NMS Lecture 4 - Mylenation, Synaptic Transmission

How can the speed of an actual potential propagation speed up?

Myelination of the neuron

Role of Oligodendrites

Create myelin for the central nervous system

Role of Schwann cells

Create myelin for the peripheral nervous system

Myelin composition

80% lipid, 20% protein

Axon ensheathing

Axon is wrapped in myelin by an oligodendrite or a Schwann cell

What cell ensheaths interneurons

Oligodendrites - interneurons are part of the CNS

What cell ensheaths efferent nerves

Schwann cells - part of the PNS

Role of mylenation

Acts as an insulator and precents ions from moving across the membrane

What parts of the neuron are covered in myelin

Only the axon

Nodes of Ranvier

Regions of an myelinated axon which are unmyelinated. Contain high concentration of voltage-gated ion channels to propogate the action potential

Where are the voltage-gated channels present in a myelinated neuron

Nodes of Ranvier and at the axon hillock

How many segments can a Schwann cell mylelinnate

A Schwann cell can myelinate one section of one axon

How many segments can an oligodendrite cell myelinate

An oligodendrite cell can myelinate multiple sections of multiple axons

Advantage of myelinated neuron vs unmyelinated neuron

Signal travels significantly faster down the axon of a myelinated axon, as the action potential is only stimulated at the nodes of ranvier

Saltatory conduction

Depolarization at note of Ranvier causes depolarization at neighboring nodes of Ranvier further away

Electrotonic conduction

Depolarization of axon causes neighboring sections to depolarize

What causes multiple sclerosis

Degeneration of myelin sheath, slowing down neurotransmission

2 factors which determine the speed at which an axon propagates along an axon potential

1. Size of the axon - thicker the diameter, faster it propagates an action potential
2. Myelination - myelinated axons propagate action potentials faster than unmyelinated axons

Why does signal propagation only travel down to the axon terminals

Refractory period is too long for the neighboring action potential to re-stimulate the cell

Speed of AP conduction down a myelinated axon

12-130 m/s

Speed of AP conduction down an unmyelinated axon

0.5-2 m/s

Synaptic transmission

The process whereby one neuron communicates with other neurons or effects, such as a muscle cell, at a synapse

Electrical synapse

Synapse where cells are connected by gap junctions, allowing the passage of ions and small molecules

Connexin

Gap junction protein connecting the two cells

How many subunits if a connexin made of

6

Use of electrical synapse in neurons

Synchronizes large clusters of neurons by physically connecting them. When the gap junction (connexins) are open, neurons can fire at the same time

What direction to electrical synapses run

Bi-directional - ions can move between each cell freely

Where are electrical synapses connected on a neuron

Via the dendrites - dendrite-dendrite, dendrite-cell body, etc.

Chemical synapse

a type of synapse at which a chemical (a neurotransmitter) is released from the axon of a neuron into the synaptic cleft, where it binds to receptors on the next structure (either another neuron or an organ)

What direction do chemical synapses run

From pre-synaptic to post synaptic neuron

To the neurons touch in chemical synapses

No. There is a 40nm gap

Steps of chemical synapse

1. Neurotransmitter is stored in the presynaptic terminal in synaptic vesicles
2. Neurotransmitter is released from the vesicles and enters into the synaptic gap and binds to receptors on the postsynaptic cell
3. Binding of neurotransmitter to receptors on the postsynaptic cell opens ion channels on the postsynaptic membrane, resulting in depolarization or hyperpolarization

inhibitory neurotransmitters

NTs that bind to receptors and cause the cell to become more negative, making it harder to activate

Excitatory neurotransmitters

NTs that bind to receptors and open sodium channels, depolarizing the neuron