neuroglial cells and neurotransmitters

nerves

consists of hundereds of thousands of axons wrapped together in a connective tissue

ganglia

are part of a single nerve

• in the PNS the cell bodies of neurons are grouped together in masses

neuroglial cells

are highly important for neuron function

• are support cells for neurons

• neurons require a highly specific environment to survive and conduct electrical impulses efficiently

  • help support neurons to enable them to thrive in their needed environment

schwann cells

coats perihperal nerve axons in short sections

• are a special type of neuroglial cell found in the peripheral nervous system and composed of a white fatty layert called the myelin sheath

• protects nerve axons

  • increases speed of nerve impulses

nodes of ranvier

gaps between Schwann cells, at these exposed nodes, the nerve impulse is forced to jump to the next node

• enables saltatory conduction

saltatory conduction

when nerve impulses are forced to jump to the next node, greatly increasing the nerve impulse transmission along the axon

satellite cells

surround the cell bodies of peripheral neruons, helping to regulate the cell body environment

myelin sheath

rolled around the axon, insulating the nerve fiber from others and increasing the speed of nerve impulses

• there are also unmyelinated fibers, which are common in the gray matter of the brain and spinal cord

  • schwann cells do not wrap around the axon but are just loosely associated with the axon

axonal regeneration

if a peripheral nerve axon is severed, the schwann cells grow ahead of the axon, creating a path for the axon to follow as it grows

• it is possible in many cases for axons to regrow after an injury to a peripheral nerve; although the growth is very slow

ependymal cells

circulate cerebrospinal fluid and allow fluid exchange between the brain, spinal cord, and cerebrospinal fluid

oligodendrocytes

act as the insulation for CNS axons

astrocytes

control the chemcial environment of neurons by wrapping around the blood capillaries

blood-brain barrier

the physical barrier between neurons and capillaries, allows the passage of only certain substances into the CNS

microglial cells

protect the CNS by scavenging dead cells and infectious microorganisms (pathogens)

action potentials

the nerve impulse is an electrochemcial charge moving along an axon created by the movement of unequally distributed ions on either side of an axon’s plasma membrane

polarized

the plasma membrane at rest, one side has a different charge than the other side

resting potential

phase 1

during this phase, both sodium and potassium gates are closed

when the axon is not conducting an impulse, the difference in electrical charge or the resting state of a neuron is equal to about -70mV

sodium-potassium pump

resting potential is maintained by this, it uses active transport to carry ions across the plasma membrane

• works by using an integral carrier protein that, for every three sodium ions out, two potassium ions are in

• must keep in constant operation because the sodium and potassium ions will naturally diffuse back to where they originated

• plasma membrane is more permeable to potassium diffusing outward and because more sodium ions are being pumped outwards than K pumped inward

• a relative charge develops and is maintained on the outside of the membrane

action potential

the change in polarity

• the resting potential becomes an action potential if the membrane becomes depolarized

  • once an action potential occurs, it continues through the entire length of the axon

depolarization

phase 2

sodium gates open, and sodium rushes into the axon during this phase of the action potential. voltage travels to 0 and then up to +40mV

first, the membrane potential becomes more positive, indicating that the inside of the membrane is now more positive than the outside

repolarization

phase 3

The sodium gates close, and the potassium gates open, allowing potassium to rush out of the axon.

This returns a negative voltage to the inside of the axon

The axon potential returns to normal, indicating that the inside of the axon is negative again

afterpolarization

phase 4

also called hyperpolarization

• potassium gates are slow to close, and there is an undershoot of the potential

  • the voltage drops below -70mV and then returns to -70mV as the resting state begins

self-propagating

The action travels along the length of an axon like a wave

• because the ion channels are prompted to open whenever the membrane potential decreases (depolarized) in an adjacent area

all-or-nothing: an action potential is this response, either occurring or not

The intensity of a sensation is distinguished by the number of neurons stimulated and the frequency with which the neurons are stimulated

electrochemical

the transmission of the nerve is electrochemical in nature as chemicals called neurotransmitters allow the signal to jump the synpatic gap

• the signal moves from electrical (through the neuron) to chemical (in the synapse) to electrical again once the signal reaches the next neuron

calcium channels

when a nerve impulse reaches the end of an axon, voltage gated channels open

• as calcium ions rush in, it causes vesicles containing the neurontrasmitters to fuse with the plasma membrane and release the neurotransmitter into the synpase

• when the neurotransmitter released binds with a receptor on the next neuron, sodium ion channels in the receiving dendrites open

  • depolarization occurs in the next neuron, and the impulse is propagated forward to another neuron or to a target organ, always in one direction

neurotransmitter released into a synapse

some synapses contain enzymes that rapidly inactivate the neurotransmitter

• for example, acetylcholinesterase, an enzyme or cholinesterase, breaks down the neurotransmitter acetylcholine

  • In other synapses, the synaptic ending rapidly reabsorbs the neurotransmitter

  • some neurons repackage the neurotransmitters in synaptic vesicles, while others chemically break down the neurotransmitters

  • the short existence of neurotransmitters in the synapse prevents continuous stimulation of postsynaptic membranes

    • inhibition: prevention of continuous stimulation

norepinephrine and epinephrine

are neurotransmitters produced by the adrenal glands

dopamine

is a specialized brain neurotransmitter to help regulate emotional responses and muscle tone

acetylcholine

a neurotransmitter found at the neuromuscular junction in the peripheral nervous system

• the NMJ is located where a motor neuron ends on a muscle instead of another neuron

  • for a muscle to contract, the nervous system must work together with the muscular system

the neuromuscular junction

the nervous system interacts with the muscular system at the neuromuscuilar junctions, to enable muscula contraction

• first, a nerve impulse must be sent to the muscle by the presynaptic motor neuron and muscle tissue

• once the nerve impulse reaches the muscle fiber (at the neuromuscular junction), acetylcholine is released into the synpase

  • acetylcholine binds to receptors on the muscle fiber that cause sodium channels to open. sodium rushes into the muscle cell, tiggering an action potential that reaches the sacroplasmic reticulum

the sarcoplasmic reticulum

a specialized type of smooth ER found within striated muscle tissue

• calcium ions are released from the sarcoplasmic reticulum of the muscle cell, causing the muscle to contract