Neural transmission Flashcards
what are neurons
What are glia?
Neurons are the main information processing cells −
Glia mainly for support and maintenance
− Small, low contrast -special techniques needed to visualize cells
• Static staining
• Fibre tracing
Give the structure of a neuron
Dendrites (from Greek dendron, tree)
• Cell body (soma)
• Axon hillock
• Axon (from Greek axis)
• Myelin sheath − Created by Glial cells
• Oligodendroctyes(CNS) and Schwann cells (PNS)
• Axon terminals (connect with other neurons at synapses)
give the 4 zones of a neuron
Input zone − Dendrites–where neurons collect and integrate information from other cells
- Integration zone − Cell body (soma or somata) – where the decision to produce a neural signal is made
- Conduction zone − Axon –where information is transmitted over great distances
- Output zone − Axon/synaptic terminal –where the neuron transfers information to the other cells
outline 3 different types of neuron
Multipolar − Many dendrites, 1 axon
− Multipolar neurons are most common in the brain
• Motor neurons –muscle control
• Interneurons –relay and integrating information for learning and memory
- Bipolar − 1 dendrite + 1 axon − common in sensory systems, such as vision
- Uni/Monopolar − 1 branch leaves cell body, spreads in 2 directions − also seen in sensory systems, such as touch
More then 200 shapes and types of neurons
give the physics of a neuron
Atoms are held together by electrostatic force: − Opposite charges attract one another − Same charges repel one another
When salts are dissolved in water, positive and negative ions separate and move about freely
• However, the ions are still influenced by electrostatic forces
give some ions that exist in the nervous system
• Positive ions (cations):
− Sodium (Na+): generating action potentials (nerve impulses)
− Potassium (K+): maintaining resting potential
− Calcium (Ca2+): synaptic transmission
• Negativeions (anions):
− Chloride ions (Cl-): suppressing action potentials − Proteins (An-): maintaining resting potential
which neurons are extracellular and which are inner?
Positive ions (cations): − Sodium (Na+): mainly outside (extracellular) − Potassium (K+): mainly inside (intracellular) − Calcium (Ca2+): (almost) exclusively outside (extracellular)
• Negativeions (anions):
− Chloride ions (Cl-): mainly outside (extracellular) − Proteins (An-): mainly inside (intracellular)
Define diffusion
the movement of particles (atoms, ions, molecules) in a gas or liquid (e.g., water) from regions of high concentration to regions of low concentration
- Diffusion is caused by the random movement of particles
- The speed of diffusion depends on the temperature, the size of the particles, and how difficult it is for particles to travel through the liquid (viscosity)
what is an ion channel?
Neuronal cell membranes contain “pores” made up of large proteins that allow certain ions to pass through the membrane
- These pores are called ion channels
- Ion channels are selective: they only allow one type of ion through
- There are different ion channels for K+, Na+, Cl-, etc.
- Because of ion channels, neuronal cell membranes are semipermeable: they only allow some types of ions to diffuse through
what is meant by electric potential
what is voltage?
Neurons have an electric potential across their membranes
• This potential –the membrane potential –is the result of 1. differences in ionic concentrations between the inside (intracellular) and outside (extracellular) of the neuron, and
2. ion channels in the neuronal cell membrane that only allow certain ions to pass in and out of the neuron • The membrane potential can be measured using tiny electrodes
All charged particles (such as ions) are surrounded by an electric field
• The strength of this electric field is the electric potential
• Usually one is more interested in the electric potential differenceor voltage
• Voltage is measured in volts (V)
• For example, the voltage of a battery is the electric potential difference between the plus (+) and minus (-) poles
Voltage= electric potential difference
what is the resting meembrane potential?
• When neurons are at ‘rest’, there is slightly more negative charge inside the neuron than outside the neuron
• This creates a difference in electric potential –a voltage – across the neuronal cell membrane
• This voltage is called the resting (membrane) potential
The cell membrane is semipermeable
- When neurons are at ‘rest’, only potassium ions (K+) can freely cross the cell membrane (because only K+ ion channels are open)
- Because the concentration of K+ ions is higher inside the cell than outside, some K+ ions leave the cell by diffusion
- Negative protein ions (An-) cannot cross the membrane and remain inside the cell – leaving more negative than positive ions inside
- This causes a negative charge to build up insidethe neuron
- The resting potential is about -70 millivolts (mV)
what causes resting potential?
Determined by 2 opposing forces: 1. Concentration difference (concentration gradient)
• Drives K+ out of the cell
• This makes the inside more negative
2. Electrostatic force (electrostatic gradient)
- Because opposite charges attract, K+ is drawn back into to the negative inside of the cell
- When these forces are balanced, equal numbers of K+ ions leave and enter the cell
- The membrane potential is then at the equilibrium potential
- This is the resting potential
how does the sodium-potassium pump work
Because of the sodium-potassium pump (Na+/K+ pump) • An ion channel that pumps Na+ outof the cell and K+ into the cell • Because this is againstthe concentration gradients of Na+ and K+ it requires energy
The Na+/K+ pump uses energy from ATP to pump Na+ out of the cell and K+ into the cell • Pumps 3 Na+ out for every 2 K+ in • Most of the brain’s energy consumption is used to fuel this pump!
How was information about neurons first discovered?
Hodgkin & Huxley shared the Nobel prize in 1963 “for their discoveries concerning the ionic mechanisms involved in excitation and inhibition in the peripheral and central portions of the nerve cell membrane”
- Giant squid axons − Human axons are at their largest ~100 micrometers (but they can be under 10 micrometers). − A human hair is about 80 micrometers − Hodgkin and Huxley got around this problem by using giant squid axons (up to 1 mm in diameter)
- Voltage clamps − able to “freeze” the neuron at particular voltages − allowing them to gather details on what was happening in the neuron at each stage in the action potential.
WHat is hyperpolarization and what is depolarization?
Hyperpolarization= increase in resting potential (more negative)
- Depolarization= decrease in resting potential (less negative)
- Spread passively along membrane and diminish in size with distance
- Depolarization and hyperpolarization counteract each other
- The membrane potential is the sum of the depolarization and hyperpolarization
