area where the initial event took place is where there is going to be the largest change in membrane potential
  - takes many stimuli (ligand gated channels) to be activated to create a large graded potential
  - decay rapidly
  2. Action potentials
  - originate in the axon hillock and then travel down the axon to the axon terminals
  -long distance signals of axons because axons are long
  - axons only
  - always the same size, all or none

Changes in Membrane Potential

• Normal resting membrane potential is about- 70 mV

• if you have a postive deflection, one that moves up towards zero, you will have a depolarizing event
  - positive deflection where the cell becomes less negative

• if you have a change in membrane potential where you move away from zero and get more negative is hyper polarizing