Nervous Tissue II Flashcards
What is a synapse and the cells involved?
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
What are classifications of synapses? Describe each (x4)
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
What are characteristics of electrical synapses?
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
More connexons mean what for transmission in electrical synapses?
Faster transmission!
What are chemical synapses?
Common
Use neurotransmitters or neuromodulators to transmit the signal from the presynaptic neuron to the other cell
Slower than electrical
Allows for complex signaling
What are the parts of a chemical synapse and their characteristics?
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
What are the steps of synaptic transmission?
- Action potential reaches presynaptic membrane, causing calcium channels to open on the presynaptic membrane
- Influx of calcium into axoplasm of terminal bouton triggers exocytosis of neurotransmitters
- Neurotransmitter diffuses across the synaptic cleft and bind receptors, which are ligand gated sodium channels
- Influx of sodium into cytosol causes reversal of resting potential = depolarization of postsynaptic membrane
- Cell membrane opens potassium channels to repolarize, briefly passes through a hyperpolarized state, and return to its original state
What is a action potential?
Rapid sequence of changes in voltage/membrane potential
What are the effects of neurotransmitter?
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
What are neurotransmitters?
Bind receptors that are ligand gated ion channels
Acts directly and entire process in fast
What are neuromodulators?
Binds receptors associated with G proteins or receptor kinases and activate second messengers
Indirect and process is slower
What are the 4 classes of neurotransmitters and neuromodulators?
- Small molecule transmitters - acetycholine, amino acids, biogenic amines (serotonin) and 3 catecholamines (dopamine, norepinephrine, epineprine)
- Neuropeptides - endorphins, oxytocin; many are neuromodulators
- Gasses - NO and CO; also neuromodulators
- Other - neurotransmitter that bind to other types of receptors
What does it mean that an axon is myelinated?
Axon surrounded by myelin
What is a myelin sheath? What does it do?
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
What is the difference between CNS and PNS myelination?
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
What are nodes of Ranvier?
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
What are clefts of Schmidt-Lanterman?
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
What are myelinated regions between Nodes of Ranvier?
Internodes
How do impulses travel along unmyelinated axons?
Impulses travel slowly along axon via continuous conduction - activation of next batch of sodium channels occurs in adjacent patch of cell membrane
What does myelin enable, in regards to conduction?
Saltatory conduction - impulse is able to jump from node to node, rather than going along entire length
What 3 CT sheaths compartmentalize the Peripheral nerves?
Endoneurium
Perineurium
Epineurium
What is the endoneurium?
Thin layer of reticular CT surrounds a single axon and its myelin sheath
Made by Schwann cells
Contains fibroblasts, mast cells, and macrophages
What is the perineurium?
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
What is the epineurium?
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
What is the response of a PNS nerve fiber to injury? Anterograde change
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
What changes occur in response to a nerve fiber injury and what is the process called?
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
What is the retrograde change when nerve injury occurs?
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
What is essential for PNS nerve regeneration?
Soma must remain intact
Schwann cells, macrophages, and fibroblasts must be present
Original axonal basal laminal and some endoneurium must remain
