IC1 Histology of the CNS Flashcards
What are the functions of specialized nervous tissue in the brain?
- Are excitable → receive and generate stimuli
- Are conductive → transmit impulses
Not many tissues in the body do both of these!
What are the 2 types of cells in the brain? Explain their function
Neurons
Structural and functional unit of the nervous system
Generate/Conduct impulses and are excitable
Non-neuronal cells (glial cells)
Usually non-conducting
Support and protect neurons by providing nutrients, oxygen
10X more glial cells than neurons
What is the structure of a neuron made up of?
- Cell body / soma
- Axons
- Dendrites
Briefly describe how information can be transmitted to the target
Information comes from other neurons to dendrites → integrated in cell body → action potential generated at axon hillock (end of cell body) → travels down axon to synaptic terminals & continue on to subsequent neurons → target
The cell body of a neuron contains
- nucleus
- various cytoplasmic organelles, cytoskeletal elements, and inclusions
- including Golgi complex (near nucleus)
- & mitochondria (throughout cytoplasm), rough endoplasmic reticulum
everything the cell needs to perform its functions are in the cell body
What are axons?
- nerve fibers
- conduct impulses away from the cell body
- most axons are long slender processes that arise from the axon hillock & branch at the distal / terminal end
Length of an axon may vary from _____ to _____
as short as <1 to as long as 2m
Name the specialised structures of neurons
Axon
Dendrite
Dendritic spine
(soma components like organelles are universal - also in liver, kidney cells)
What is the main function of an axon?
relay the electrical signal to the next cell
What does the axonal cytoplasm (axoplasm) lack?
- ribosomes, RER, and Golgi apparatus
- as axon terminus lacks components for synthesizing new proteins or degrading old ones, materials must be transported back and forth between the cell body and the terminus
Explain the 2 types of transport that occurs along axons
Fast
speed of 50 to 400mm/day
transports cytoplasmic proteins and macromolecules required for metabolic and synaptic activity, like receptors (for daily functioning of axon)
Slow
speed of 1 to 4mm/day
transports cytoskeletal components down the axon (for maintenance of axon)
Both of the above are anterograde transport (forward, from cell body into axon)
Whereas retrograde is from axon into cell body, often for used materials like degrading proteins
What is the function of dendrites?
- Have receptors that sense incoming neurotransmitters and convert it into an electrical signal
- This electrical signal is conducted towards the cell body
Describe the structure of dendrites
- Relatively short
- Highly branched
- Contain all cytoplasmic components found in the cell body except Golgi apparatus (so no need for much retrograde or anterograde transport)
What are dendritic spines & their purpose?
- Protrusions coming out from dendrites
- ↑ SA → able to receive more information (mostly glutamatergic inputs)
- Majority of synapses are formed in dendritic spines
Define synapses
- regions of functional apposition where
- impulses are transmitted from one neuron (presynaptic) to another neuron (postsynaptic) or
- from a neuron to an effector cell (muscle)
What are the 2 types of synapses?
- Chemical (communication via neurotransmitters)
- Electrical / gap junctions (communication via electricity)
Describe what chemical synapses are made of
- Presynaptic axon terminal → a small, knoblike termination of the transmitting neuron
-
Postsynaptic cell with receptors sensitive to specific neurotransmitters
(upon binding, ion channels open → converted to electrical signals again & process continues until target site)
What is the main characteristic of a chemical synapse?
Contains synaptic vesicles filled with a neurotransmitter e.g. GABA, glutamate depending on the type of presynaptic cell
Most signalling in the brain is ______ except ______
- Electrical
- In the space between axon terminal & postsynaptic compartment
How does an electrical signal get converted to a chemical one at the synapse?
- When action potential arrives at an axon terminal, voltage gated Ca2+ channels are activated
- → Opens, allowing Ca2+ ions influx → ↑ systolic Ca2+ levels
- → Triggers exocytosis of neurotransmitters into synaptic cleft
What are presynaptic membranes?
A thickened region in the plasmalemma of the presynaptic axon terminal which contains voltage gated Ca2+ channels
What are synaptic clefts?
A 20 to 40nm wide space separating the pre- and postsynaptic membranes, across which a neurotransmitter diffuses
What are postsynaptic membranes?
A thickened region in the plasmalemma of a receiving dendrite, which contains neurotransmitter receptors
Describe the neurochemical processes that occur at excitatory synapses
- Neurotransmitter binds to its receptors
- → Na+ channels in postsynaptic membrane open
- → Depolarization of the membrane & action potential is generated in postsynaptic cell
Name 2 glutamate receptors
- AMPA
- NMDA
Excitatory transmissions always go ______
Forward
Describe the neurochemical processes that occur at inhibitory synapses
- Neurotransmitter binding opens K+ or Cl- channels in the postsynaptic membrane
- → Membrane hyperpolarizes (as Cl- is negatively charged)
- → No action potential in postsynaptic neuron
Name a GABA receptor commonly involved in inhibitory transmission
GABA-A
What are 3 types of glial cells in the CNS & their % composition?
- Astrocytes 60-80%
- Oligodendrocytes 25%
- Microglia 5-10%
______ are the largest neuroglial cells
- Astrocytes
- Have numerous processes with expanded end-feet (pedicles) that terminate on capillaries or pia mater
The 2 types of astrocytes are ______
- Fibrous
- Protoplasmic
Describe the location & structure of fibrous astrocytes
- Primarily in the white matter
- Are long, spindly processes w few branches
Describe the location & structure of protoplasmic astrocytes
- Grey matter
- Closely apposed to neuron cell bodies
- Are thick, lightly branched processes
Fibrous astrocytes are located in the _____ matter
White
Protoplasmic astrocytes are located in the _____ matter
Grey
What are the functions of Astrocytes?
- Structural support to neurons (specifically to synapses), scar formation 🥓
Regulate composition of intercellular environment & entry of substances into it (potassium) 🥓 - Metabolize neurotransmitters via glutamate glutamine shuttle (e.g. if diffused out of synapse) 🥓
- Mediate exchange of nutrients and metabolites between the blood and neurons 🥓
- Support BBB
- Secrete nerve growth factors
- Water transport
- Excess transport – cerebral edema
How is the role of astrocytes related to epileptic seizures?
- Astrocytes, by performing their usual function, prevent epileptic seizures
- At a high density, astrocytes express potassium channels
- When neurons are active, they release potassium → ↑ local extracellular concentration
- As astrocytes are highly permeable to potassium, they rapidly clear the excess accumulation of potassium in the extracellular space
- If uncleared → epileptic seizures
How do Astrocytes metabolize neurotransmitters?
- Astrocytes express plasma membrane transporters such asglutamate transportersfor neurotransmitters likeglutamate, ATP, andGABA
- If these neurotransmitters are not cleared up after release → enter the extra synaptic spaces → activate neighbouring synapses
How do Astrocytes mediate exchange of nutrients and metabolites?
- Astrocytes containglycogenand are capable ofgluconeogenesis
- They also store and release glucose for normal functioning of the brain
The 2 most important functions of Astrocytes are?
- Regulate blood flow in the brain, through a process called “end feet” They do this by expanding or contracting the blood vessels → control supply of oxygen and nutrients to the brain
- Regulate homeostasis of neurotransmitter glutamate at synapses
______ are the predominant type of neurogllial cell in the white matter
Oligodendrocytes
What is the function of Oligodendrocytes?
White matter
Myelin sheath formation around nerve fibers (axons of CNS) in the white matter 🥓
Grey matter
Oligodendrocytes are closely a/w neuron cell bodies, functioning as satellite cells
What is the purpose of myelin sheaths?
- Make movement of impulses along axons much faster
- Signal skips from one node of Ranvier to another, instead of slowly waiting for every channel to open & close
- So less energy will be needed bc need open less channels
🥓 “Oligodendrocytes only form myelin sheaths around axons of the CNS”, is this true or false?
True
In axons of the PNS → done by Schwann cells
Describe Oligodendrocytes
- Cell marker - CNPase
- A few processes
- 60-70 % of glial population
- Express Nogo-A, a myelin
associated neurite-outgrowth
inhibitor important for the inhibition of axonal regeneration following injury or ischemia in CNS
What is the marker and origin of Microglia cells?
- Marker- lectin, OX42 (complement type 3 receptor)
- Monocyte origin, but can continuously renew themselves & persist throughout life w/o replenishment from monocytes
What are the functions of Microglia cells?
- Primary immune cells of CNS
- Are small phagocytic cells that enlarge & become mobile after injury to the CNS
- Engulf debris, foreign particles & dead cells (“brain macrophages 🥓”)
- Are immunocompetent 🥓 → act as antigen presenters to T cells
- Neuroinflammation - secrete proinflammatory mediators like TNF-α, IL-1β in response to foreign particle / debris → activate & signal more Microglia to be activated
Why is it important for Microglia cells to act as antigen presenters to T cells?
- Most antibodies our body makes do not cross the blood brain barrier into the nervous tissue
- So it is up to the microglia to be able to recognize foreign particles and activate immune T cells
In what diseases are Microglial cells implicated in?
Diseases a/w neuronal degeneration
* Alzheimer’s disease
* Parkinson disease
* Cerebral ischemia (stroke)
