Histology Of The Nervous System Flashcards
Describe the passive functions of glial cells
- Uptake and degradation of extracellular neurotransmitters 2. Homeostasis of extracellular K+ levels 3. Neuronal energy supply 4. Contribution to the blood-brain barrier 5. Injury and immune responses
What is the difference between white matter and grey matter?
White matter consists of neuronal axons and glial cells covered by a fatty white myelin sheath.
Grey matter is comprised of the neuronal cell-body/soma, dendrites and their associated glial cells
In what ways are neurons electrically active?
The unequal distribution of charged molecules across the membrane of a neuron creates an electrochemical gradient capable of electrical signalling. This gradient is established by a variety of Na/K ATPase pumps and ion channels.
There are graded potentials ( passive and localised electronic spread of current - primarily dendritic/cell body inputs); and action potentials ( axonal output signal across long distances)
Output signal occur via the release of neurotransmitters across a synapse to a post-synaptic neuron/cell.
What are the histological characteristics of neurons?
- Electrically Active
- Morphologically Distinct
- Exhibit Rapid Communication
- Signal Over Long Distances
- Specialised Types
- High Levels of Protein Synthesis
In what ways are neurons morphologically distinct?
Neurons are of various cell shapes and configurations.
Common to all neurons, however, is the presence of dendrites, a cell body/soma, axon and axon terminals.
The axon represents the majority of total cell volume of a neuron - thus, random damage often affects the axon
Axonal transport is critical in supplying cell body derived molecules (e.g. neurotransmitters and proteins) to the axon terminals as the cell body/soma is the metabolic centre of the neuron cell.
How can the speed of conductance along an axon be increased?
Myelination and increased diameter of the axon
Do neurons exhibit low or high levels of protein synthesis?
Neurons exhibit high levels of protein synthesis.
Neurons require a large volume of protein production to synthesise ion channels, receptors and cytoskeletal proteins
Histologically, **Nissl Bodies **represent the intense granular staining of abundant ribosomes and rough endoplasmic reticulum that are responsible for protein production.
What cytoskeletal proteins are important in neurons; and what roles do they play within the cells?
- Actin (assembly/dissassembly facilitates neuronal shape changes to spines and growth cones)
- **Intermediate Filaments **(cellular structure and organisation)
- **Microtubules **(Tubulin polymers involved in axonal transport)
List the passive support functions of astrocytes in the CNS
- Neurotransmitter uptake and degradation
- K+ Homeostasis
- Neuronal Energy Supply
- Maintainance of the BBB
- Injury response and recovery
List the active functions of astrocytes
- Modulation of neuronal function
- Modulation of blood flow
Are glial cells excitable?
Glia demonstrate modulations in intracellular calcium - known as calcium waves.
These calcium waves are initiated by neurotransmitters (ATP and glutamate especially), trauma, inflammatory mediators and spontaneously.
These calcium waves are a form of excitability that enables communication between glial cells and other CNS structure including neurons and blood vessels. Synaptic vesicles and exocytosis is observed in glial cells is associated to calcium waves - demonstrating potential for communication with surrounding structures.
Calcium waves inhibit neurons - mechanism involves the release of ATP from glia following calcium wave stimulation.
Can astrocytes regulate vascular tone?
Astrocytes regulate vascular tone via calcium waves - causing vasoconstriction or vasodilation. Thus, may play a role in CNS blood flow. Astrocyte processes are intimately wrapped around blood vasculature
Describe the role of myelination
Oligodendrocytes are responsible for myelinating axons within the CNS; Schwann cells in the PNS.
Myelination provide electrical insulation to neuronal axons to provide significantly faster saltatory conduction. Faster conduction also provided via increased axon diameter.
Myelin sheath only covers parts of an axon - there are small gaps called Nodes of Ranvier between myelinated sheath sections that facilitate saltatory conduction
Oligodendrocytes extend multiple processes that wrap around parts of several axons; whereas, Schwann cells wrap around only one axon.
Oligodendrocyte cells are the predominant glial cell type of white matter in the CNS
Describe the role of microglia in the CNS
The CNS is immune privileged; protected from the outside world via highly specialised barriers. Despite this, the CNS has microglial immune cells that aid in the defence and repair of the CNS to damage
Microglia are localised defence cells derived from bone marrow and are specialised macrophages constituting 5-20% of cells in a mouse brain.
These immune cells constantly survey the CNS for signs of infection, damage or abnormalities
Microglia change and react rapidly in response to inflammation or injury to up regulate the release of cytokines and growth factors.
May be involved in development and disease process
Describe the organisation of connective tissue layers within peripheral nerves
Peripheral nerves are composite structures, often containing multiple fascicles/bundles of nerve fibres
The epineurium is the outermost outer collagenous tissue that encloses the entirety of peripheral nerve structures.
The perineurium is a collagenous tissue that encloses a single fascicle of nerve fibres
Within each fascicle, each single nerve fibre and associated Schwann cell is loosely surrounded by vascular supporting endoneurium.
