BL 19 Flashcards
How many types of nerve connective tissue is there?
3 types
What is the name of the connective tissue in nerves?
- Epineurium
- Perineurium
- Endoneurium
How is the nervous system divided? What is included in each part?
- Can be divided into two parts:
- Central nervous system (CNS):
- Brain and spinal cord
- Relay neurones
• Peripheral nervous system (PNS):
- Cranial nerves and spinal nerves
- Sensory neurones and motor neurones
What is grey matter? Where is it found? What does it consist of?
Grey matter
• Peripheral in brain and in areas called ‘nuclei’
• Central in spinal cord (H- or butterfly-shaped) Consists of:
• Nerve cell bodies
• Dendrites
• Axon terminals
• Non-myelinated axons
• Neuroglia (support cells)
What is white matter? Where is it found? What does it consist of?
White matter
• Central in brain
• Peripheral in spinal cord
Consists of:
• Myelinated material
Concentrate on the spinal cord grey and white matter - label the parts of the spinal cord (in detail)
• The gray matter is roughly in the form of a butterfly
• Ventral horns, dorsal horns, grey commisure:
The anterior and posterior prongs are referred to as ventral horns (VH) and dorsal horns (DH), respectively. They are connected by the grey commissure (GC) - connects the two halfs of the grey matter.
- Nerves come out of the dorsal root
• The white matter contains nerve fibres that form ascending and descending tracts
• Blood vessels of the outer connective tissue layer (pia mater), the ventral fissure (VF), and some dorsal roots of the spinal nerves are visible in the section
Neurone - basic structure - which parts in CNS, which parts in the PNS?
- Normal complement of cell organelles
- Cytoplasmic projections – Many dendrites, single axon
- The main cell body (soma), dendrites, and proximal part of the axon are within the CNS
- Distal axon and arborisations are within PNS
- Often coated with insulation (myelin)
- In the CNS, the myelin for the axon is produced by, and is part of, an oligodendrocyte
- in the PNS, the myelin is produced by, and is part of, a Schwann cell
Oligodendrocyte = little branch cell
Types of neurones
Motor
Location: CNS to periphery
Function: to send signals to effector tissues
Sensory
Location: Periphery towards CNS
Function: to send environmental signals to integrative centre
Integrative
Location: CNS
Function: collate all information (and make decisions about where it’s going to go)
- Pyramidal cell: huge number of dendrites attached, takes all info, then sends it on to brain
- Intarneurones: Take info in, then sends on to another neurone that sends it on again to its final destination (target)
- Purkinje cell: bipolar body between dendrites and axon ends
Anaxonic
Location: retina (some parts of CNS)
Function: act as relays
Dendrites, no axons. Found in sense organs e.g. in retina. Dendrites interact with other dendrites
Cell bodies and types of neurones
• Located outside the CNS: Pseudounipolar (unipolar), bipolar, and postsynaptic autonomic neuron cell bodies are located outside the CNS
• Located in the CNS: Purkinje and pyramidal cells are restricted to the CNS; many of them have elaborate dendritic arborisations that facilitate their identification
(i.e. integrative neurones)
• The majority of nerves in the CNS are interneurons
(i.e. most motor and sensory)
Variations of neurones - lots of variations in neurone structure
Neurone cell body structure in the CNS
What do neurone cell bodies have a lot of in the CNS?
- Lots of rough endoplasmic reticulum
- Lots of Golgi apparatus
- Many free ribosomes (to boost the number of ribosomes)
- Lots of mitochondria
This is because the nerve cell spends a lot of time
- making substances that then form neurotransmitters and packaging them into vesicles
- making enzymes that are involved in making neurotransmitters
- making poteins that are involved in the transport of the vesciles and neurotransmitter from the cell body down the axon.
What is the axon hillick?
Specialised part of the neurone connected to the axon
What is a nissl body?
Nissl body, also known as Nissl substance and Nissl material, is a large granular body found in neurons. These granules are of rough endoplasmic reticulum (RER) with rosettes of free ribosomes, and are the site of protein synthesis.
Visualise neurone cell body
What are the two types of transport system down an axon? Briefly how do these systems work?
- Anterograde and retrograde transport
- Anterograde and retrograde vesicles use a microtubule ‘shuttle’ system to move from soma to synapse and back
Explain how the anterograde and retrograde transport system works
Microfilament is present.
Neurofilament attach to the microfilament.
Microtubules attach to the neurofilament
- In the anterograde direction, kinesin is attached to the microtubules
- In the retrograde direction, dynactin/dynein is attached to the microtubule
Both kinesin and dynactin/dynectin move down the microtubule by a ‘walking’ fashion. They attach one leg, then detach the other etc. This process requires ATP
The microtubules assemble when they need to move cargo down the axon
Anterograde - movement from cell body (soma) to the terminal
ANTEROGRADE -
- The microtubule is attached to kinesin
- The kinesin is attached to vesciles and mitochondria
- Vesicles contain enzymes involved in making neurotransmitter inserted in their cell membranes. As the vesicles travels down the length of the axon, the neurotransmitter is synthesised.
Retrograde - movement from terminal to cell body
- The microtubule is attached to dynactin/dynein
- The dynactin/dynein is attached to empty vesciles (no neurotransmitter in it). Mitochondria are not transportted back (stay at the terminal). The vesicles are then either: recyled or broken down
Neurotransmitter synthesis
Immature vesicle contains only enzyme in its membrane
As it travels the length of the axon it starts to synthesise the neurotransmitter (number 2 in the diagram attached)
Vesicle has two fates:
- Reprocess the material from the synaptic cleft
- Lost to neurolemma (when vesicles fuse with the membrane to release the neurotransmitter, it can be lost)
Note - neurotransmitter is reused in the terminal of the axon, it is not taken back up to the cell body
Transport using microtubules (as stated on earlier slides)
Different types of synapses in the CNS
a. Axodendritic or axosomatic - nerve axon bouton and muscle or gland.
directly to the plasma membrane of nerve or cell
b. Axodendritic - axon bouton and dendritic spine. axon terminal synapses with a dendritic spine
c. Axoaxonic - two axon boutons.
synapse at the axonic bouton - The axoaxonic synapse may enhance or inhibit the axodendritic (or axosomatic) synapse
d. Dendro-dendritic - two dendrites
(important if the nerve doesn’t have an axon)
e. Axo-axonal - impinging dendritic/axonal synapse usually inhibits other inputs (this can occur at the cell body or at the axon hillick)
in the diagram -
M means muscle
G means gland
D means dendrite
DS means dendrite spine
More in depth info about the 4 types of connective tissue in peripheral nerves
Endoneurium
• Loose connective tissue
• Surrounds single nerve cells
Perineurium
• Loose connective tissue (slightly thicker than endoneurium)
• Maintains ionic composition
• Surrounds clusters of axons (fascicle)
Epineurium
• Dense irregular connective tissue (lots of collagen)
• Separates different types of nerves and fills spaces between fascicles
Paraneurium
• Fascia that separates nerves from surrounding structures
• Holding the nerves and blood vessel bundles together
• Also acts as a ‘shock absorber’ between the nerves and surrounding tissues, as most of the peripheral nerves are very close to bone
How many types of peripheral nerves are there?
3 types:
- Somatic motor neurone (myelinated)
- Somatic sensory neurone (myelinated)
- Autonomic neurone (mix between unmyelinated and myelinated - cover this in lecture 20)
Peripheral nerves - histology
Look at pics attached
Multiple cells types with different diameters
• Some covered with myelin (A)
• Some not (*) - they have a thin membrane around them instead
Diagram on the left is a single axon:
- Mt is the mitochondria
- small dots - are the transport mechanisms discussed in lecture 1 (transport proteins, microtubules and microfilaments)
- the myelin (made up of Schwann cell cytoplasm)
- M stands for myelin
Peripheral nerve structure - myelinated nerves and unmeylinated nerves - what is this?
Unmyelinated:
Schwann cells are at the centre. There is lots of cytoplasm and the axons are ‘sitting’ in the edges of the cytoplasm, in a cleft, the axons are not shielded by anything in this structure.
All nerves in the ‘group together’ will work as a functional group = they all have the same function e.g. all will be sensory or all will be motor
In pic on top right, there are 5 axons embedded in the Schwann cell
Histology - difference between unmyelinated and myelinated nerves?
How does myelination occur?
- The axon sitting in a groove is surrounded by a Schwann cell
- The edges of the Schwann cell start to move around the axon (these edges are called mesaxons). The mesaxon membrane initiates myelination by surrounding the embedded axon
- Axon puts protein on it’s surface, which drives the movement. A sheet-like extension of the mesaxon membrane then wraps successively around the axon, forming multiple membrane layers
- Cytoplasm is extruded from between the two apposing plasma membranes of the Schwann cell, which then become compacted to form myelin (19-20 rounds)
M-A means mesaxon
A means axon
O abaxonal domain - outside abaxonal domain
I abaxonal domain - inside abaxonal domain
Electrical conduction -
Myelination vs unmyelinated nerve:
Myelinated nerve much faster due to saltatory conduction
Larger diameter = faster
This is because you can get more material in and out at the node of Ranvier during condution, this allows larger internodal distance.
Larger internodal distance vs shorter internodal distance - this is the distance between nodes of Ranvier. Larger the internodal distance = faster conduction
Unmyelinated nerve cells - affecting speed of conduction
- The individual axons (A) are engulfed by the cytoplasm of a Schwann cell
- The arrows indicate the site of mesaxons. In effect, each axon is enclosed by the Schwann cell cytoplasm, except for the intercellular space of the mesaxon (Some of the axon, isn’t insulated by the Schwann cell)
- In the upper part of the micrograph, myelin (M) of a myelinated nerve is also evident
- Slower propagation of action potential (sensory neurones and some autonomic neurones are often unmyelinated, so this is why they tend to be slower compared to myelinated motor neurones).
Oligodendrocyte
• These cells are the reason why conduction is much faster in the CNS compared to the PNS (whereas Schwann cells does all of the myelination in the PNS)
• The oligodendrocyte does the same thing as a Schwann cell, but in the CNS
• Cytoplasmic processes from the oligodendrocyte cell body form flattened cytoplasmic sheaths that wrap around each of the axons
• The relationship of cytoplasm and myelin is essentially the same as that of Schwann cells
• Key difference: wraps around more than one axon simultaneously. Wrapping legnth is therefore much longer than with a Schwann cell, internodal distance is therefore greater, as myelin stretches longer alonge the axon = this is why conduction is much faster in the CNS compared to the PNS
- An oligodendrocyte also interacts with several axons at one time, whereas, a Schwann cell interacts with only one axon.
Astrocyte
• Star-like structure
• Have ‘perivascular (feet on blood vessels)/perineural feet (feet on axon or node of Ranvier)’ that contain gap junctions
- biophysical support for endothelial cells
- transport of nutrients (lactate, ketones) from blood to nerve cells
• Regulate nerve impulses by releasing gamma amino butyric acid (near to Node of Ranvier) - this is an inhibitor of neurones
• Contribute to the blood-brain barrier (the perivascular feet sit over the endothelial cells)
• Many other [conjectured functions] e.g. calcium regulation?
Satellite cells
ONLY FOUND IN PNS (NOT FOUND IN CNS)
- different to satellite cells in muscles!
- ‘look after the cell body’. They produce nutritional material for the cell to use
- Only found on sensory neuron cell bodies
- Only in dorsal root ganglion
- Cover almost the entire surface Axon
- May have similar functions to an astrocyte.
Microglial cell
Only CNS
• Large cells with elongated nucleus and relatively few processes emanating from the cell body
• Found throughout the CNS
• Resident macrophage (within the brain)
– Immune function (phagotitic cell)
– Remove damaged nerve cells
– Sense increased K+ ions (damage nerve cells release K+, will move towards the damaged nerve)
• NB. Also thought to digest protein tangles that are associated with senile dementia and Alzheimer’s disease. These cells also digest any damaged proteins that you have produced when you have been awake.
NB - it’s flattened nucleus
Ependymal cell - position
(just CNS!)
Ependymal cells line the spinal canal
Spinal canal is located at the end of the ventral medium fissure (VF on the diagram)
CC - means central canal
Ependymal cell - Structure
Ependymal cells are of derived from the neural crest and are epithelial tissue (during embryonic development)
They look like columnar epithelial cells lining of the spinal canal and ventricles of the brain, but they are actually simple cuboidal
They are joined by a junctional complex (JC) that separates the lumen of the canal from the lateral intercellular space
The apical surface has both cilia (C) and microvilli (M)
- They have very large nucleus
- They have tight junctions near their apical surface, it stops the fluid from the central canal from leaking into the central nervous system
- It has the ability to open and close, to let fluid in and out as it needs to
Ependymal cell - Function
• Synthesise and secrete CSF (cerebrospinal fluid) in the ventricles (choroid plexus)
• Cilia move cerebrospinal fluid (CSF) through the ventricles (ventricles in the brain are also lined with the ependymal cells) to the spinal cord
• Microvilli absorb CSF for removal of pathogens
- Present pathogens to microglial cells (looking for pathogens) and astrocytes
• Modified tight junctions between ependymal cells control fluid release into brain (preventing cerebral oedema)
- Molecular and cellular contents of brain can be monitored – ‘spinal tap’ (CSF - lumber puncture)
Multiple sclerosis (where does it effect?)
CNS NOT PNS
• Remitting and relapsing disease (go into remission, then relapse etc…)
• Degenerative
• Caused by autoimmune degradation of myelin (myelin in the CNS)
- [Probably against EBV - Epistein Bar Virus. Antibodies that destroy the CNS myelin, are probably against the EPV - not much evidence to support this though]
Symptoms:
- Fatigue
- Vision problems (diplopia)
- Slurred speech (dysarthria)
- Numbness and tingling sensations (paraesthesia) - Mobility issues (muscle spasms)
- Urinary retention
- Constipation
All symptoms caused by loss of conduction velocity. As the myelin is breaking down, detroying the internodal distance between the nodes of Ranvier. During remission, there is some repair of the nerve.
