Nervous Tissue II

Question 1

What is a synapse and the cells involved?

Answer 1

Synapses are sites where nerve impulses are transmitted - electrical or chemical signal

First cell = presynaptic cell, always a neuron

Second cell = postsynaptic cell, often another neurons

Question 2

What are classifications of synapses? Describe each (x4)

Answer 2

Can only be neuron to neuron:
Axodendritic synapse - between axon and dendrite, common in CNS

Axoaxonic synapse - rare, between 2 axons

Dendrodentritic synapse - between 2 dendrites

Does not have to be neuron to neuron:

Axosomatic synapse - between axon and cell body

Question 3

What are characteristics of electrical synapses?

Answer 3

Rare

In cerebral cortex, brainstem, and retina

No neurotransmitters

Gap junctions allow electric current to pass directly from cell to cell - free movement of ions

Impulse transmission is much faster compared to chemical synapse

Question 4

More connexons mean what for transmission in electrical synapses?

Answer 4

Faster transmission!

Question 5

What are chemical synapses?

Answer 5

Common

Use neurotransmitters or neuromodulators to transmit the signal from the presynaptic neuron to the other cell

Slower than electrical

Allows for complex signaling

Question 6

What are the parts of a chemical synapse and their characteristics?

Answer 6

Terminal bouton of presynaptic neuron - lots of mitochondria, some SER, synaptic vesicles contain neurotransmitters, and presynaptic membrane where neurotransmitter exocytosed

Synaptic cleft - space between membranes

Postsynaptic membrane - receptors for neurotransmitters

Question 7

What are the steps of synaptic transmission?

Answer 7

1. Action potential reaches presynaptic membrane, causing calcium channels to open on the presynaptic membrane
2. Influx of calcium into axoplasm of terminal bouton triggers exocytosis of neurotransmitters
3. Neurotransmitter diffuses across the synaptic cleft and bind receptors, which are ligand gated sodium channels
4. Influx of sodium into cytosol causes reversal of resting potential = depolarization of postsynaptic membrane
5. Cell membrane opens potassium channels to repolarize, briefly passes through a hyperpolarized state, and return to its original state

Question 8

What is a action potential?

Answer 8

Rapid sequence of changes in voltage/membrane potential

Question 9

What are the effects of neurotransmitter?

Answer 9

Either excitatory or inhibitory on a postsynaptic membrane

Excitatory - if binding postsynaptic receptors causes the target cell to initiate an action potential

Inhibitory - if binding postsynaptic receptors prevents the target cell from initiating an action potential

Question 10

What are neurotransmitters?

Answer 10

Bind receptors that are ligand gated ion channels

Acts directly and entire process in fast

Question 11

What are neuromodulators?

Answer 11

Binds receptors associated with G proteins or receptor kinases and activate second messengers

Indirect and process is slower

Question 12

What are the 4 classes of neurotransmitters and neuromodulators?

Answer 12

1. Small molecule transmitters - acetycholine, amino acids, biogenic amines (serotonin) and 3 catecholamines (dopamine, norepinephrine, epineprine)
2. Neuropeptides - endorphins, oxytocin; many are neuromodulators
3. Gasses - NO and CO; also neuromodulators
4. Other - neurotransmitter that bind to other types of receptors

Question 13

What does it mean that an axon is myelinated?

Answer 13

Axon surrounded by myelin

Question 14

What is a myelin sheath? What does it do?

Answer 14

Concentric layers of oligodendrocyte or Schwann cell membrane that the cells have wrapped many times around the axon

Insulates axons

Increases speed of electrical impulses as they are propagated down the axon

Question 15

What is the difference between CNS and PNS myelination?

Answer 15

CNS neurons may have:
Unmyelinated axons - no covering at all
Myelinated axons - myelinated by oligodendrocytes

PNS neurons may have:
Unmyelinated axons - partially surrounded by Schwann cells, Schwann cells can only myelinate one axon
Myelinated axons - myelinated by Schwann cells

Question 16

What are nodes of Ranvier?

Answer 16

Gaps in myelin along a single axon between adjacent oligodendrocytes or adjacent Schwann cells - uninsulated

Where electrical impulses regenerate, enabling much faster propagation down an axon

highest density of voltage gated sodium channels in entire nervous system

Question 17

What are clefts of Schmidt-Lanterman?

Answer 17

As Schwann cells wrap themselves around the axon, some bits of cytoplasm get caught/trapped in the concentric layers of membrane and these regions are called clefts of Schmidt-Lanterman

Question 18

What are myelinated regions between Nodes of Ranvier?

Answer 18

Internodes

Question 19

How do impulses travel along unmyelinated axons?

Answer 19

Impulses travel slowly along axon via continuous conduction - activation of next batch of sodium channels occurs in adjacent patch of cell membrane

Question 20

What does myelin enable, in regards to conduction?

Answer 20

Saltatory conduction - impulse is able to jump from node to node, rather than going along entire length

Question 21

What 3 CT sheaths compartmentalize the Peripheral nerves?

Answer 21

Endoneurium

Perineurium

Epineurium

Question 22

What is the endoneurium?

Answer 22

Thin layer of reticular CT surrounds a single axon and its myelin sheath

Made by Schwann cells

Contains fibroblasts, mast cells, and macrophages

Question 23

What is the perineurium?

Answer 23

Surrounds fascicles (bundles of adjacent axons)

Specialized CT, lined by several layers of epithelioid cells and surrounded by a basement membrane

isolates the neural environment by creating a blood nerve barrier

Question 24

What is the epineurium?

Answer 24

Loose (areolar) CT surrounds the entire nerve

Bundles all fascicles into one nerve

Contains the blood vessels and tiny nerves that supply a nerve - vasa nervorum and nervi nervorum

Question 25

What is the response of a PNS nerve fiber to injury? Anterograde change

Answer 25

Local changes occur right around the site of axon injury

Severed ends of axons retract away from each other

Cut ends grow membrane to prevent loss or axoplasm

Schwann cells proliferate and begin to break down myelin sheath - forms hollow cylinder surrounded by original axonal basal lamina and endoneurium

Fibroblasts and macrophages arrive to clean up debris in endoneurium and begin CT repair - macrophages phagocytose debris

Question 26

What changes occur in response to a nerve fiber injury and what is the process called?

Answer 26

Anterograde changes occur distal to the location of axon injury

Retrograde changes and neural regeneration occur proximal to the location of axon injury

Process is called Wallerian degeneration = axonal degeneration and elimination of debris

Question 27

What is the retrograde change when nerve injury occurs?

Answer 27

Soma swells and undergoes chromatolysis - reduction and redistribution of Nissl bodies within 1-2 bodies

Nissl bodies and nucleus move to periphery of soma

Protein synthesis begins in soma and axon stumps grows sprouts - grow toward Schwann cells and enter tube

Schwann cells on other end of tube continue to proliferate and grow towards the original target cell

As axon lengthens, Schwann cells begin to reform the myelin sheath around it

First sprout to reach the target forms the synapse, other sprouts degenerate

Question 28

What is essential for PNS nerve regeneration?

Answer 28

Soma must remain intact

Schwann cells, macrophages, and fibroblasts must be present

Original axonal basal laminal and some endoneurium must remain