Neuron structure, function and connectivity - bio week 3 Flashcards
What is the neuron and its function?
Neuron - the basic unit of structure and function in the nervous system. The function is to receive, integrate and transmit information.
The history of the neuron hypothesis?
Galen C2-Descartes C17- NS a continuous network of pipes
Late C19 Cajal- NS made up of discrete components (neurons)
Neuron Doctrine (Waldeyer, 1891) –
NS comprised of individual nerve cells
Information is transmitted from neuron to neuron across a tiny gap (synapse)
Sherrington C20 – theory of discrete inter-connected cells supported
What are the types of neuron?
Sensory – respond to external stimuli transmit sensory stimuli to CNS
Motor – carry messages from CNS to muscles and glands
Interneurons – transmit messages to other neurons
what is the structure of the neurons input zone?
Cell body:
- contains nucleus (DNA),
- mitochondria (energy)
- endoplasmic reticulum (protein synthesis)
- contact with other neurons
Dendrites:
- branch out from cell body and increase surface area.
- receive input from other neurons
What is the structure of the neurons conducting zone?
Axon – carries information (nerve impulse) from the cell body to the axon terminal buttons
-afferent axons (sensory)
-efferent axons (motor)
Neurons can also be classified on how the axon and dendrites leaves the soma:
- multipolar
- bipolar
- monopolar
The axon is insulated by - Glial cells – mylelin sheath
- Oligodendrocyte cells-CNS
- Schwann cells-PNS
- Protector
- Helps speed up the transmission of impulse aided by the Nodes of Ranvier.
What is the structure of the neurons output zone?
Presynaptic Terminal -
found at the end of branched axon.
communicate with dendrites of other neurons.
when a nerve impulse reaches the axon terminal button they release a chemical called a neurotransmitter.
How is a neuron dual functioning?
Primary function of a neuron is to transmit information.
There is 2 ways:
- Within the neuron – electrical transmission.
from the cell body - down the axon - to nerve terminals
Between neurons - Chemical Transmission - from nerve terminals - across the synapse – to receptors on adjacent target cell
Electrical transmission 1 - the cell membrane?
Outer layer of the nerve cell is called the cell membrane. The cell membrane defines boundaries of cell and plays an important role in electrical transmission. The nerve/cell membrane creates the intercellular and extracellular compartments.
Electrical transmission 2 - membrane potential?
at rest the inside of the neuron (intracellular fluid) is electrically negative with respect to the outside (extracellular fluid) which is positively charged. The membrane resting potential is the difference between the charges. At rest the electrical difference is -70 millivolts. And when the cell is at rest the neuron is polarised.
Electrical Transmission 3 - intracellular and extracellular fluids
Intracellular and extracellular fluids contain positively and negatively charged ions. The resting membrane potential is a result of the unequal distribution of these ions across the nerve membrane.
Most important ions to the RP are:
Sodium ions (Na+)
Potassium ions (K+)
Chloride ions (Cl-)
Negatively charged protein ions (organic anions) (A -)
Electrical transmission 4 - why are the ions unequally distributed?
Selective permeability – some ions can pass through (E.g k+) but others can’t (A-)
Concentration gradient – osmosis – high concentration to low
Electrical forces – electrostatics : 2 positives repel – a negative and a positive attract
What is the sodium-potassium pump (SPP)
SPP – is voltage dependent and is maximal at resting potential (-70mV).
If the membrane potential becomes less negative the SPP breaks down-this occurs when the neuron is stimulated.
Electrical transmission 5 - the action potential?
When the neuron is activated it becomes a little less negative. If the strength of electrical potential reaches -55 mV then SPP breaks down -threshold of response reached - cell fires = action potential. Membrane potential becomes + 50mV and the neuron is depolarised.
Electrical transmission 6 - action potential stages
- Sodium channels open causing influx of positive ions into the cell - leading to depolarisation - action potential.
- Potassium channels open allowing it to flow out of cell
- AP reaches peak after 1msec. sodium channels blocked
- potassium channels still open so membrane potential returns to normal as potassium still flowing out
- Relative Refractory period - brief time when membrane potential more negative than RP, Potassium ions close and sodium reset ready for next stimulation
- RP reached
electrical transmission 7 - all or none law
A neuron will only fire if stimulation reaches the threshold of response. Once reached, action potential will remain the same size regardless of its original strength. Once action potential is created it will travel down the axon to the terminal buttons. How an action potential is conducted is dependent on presence of Myelin Sheath.
Unmyelinated Axon-slow conduction.
Myelinated Axon-rapid saltatory conduction. There are nodes of Ranvier where the is no myelin sheath. So the signal has to ‘jump’ to the next which speeds up the process.
