Nervous Systems (2) Flashcards
Information processing of nervous systems
- Sensory input
- Integration
- Motor output
Components of a neuron
Nucleus Cell body Axon hillock Dendrites Axon Synaptic terminals Synapse (junction between synaptic terminals and cell body)
Structural diversity of vertebrate neurons
Sensory neuron
Interneuron
Motor neuron
A nerve
Consists of many neurons
- axons
- connective tissue
- blood vessels
Glia
Supporting cells vital for structural integrity and normal function.
10-50 times more glia than neurons in the mammalian brain.
- Astrocytes
- Oligodendrocytes & Schwann cells
Astrocytes
- CNS
- Structural support
- Regulate extracellular concentration of ions and neurotransmitters
- Formation of the blood-brain barrier
Oligodendrocytes
- CNS
- Form myelin sheaths around axons
- Lipid membranes: insulator
- Have nodes of Ranvier
Schwann cells
- PNS
- Form myelin sheaths around axons
- Lipid membranes: insulator
- Have nodes of Ranvier
Gradients of ions across the membrane
The resting membrane potential is negative
- The inside membrane is negative relative to the outside (120mM Cl- ECF but 100mM A- anions cytosol)
Na+/K+-ATPase
- Pumps 3 x Na+ out of the cell
- Pumps 2 x K+ into the cell
(More Na+ in the extracellular fluid than in the cytosol, more K+ in the cytosol than in the extracellular fluid)
The membrane at rest has many open K+ channels but few open Na+ or Cl- channels.
Build up of -ve charge in neuron: limited by electrical gradient vs. chemical gradient of K+.
Eq’m potential in neuron: approx. -70mV
Process not dependent on voltage-gated ion channels- which are required for action potentials.
Excitable cells
All cells have a membrane potential.
Rapid changes in membrane potential occur in these cells:
Neurons
Myocytes
Pancreatic beta cells
Membrane potential
Voltage of inside of membrane relative to outside - equilibrium
Hyperpolarisation
Inside of membrane becomes more negative
- Opening of voltage-gated K+ channels
- K+ out
Depolarisation
Inside of membrane becomes more positive
- opening of voltage-gated Na+ ion channels
- Na+ in
Resting potential
-70mV
Threshold
-55mV
Action potentials
General info:
Magnitude is independent of the strength of the original stimulus - all or nothing response depending on if threshold is reached.
Steps:
1. Resting state:
Voltage-gated ion channels are closed
- A stimulus:
Causes a few Na+ channels to open, Na+ rushes in - Depolarisation:
If threshold is reached:
Lots of Na+ channels open
Lots of Na+ rushes in - Repolarisation
K+ channels open
K+ rushes out
Na+ channels INACTIVATED and then start to close - Undershoot
Small hyperpolarisation
Also need Na+/K+-ATPase to restore Na+ and K+ concentrations
Usually start at the axon hillock
Travels long distances by regenerating itself along the axon
Voltage-gated Na+ channels
Three states: Fast
Closed —> Depolarisation
Open
Inactivated
General anaesthetics prevent VG Na+ channel transition from inactivated to closed state
Voltage-gated K+ channels
Two states: Slower
Closed —> Depolarisation
Open
Absolute refractory period
From start of depolarisation at threshold to start of undershoot.
No action potential can be generated
- Na+ channels open for first half
- Na+ channels inactivated for the second half
Relative refractory period
From start of undershoot to the end.
AP only occur if a large enough stimulus is applied
-Because some Na+ channels are in the closed state again
Refractory periods
Limit firing frequency
The action potential can only travel in one direction
Factors affecting action potential conduction speed
Axon diameter
-Larger diameter, less resistance
Temperature
-Chemical reactions occur faster at warmer temperatures
Degree of myelination
-Insulates the axon membrane in vertebrates to enable faster conduction speed
Conduction speed is affected more by myelination than by axon diameter
Saltatory conduction and smooth conduction
Nodes of ranvier restrict sites where Na+ and K+ channels can open and close (120 m/sec) - salutatory conduction
Unmyelinated nerve fiber - smooth conduction
Presynaptic and postsynaptic neurons
Presynaptic neuron —> Postsynaptic neuron
Presynaptic neuron —> effector cell (muscle)