Neurones & glia

Seite 1

4 types of neurons

Sensory

Unipolar

Motor

Multipolar

Bipolar

Interneuron

Pyramidal

Brain

Seite 2

Types of membrane potentials

i) Resting membrane

Generally between minus 60 & minus 80 mV

Charge inside cell less positive than outside

Electrical current is generated by movement of ions across membrane

ii) Equilibrium

When concentration gradient for K+ = electrical gradient pulling K+ in to cell

Approx minus 80 mV for K+

K+ diffuses out of cell (high conc.) into extracellular fluid (low conc.)

Electrical gradient generated by increasing negative charge inside cell (K+ leaving but large negative proteins remaining) & increasing positive charge outside cell (K+ adding onto Na+)

Approx +50 mV for Na+

Nernst equation

iii) Transmembrane

Voltage difference across a cell membrane

Due to unequal distribution ions across cell membrane

Greater permeability to K+ than Na+

Na+/K+ transmembrane pump (3 sodium out, 2 potassium in) ensures voltage difference remains

Seite 3

Ion channels & receptors

i) Voltage-gated

ii) Ligand-gated

Ionotropic

Fast activation

Short duration of action

iii) Mechanosensitive

iv) G-protein coupled receptors

Metabotropic

Slow activation

Long duration of action

Seite 4

Action potentials & signal propagation

i) AP criteria

Must reach threshold

All or nothing principle - if threshold isn't reached, the signal isn't travelling

Self-propagating

​​​​​​​Signal travels down axon

Refractory period

Creates a gap between APs

2 types:

​​​​​​​Absolute refractory period

No AP can be sent at all under any circumstances​​​​​​​

Relative refractory period

​​​​​​​​​​​​​​AP could be sent but requires a stronger than normal stimulus

​​​​​​​​​​​​​​Prevents overstimulation of neuron

ii) Ion channels involved in APs

Voltage gated Na+ channels

Upon reaching threshold, Na+ channels open

Allows Na+ to enter cell

Depolarisation

Approx 5000 fold increase in Na+ conductance at AP

Voltage gated K+ channels

​​​​​​​Repolarisation

Open to release K+ ions

Repolarisation

iii) Local circuits

Na+ influx causes depolarisation for up to 3mm along axon

These adjacent areas reach threshold

OCCURS IN ONE DIRECTION ONLY!!

Allows signal to propagate along axon

Seite 5

Nerve classes

i) General features of nerves

All possess nodes of Ranvier @1-3 mm intervals

Saltatory conduction occurs along NOR

APs 'hop' from one NR to another

Increases conduction velocity of signal

Most are myelinated

Composed of layers containing sphingomyelin

Myelinated axons = faster signal transduction compared to unmyelinated (see GIF below)

CNS

Oligodendrocytes myelinate axons

PNS

Schwann cells myelinate axons

ii) Nerve classifications

A type nerves

Subdivided into alpha, beta, gamma & delta

B type

C type

Sensory nerve classification

Type I (A&B)

Type II

Type III

Type IV

Seite 6

Synaptic transmission

i) MOA

AP propagation to axon terminal

Activation of voltage -gated Ca+2 channels at pre-synaptic terminal

Ca+2 activates Ca+2 calmodulin -dependent protein kinase II

Primes vesicles for mobilisation

Causes vesicle fusion to docking sites on membrane

Exocytosis

ii) Receptors on post-synaptic membranes

iii) Post synaptic potentials

Excitatory neurotransmitters produce depolarisation of post-synaptic membrane

Excitatory postsynaptic potential

Does not necessarily guarantee an AP will occur

No propagation

Inhibitory neurotransmitters produce hyperpolarisation of post-synaptic membrane

Inhibitory post-synaptic potential

Makes it more difficult to trigger an AP

iv) Spatial & temporal summation

Seite 7

Glia

i) What are glia?

Provide protection & support to cells of neurones

Heighten functional capacity of neurones

10-50 times more glia than neurones

What are the different types of glial cells?

Schwann cells

Oligodendrocytes

Astrocytes

Ependymal cells

Microglia

Radial glia

ii) Function

Schwann cells & oligodendrocytes make myelin

See pg 5.

Schwann cells in periphery trigger neuromuscular junction formation

Astrocytes & ependymal cells control extracellular environment around neurones

Provide physical barrier between cells (BBB)

Maintain ionic & pH equilibrium of extracellular space

Astrocytes actively control synaptogenesis

Release protein signals which trigger synapse formation

Regulate synapse numbers, function & stability

Microglia CANNOT do this

Radial glia involved in brain development

Microglia have immune function

When nervous tissue damaged, microglia migrate to site of injury

Phagocytose cellular debris

iii) Macroglia vs microglia

Macroglia derived from neural stem cells

Microglia derived from macrophages outside CNS