L4 - Neurons Flashcards Preview

PSYC1020 - Introduction to Psychology - Minds, Brains and Behaviour > L4 - Neurons > Flashcards

Flashcards in L4 - Neurons Deck (11):
1

Cells in the Brain:

The brain consists of glial cells and neurons

2

The Neuron:

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

3

Glial Cells:

  • 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

4

Neuron cell body (soma) =

Contains Nucleus and other structures for cell function

5

Dendrites =

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

6

Axons =

  • 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)

7

Axon terminal / Terminal boutons =

  • “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

8

Synapses =

  • 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

9

How do Neurons Talk? (Electrical Signals)

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

10

Membrane Potential:

Resting Potential:

Action Potential:

  • 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

11

Fixed Size and All-or-None principle:

  • 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