L4 - Neurons

Question 1

Cells in the Brain:

Answer 1

The brain consists of glial cells and neurons

Question 2

The Neuron:

Answer 2

• Neuron cell body (soma)
• Dendrites
• Axons
• Axon terminals
• Synapses

Question 3

Glial Cells:

Answer 3

• Oligodendrocytes
  
  • Produce myelin sheath which wraps around axons (Multiple Sclerosis = No myelin)
• Astrocytes
  
  • Transfer nutrients from the blood to neurons & maintain the BBB
• Microglia
  
  • Brain’s immune system; clean-up foreign/toxic substances that pass through the BBB

Question 4

Neuron cell body (soma) =

Answer 4

Contains Nucleus and other structures for cell function

Question 5

Dendrites =

Answer 5

Many on a single neuron to receive signals (inputs) from multiple other neurons

Question 6

Axons =

Answer 6

• unique to neurons
• Sends signals (starts at axon hillock – the swelling at the junction of the axon and cell body)
• One per neuron – only one axon for output
• Wrapped in myelin - to prevent leakage of depolarisation waves = rapid signal transmission (100x)

Question 7

Axon terminal / Terminal boutons =

Answer 7

• “terminal boutons”, a.k.a. axon terminal
• Forms synapses with another neuron
• Sends information to that neuron
• Secretes neurotransmitters when an Action Potential reaches them

Question 8

Synapses =

Answer 8

• Axon Terminals (neuron 1) to Dendrites (neuron 2)
• Join axon terminals of one neuron to dendrites of another neuron for transmission of signals
• Neural signals go one-way:
  
  • Pre-synaptic: From cell body to axon terminal
  • Post-synaptic: From dendrite to cell body

Question 9

How do Neurons Talk? (Electrical Signals)

Answer 9

Step 1

• Neurons are surrounded by a fatty layer that regulates the flow of charged particles (ions) in and out of the cell – Works like a battery
• Regulating ion flow creates a resting membrane potential
• Voltage difference (-75 Millivolts)

Step 2

• Excitatory – Depolarises (gets more positive)
• Inhibitory– Polarises (Gets more negative)
• These change the resting potential
• Each neuron has a threshold (~55MV)

Step 3

• When the neuron’s potential reaches the threshold, there is a quick depolarisation and repolarisation at the axon hillock (takes about 2 milliseconds, which is two 100ths of a second)
• Creates a current called an Action Potential
• When it depolarizes, it ‘overshoots’ neutral – slows down action potential
• When it repolarizes, it ‘undershoots’ the resting potential – makes it harder for there to be another ‘action potential’ again directly afterwards

Step 4

• The Problem: The current is so small that it normally wouldn’t reach the end of the axon
• Solutions:
  
  • Myelin Sheath – Plugs holes and forces ions (hence current) down the axon (similar effect to duct tape on a leaky hose)
  • The gaps between the sheaths are called the nodes of ranvier, and they act as amplifiers that regenerate the action potential
    
    • They are like additional axon hillocks

Step 5

• Ultimately, the current reaches the end of the axon
• While so far, it is all electric, this isn’t how the two neurons finally communicate
• The arrival of the action potential at the presynaptic axon terminal causes a chemical to be realised (neurotransmitters) which go across the synaptic cleft to the postsynaptic dendrite

Step 6

• These neurotransmitters enter the synaptic cleft and then float away down through cerebral-spinal fluid
• Depending on the neurotransmitter, it’ll either cause an:
  
  • Excitatory Post-Synaptic Potential (EPSP) or
  • Inhibitory Post-Synaptic Potential

Question 10

Membrane Potential:

Resting Potential:

Action Potential:

Answer 10

• Membrane Potential = Difference in the electrical charge (voltage) between inside and outside cell, across cell membrane wall
• Resting Potential = At rest (i.e. NOT during an action potential) more positive ions outside than inside the cell gives overall negative potential (voltage) inside compared with outside the cell
  
  • Difference in electrical charge (voltage) at rest = -70mV
• Action Potential = Transmission of electrical signal along axon.
  
  • Input from other neurons (via synapses on dendrites) increases membrane potential. If voltage exceeds threshold, triggers action potential
    
    • Depolarisation of cell: membrane potential goes back to zero
    • Repolarisation: membrane potential back to -70mV resting potential

Question 11

Fixed Size and All-or-None principle:

Answer 11

• If threshold level is reached, action potential of a fixed sized will occur. The size of the action potential is always the same for that neuron.
• All-or-None: Either a full action potential is “fired” (if membrane potential reaches threshold) or there is no action potential. There are no “large” or “small” action potentials.
• The strength of the neuron signal is determined by the rate of repeated action potentials