Neurons and Glial Cells Flashcards
What are connections between neurons called?
synapses
What is the nucleus?
a cluster of neurons with similar functions (ganglia)
How many more glial cells are there than neuronal cells?
10x
What is nervous tissue specialised for?
to receive and generate stimuli (excitable) and transmit impulses (conductive)
What are neurons made up of?
cell body (soma) with extensions of axons and dendrites
What do neurons do?
create and maintain a potential difference from the inside to the outside (inside is more negatively charged compared to the outside)
What are the structural and functional units of the NS?
neurons; they can generate and conduct APs and are excitable
What is the inside of the neuron separated from the outside by?
the neuronal membrane
How much longer are axons than dendrites?
2000x
What is the diameter of the cell body of a typical neuron?
20µm
Which organelles are within the cell body of a neuron?
- nucleus
- RER
- SER
- Golgi apparatus
- mitochondria
What does the neuronal membrane serve as?
a 5nm barrier to enclose the cytoplasm inside the neuron and to exclude certain substances that float in the fluid that bathes the neuron
What gives structure to neurons?
the cytoskeleton
What are most axons and where do they arise and branch?
long, slender processes that arise from the axon hillock in the cell body and branch at the distal (terminal) end
What is the main function of an axon?
to relay the electrical signal to the next cell
How does the axon differ from the soma?
the axonal cytoplasm (axoplasm) does not contain any RER that is usually found in the soma i.e. no protein translation can occur
What must happen since there is no protein translation in the axon?
all the proteins, nutrients and cytoskeletal elements found in the axon have to be transported from the soma to the far end of the axon
What are the 2 types of axonal transport?
- anterograde - forward (cell body to axon)
- retrograde - backward (axon to cell body)
What are the 2 types of anterograde transport?
- fast transport, where cargo is moved at the rate of 50-1,000 mm/day
- slow transport, where the cargo moves at a speed of 1-10 mm/day
What are the cargoes in fast and slow anterograde transport respectively?
- fast = metabolites and receptors
- slow = building blocks that are required for maintenance of the axon
What are dendrites?
specialised structures that arise from the soma and branch
What do dendrites do?
conduct impulses toward the cell body
How do electrical signals move through neurons?
- receptors sense the incoming neurotransmitters and convert them into electrical signals
- the signal is passed down to the soma and further into the axon and to the next cell
What is the direction of information flow in axons?
unidirectional; from dendrites and cell body down to the axon
How are impulses transmitted?
from one neuron (presynaptic) to another neuron (postsynaptic)
What are the 2 types of synapse?
chemical and electrical
How does communication occur at chemical and electrical synapses?
- chemical = chemical neurotransmitter release
- electrical = electricity
What is a synapse?
a junction where the axon from one cell meets the dendrite from another cell, so as to relay the message the first cell (presynaptic cell) to the second cell (postsynaptic cell)
Give examples of neurotransmitters
- GABA
- glutamate
- ACh
Where is the only place where transmission is chemical in nature?
synaptic cleft
What do the pre and post synaptic terminals have respectively?
- pre = vesicles and voltage gated ion channels
- post = receptors
Where is an AP generated and where does it go?
axon hillock of cell 1 then travels down the axon to synapse mediated by depolarisation via sodium channels
What follows the cleft?
the postsynaptic density (dendroplasm) which contains all of the receptors
What happens in excitatory synapses?
- binding of neurotransmitter to its receptors opens sodium channels in the postsynaptic membrane
- depolarisation of the membrane to ~-45mV
- AP in the postsynaptic cell
What happens in inhibitory synapses?
- binding of neurotransmitter to its receptors opens chloride or potassium channels in the postsynaptic membrane
- hyperpolarisation of the membrane to ~-45mV
- no AP in the postsynaptic cell
Why can neurons in the CNS not undergo further cell division?
they are postmitotic and cannot regenerate
What is the first thing that happens when CNS axons are cut due to an accident?
the nucleus moves away from the cell body to the periphery and the Nissl body moves away from the nucleus and are dispersed
What are the 7 stages of axon regeneration after an accident in the PNS?
- Nissl bodies disperse which leads to loss in the cytoplasmic basophilia
- Wallerian degeneration occurs in the distal portion of the axon
- the axon begins to swell and degenerate, the myelin sheath fragments, and phagocytes remove cellular debris
- after ~3 weeks, Schwann cells proliferate and form a tube of cells distal to the injury
- if the lesion in a peripheral neuron is sufficiently distant from the cell body, the nerve may recover
- axonal sprouts are produced at the distal end of the axon stump
- the axon stump elongates, and the sprouts seek the tubes formed by Schwann cells
What happens when the axon is not completely able to reattach?
it makes random connections which can sometimes lead to phantom pain
What are the 3 types of glial cells?
- oligodendrocytes
- astrocytes
- microglia
What is the percentage and function of oligodendrocytes?
- 60-80%
- myelin sheath formation
What is the percentage and function of astrocytes?
- 25%
- blood-brain barrier, structural support, scar formation, secretion of nerve growth factors, water transport
What is the percentage and function of microglia?
- 5-10%
- brain macrophages, secretion of proinflammatory cytokines, antigen-presenting function and immunocompetent cells of the CNS
What can excess transport by oligodendrocytes lead to?
cerebral oedema
What are the largest neuroglial cells?
astrocytes
What do the processes of astrocytes do?
terminate on capillaries near the pia mater
Where are fibrous astrocytes located?
white matter
Where are protoplasmic astrocytes located and what are they?
grey matter and are thick, lightly branched processes that are closely opposed to neuron cell bodies
What do astrocytes express?
potassium channels that release potassium, increasing the local extracellular concentration when neurons are active
What do synapses contain?
glycogen for gluconeogenesis
What happens upon injury to nerve cells within the CNS?
astrocytes rush to the injury site and fill up the space to form a glial scar
What can happen if potassium is not cleared from the extracellular space?
epileptic seizures
What are the 2 most important functions of astrocytes?
- regulate blood flow in the brain by end feet i.e. they control the supply of oxygen and nutrients to the brain
- regulate homeostasis of neurotransmitter
What is a tripartite synapse?
a presynaptic terminal synapsing on a post synaptic terminal
What does the tripartite synapse do?
mop up excess neurotransmitter at the CNS only, mostly at excitatory synapses
What are the most important type of neuroglia cells?
oligodendrocytes
What do oligodendrocytes in grey matter do?
act as satellite cells to neurons
What do myelin sheathes allow?
increased conductivity and AP velocity via APs jumping from one node of Ranvier to another skipping the unnecessary channels in the internode
Where do microglia originate from?
peripheral monocytic precursor cells
Why do microglia cells have to act as antigen presenting cells?
most of the antibodies that the body makes do not cross the blood brain barrier into the nervous tissue, so it is up to the microglia to be able to recognise foreign particles and activate immune T cells
What do microglia cells secrete?
cytokine-like TNF α and IL1 β that signal more microglia to be activated
When are cytokine-like TNF alpha and interleukin 1 β secreted?
in response to a foreign particle or debris
What are microglia cells implicated in?
a wide variety of diseases associated with degeneration of neurons, including Parkinson’s, Alzheimer’s disease etc.
What are the 3 components of the PNS?
- neuron processes and cell bodies located outside the CNS
- neuroglial cells (Schwann cells and satellite cells)
- nerve endings
What is a peripheral nerve comprised of?
bundles (fascicles) of nerve fibres (axons) that are surrounded by myelin sheathes or Schwann cells
What are the 3 connective tissue elements of a peripheral nerve?
- epineurium - surrounds the entire nerve
- perineurium – dense layer around each fascicle of nerve fibres
- endoneurium – thin, reticular layer that surrounds each individual nerve fibre and contains Schwann cells
In myelinated fibres, where does the endoneurium lie?
outside and encloses the myelin sheath
What do Schwann cells form?
the myelin sheath around myelinated axons
How does myelination work in the CNS and PNS respectively?
- CNS = oligodendrocytes myelinate portions of several axons
- PNS = individual Schwann cells myelinate portions of only a single axon
What are ganglia?
encapsulated collections of neuron cell bodies, located outside the CNS
What do ganglia contain?
satellite cells and connective tissue elements along with neurons
What do satellite cells do?
form a capsule of cells around neuron cell bodies located in the peripheral ganglia
