Histology Flashcards
1
Q
Discuss the characteristics of nerve tissue
A
- provides rapid and specific communication between organs in the body
- 2 major components of nerve tissue are high specialized cells called neurons and support cells called neuroglia/glial cells
- neurons are electrically excitable
- functional categories of neurons
- sensory
- interneurons
- motor
2
Q
Describe the structure of a neuron.
A
- large, rounded, euchromatic nucleus with prominetn nucleolus
- perikaryon varies in size (5-135um)
- Nissal bodies formed from well developed RER (dense structures that are parallel arrays of RER)
- well-developed Golgi complex
- good RER and Golgi reflect the need for the neuron to produce membrane and neurotransmitter in large quantities
- many mitochondria
- lysosomes in cytoplasm
- lack centrioles (can’t divide)
- cytoskeleton made of neurofilaments, microfilaments, and microtubules
3
Q
Describe the axon
A
- each neuron has only one axon per cell
- axons convey signals from perikaryon to the next neuron or effector cell, ending with axon terminal
- usually fairly long with constant diameter
- begin at axon hillock
- Nissal bodies are not present in axonal cytoplasm, nor are ribosomes. does have smooth ER
- axons insulated with myelin sheath
- enclosed by the continuation of the plasma membrane called axolemma
- cytoskeleton of axon formed by numerous microtubules and neurofilaments
4
Q
Describe axonal transport
A
- anterograde flow
- from perikaryon to periphery
- allows transport of actin filaments, proteins, organelles, such as mitochondria, and vesicles
- kinesin is the motor used
- slow axonal transport - 1-6mm/day moves tubulin molecules, actin molecules, and proteins that form neurofilaments from the perikaryon to the end of the axon
- fast axonal transport - 100-400mm/day moves membrane bound organelles, such as SET compartments, synaptic vesicles, and mitochondria
- retrograde flow
- distal part of axon towards perikaryon
- transport materials taken up by endocytosis at axon terminal
- some viruses and toxins use this pathway, eg herpes simplex, rabies, and tetanus
- motor for this type of transport is dynein
5
Q
Discuss dendrites
A
- most neurons have several dendrites per cell
- designed to deliver signal from periphery to the perikaryon
- numerous, thick, short, and tapered
- branch profusely to form dendritic tree, to increase area for synaptic contacts (up to 200,000 contacts per tree)
- surface is covered with dendritic spines where synapses with axonal processes of other neurons are formed.
- mushroom shaped head where most post-synaptic receptors are located
- not myelinated
- cytoplasmic composition similar to that of perikaryon
- have ribosomes and RER, but no Golgi apparatus
6
Q
Classify neurons based on shape and number of cellular processes
A
- psuedounipolar
- primarily sensory neurons
- single large process from perikaryon that branches into peripheral and central processes
- PP reaches sensory area and collects info
- info delivered to the CNS via CP
- conduct as one axon
- typical location: dorsal root ganglia and some cranial nerve ganglia
- psuedo bc initially created as 2, but fuse into 1 process
- bipolar neurons
- sensory neurons
- limited distribution
- found in major sense organs: eye retina, olfactory mucosa, cochlea and semicircular canals of the inner ear
- 2 processes from cell body, axon and dendrite
- dendrite acts as sensory receptor
- axon delivers to CNS
- multipolar neurons
- most common type of neuron
- both motor and interneuron belong in this type
- have one axon and many dendrites
- depending on length, get classified as
- Golgi type I cells - long axon - large motor neurons in motor nuclei of CNS
- Golgi type II cells - short axon - smaller interneurons found in CNS
7
Q
Describe the basic electrophysiology of the nerve tissue
A
- plasma membrane like electric capacitor
- similar to muscle cells’ sarcolemma
- voltage on inner side of plasma membrane is negative ( ~- 70mV) relative to outer side so there is negative membrane potential in resting cell
- possible because Na+ ions are actively pumped out of cell
-
action potentials are brief positive going changes in the membrane potential that are propagated along the length of the membrane at a speed up to 120m/sec
- considered waves of depolarization
- as they travel, they open the voltage sensitive channels and let Na+ diffuse into the cell, decreasing the membrane potential
- membrane is hyperpolarized when the membrane potential becomes even more negative, making the membrane more difficult to depolarize
8
Q
Describe the structure of a synapse
A
- presynaptic terminal
- contains synaptic vesicles with neurotransmitter capable of binding to receptor to generate wave of depolarization
- have mitochondria
- no RER, but could have smooth ER
- synaptic cleft - between plasma membranes
- postsynaptic region
- contains receptors for neurotransmitters
9
Q
Classify synapse as either electrical or chemical
A
- electrical
- represented by gap junctions
- allow direct passage of ions from one cell to another to transmit wave of depolarization
- chemical
- principal type found in mammals
- no protoplasmic continuity between the two cells
- the signal is transmitted by release of a chemical (neurotransmitter) by one cell
- binding to receptor causes either depolarization (excitatory) or hyperpolarization (inhibitory)
10
Q
Describe the impulse transmission in a chemical synapse
A
- actional potential propagated along the membrane of the presynaptic cell from perikaryon towards axonal terminal
- reaches presynaptic terminal, opens Ca++ channels briefly
- influx of Ca++ causes vesicles to migrate to and fuse with the membrane (causes exocytosis)
- neurotransmitter diffuses across the cleft
- neurotransmitter bound by receptors on membrane, starting local depolarization of postsynaptic cell
- extra plasma membrane formed from synaptic vesicles fusing is removed by endocytosis using clathrin coated vesicles
- neurotransmitter deactived by 2 ways
- recapture - high affinity reuptake - for most catecholamines; NT is reincorporated by endocytosis into vesicles that are ready for repackaging
- degradation - enzymes break down remaining NT that is left in the cleft; ACh is broken into acetate and choline in the cleft
- clinically - inhibition of the enzyme that breaks down NT norepinephrine or inhibition of high-affinity reuptake, has beneficial effect in tx of depression
11
Q
Discuss morphotypes of synapses
A
- differentiate based on connections
- axodendritic - connect bw axon and dendrite
- axosomatic - connect bw axon and perikaryon
- axoaxonic - connect bw axon and another axon (commonly find inhibitor synapses)
-
motor-end plate - represents neuromuscular junction and is specialized type of synapse consisting of
- axon terminal that contains presynaptic vesicles with NT ACh
- synaptic cleft bw nerve cell and muscle cell
- sarcolemma of a muscle cell forms multiple junctional folds in the area of the motor end plate which hold the receptors for ACh
- neuroglandular junction - connects axon to gland
12
Q
Discuss support cells in the PNS
A
- Schwann cells
- form lipid layer called myelin sheath that surrounds axons in PNS
- “envelope” unmyelinated axons
- myelin sheath isolates axon from the surrounding tissue and provides electrical insulation, necessary for rapid conduction of electrical impulses
- support both myelinated and unmyelinated nerve fibers
- myelinated - single axon wrapped in lipoprotein complex - saltatory conduction - fast
- unmyelinated - several axons enveloped into simple clefts of Schwann cell, in middle of nerve bundle; action potential is wave-like
- Satellite cells
- found in ganglia of the PNS
- surround bodies of individual neurons
- create microenvironment and provide weak electrical insulation
- do not have myelin
- provide a pathway for metabolic exchange necessary for the neurons
13
Q
Describe the myelin sheath
A
- created by Schwann cells
- wraps around axon, plasma membrane wraps with it, squeezing the cytoplasm out
- forms gaps where there are interruptions in the myelin sheath - nodes of Ranvier
- represent spaces in adjacent Schwann cells
- axolemma of myelinated nerve fibers in the area of nodes of Ranvier have high concentration of Na+ channels
- action potentials travel via saltatory conduction, meaning membrane is only depolarized at the NoR.
- myelin sheath insulates well enough to allow depolarization at one node to elevate the voltage at the next node to the level necessary to generate an action potential
- NoR are also axoaxonal synapse sites and branches
14
Q
What are the support cells of the CNS?
A
neuroglia
- astrocytes
- oligodendrocytes
- microglial cells
- ependymal cells
15
Q
Describe astrocytes and discuss a clinical correlation
A
- largest neuroglial cells, support neurons and CNS
- granular cytoplasm
- extend b/t neurons and capillaries
- move metabolic substances b/t blood and nerve cells
- with endothelial cells of blood capillaries, they form the blood-brain barrier
- stain positive for glial fibrillary acidic protein (GFAP)
- 2 main types of astrocytes
- protoplasmic astrocytes - found in GRAY matter of the brain; numerous, short, branching processes, form structures called perivascular feet along blood capillaries
- fibrous astrocytes - more prominent cytoskeleton than protoplasmic, found in WHITE matter of the brain; fewer processes with less branching
- tumors derived from astrocytes are astrocytomas - most common in the brain, representing 20% of brain tumors. astrocytes give rise to 80% of tumors that originate in the brain
- in case of local damage to the brain, astrocytes are responsible for process called gliosis, resulting in glial scar
