Axons and Neuroplasticity

Dendrites

Branches that are main input site for neuron

Soma

Cell body that synthesizes proteins and neurotransmitters

axon

long branch that is the output unit of the neuron

presynaptic terminal

transmit neurotransmitters of neuron to synapse

Organelles in the soma

nucleus, golgi apparatus, rough ER

Organelles throughout the neuron

mitochondria, lysosomes, smooth ER

anterograde transport

from the soma towards the presynaptic terminal

retrograde transport

from the synapse back to the soma

Example of multipolar neuron

spinal motor neuron, perkinje cell in cerebellum

Example of bipolar neuron

retinal bipolar cell in eye (fast relay back and forth of vision info

Example of Pseudounipolar neurons

somatosensory neurons very common in PNS

How neurons generate action potential

concentration and electrochemical gradient

Na+ / Cl- / Ca2+

Ions wanting to move into neurons (higher concentration outside of cell)

K+

Ions wanting to move out of the cell (higher concentration inside of neuron)

Electrochemical gradient

Interior of the neuron is negatively - charged which affects what charge of ions attracted

Na+ / K+ / Ca2+

attracted to the interior of the neuron due to + charge

Cl-

Attracted to extracellular space due to - charge

ligand-gated channel

opens when a neurotransmitter binds to the binding site

voltage-gated channel

open in response to a voltage change across the membrane

modality-gated channel

open in response to mechanical forces such as stretch, pressure, chemical, or temperature changes

leak channels

always open and small number of ions leak at a continuous rate which maintains homeostasis and sets the electrical potential (-70 mV) of the cell

K+ channel

most common type of leak channel

-70 mV

Resting potential (interior more negative than extracellular space)

Na+ / K+ pump

uses ATP to move 2 K+ into the cell and 3 Na+ out of the cell (against the electrochemical gradient)

depolarization

membrane potential becomes less negative than -70 mV

hyperpolarization

potential is more negative than -70mV (refractory period)

Local potential

spread passively, are confined to small area of membrane, amplitude decrease with distance

Action potential

large depolarizing signal which is actively propagated along an axon and generates a signal

-55 mV

threshold for fast depolarization where Na+ channels open

Increased diameter of axon and myelination

cause faster action potential production

Node of Ranvier

1-2 mm along the axon which contains high densities of Na+ and K+ channels

myelinating glial cells

oligodendrocytes and schwann cells

oligodendrocytes

only in CNS, can myelinate multiple axons with different branches

Schwann cells

found in PNS and fully wrap around 1 axon or partially wrap around multiple axons

astrocytes

signaling/nourishing/cleaning glial cells

microglial cells

defending cells which function as phagocytes

satellite cells

thin glial cells that cover somas in PNS

Guillen-Barre Syndrome

demyelination of PNS motor and sensory neurons, usually full reversal of symptoms

Multiple sclerosis

progressive degeneration of myelin in CNS due to antibodies attacking oligodendrocytes

Neuroplasticity

the ability of neurons to change the function, chemical profile, and/or structure

Habituation

decrease in response to a repeated, benign stimulus

Experience-dependent plasticity

learning process which involves persistant, long-lasting changes in the strength of synapses between neurons and in neural networks

Long-term potentiation

silent synapses are converted to active as AMPA receptors are inserted into the synaptic membrane

Long-term depression

active synapses are converted into silent as AMPA receptors are removed from the postsynaptic membrane

Astrocytes and neuroplasticity

neuron releases neurotransmitter which stimulates the release of gliotransmitters that modulate neuronal activity and synaptic transmission/synaptogenesis

excitotoxicity

cell death caused by overexcitation of neurons as excess glutamate is produced

Wallerian degeneration

breakdown of axon programmed after axon is separated from soma due to outside injury

1-3 mm a day

1-3 in a month

axonal regeneration rate

synkinesis

different synaptic target is innervated through either collateral or regenerative sprouting

Axonal generation in CNS does not occur because…

development of glial scars, limited nerve growth factor, presence of Nogo

Synaptic changes following CNS injury…

1. recovery of synaptic effectiveness

2. denervation hypersensitivity

3. synaptic hyper-effectiveness

4. unmasking of silent synapses

Effective therapeutic ingredients for tx…

• numerous reps with little or no break

• practice actual tasks

• appropriate inc in difficulty

• focus on effect of movement

• observation

• correct timing and dosage

• task-specific training