Cells of the Nervous System and the Neuromuscular Junction Flashcards
What is the CNS?
The Central Nervous System
Name the 4 main components of the CNS and their subcomponents?
- Cerebral hemispheres (brain):
4 functional regions - frontal, occipital, temporal and parietal
Gyri and valleys - meandering hills
- Brainstem:
Hindbrain separated into 3 main sections - in ascending order: midbrain, pons and medulla
Target of the cranial nerves - responsible for majority of the autonomic functions
- Cerebellum: At the back of the brain
Used for co-ordination - adjusting fine movement (people can survive without a cerebellum, just very unsteady)
- Spinal Cord:
Forms the PNS
Nerve cells protected by the vertebrae - important in reflex actions (don’t require thought processes)
What are the 2 broad types of cells in the CNS?
Neurons
Neuroglia
What are the main cells of the nervous system?
Neurons
What are neurons and what do they consist of?
Main excitable cells of the NS - can generate an action potential
Generally considered ‘non-dividing cells’ - an older view, as newer research suggests some neuronal cells are able to divide, and perhaps depression is caused by non-diving neuronal cells in the hippocampus
All have similar features: the cell body (soma / perikaryon) - main part containing the nucleus and neurofilaments (important for structure and cell support).
Always have one axon (regardless of the morphology), which projects out of the cell body. Normally covered in myelin
Cell body may have other projections too, e.g. dendrites - highly branched cell body
What are the 4 types of neurons in the nervous system and describe their features?
- Unipolar - Only one axonal projection, nothing else protruding out of the cell body, used for receiving visual stimuli (usually)
- Pseudo-unipolar - also only have one projection from the main cell body, but the one projection spreads out into many parts, used for sensory NS e.g. pain stimuli
- Bipolar - 2 projections, only one is an axon, and the other is e.g. a dentrite
- Multipolar - most common morphology, numerous projections form the cell body, but still only ever has one axon, others are usually dendrites; has multiple connections with multiple cells (vital for its function)
What are the 3 types of multipolar neurons?
Pyramidal cells - ‘pyramid’ shaped cell body
Purkinje cells - GABA neurons found in the cerebellum
Golgi cells - GABA neurons found in the cerebellum
How are dendrites different compared to axons?
Dendrites - involved in receiving signals
Axons - involved in the transmission of signals
What are the 2 types of neuroglia cells?
Astrocytes
Oligodendrocytes
What are astrocytes and what are their most important roles?
Most abundant in the NS (more than neurons)
Appearance similar to a multipolar nuronal cell
Have foot processes important for areas of the brain such as the blood brain barrier
Important for structure - forms blood brain barrier and holds the neurons in the correct places (structure of NS)
Important for growth, maintenance and repair of the cells of the CNS (neurons and other astrocytes) by synthesising neutrophic factors
Important to maintain homeostasis i.e. mop up neurotransmitters, other substances released etc. floating around the CNS
What are oligodendrocytes and what are their functions?
Produces myelin for CNS - coats the axons of the neuronal cells
Has a cell body and cell projects to myelinate the axons of neuronal cells
One oligodendrocyte tends to myelinate parts of many axons
What is a schwann cell and role?
Has the same function as the oligodendrocyte but for the neurons part of the PNS
In the diagram, the axon goes through the centre (circular fashion)
What are some other cells that are part of the CNS?
Microglial - immune cells of the CNS, specialised cells, macrophages
Ependymal - cover ventricles, essentially epithelial cells within the nervous system
What would the CNS anatomy together with all the cells look like?
First the neurons
Next the oligodendrocyte covering / providing myeline for these axons on the neurons
Astrocytes (much smaller than the neurons) closely associated with the neuron
Microglia - neuronal macrophages
Ependyma - cover the ventricles and blood vessels
Zoom out = grey and white matter of the brain
What are the 4 ions involved in maintaining RMP and causing an AP?
K+
Cl-
Na+
Ca2+
How do neuronal cells maintain their resting membrane potential (RMP)?
What do the MP values mean in terms of excitability?
What factors contribute to MP?
Uneven distribution of the ions inside and outside the cell
Regulated by channels and pumps inside cell (ions cannot freely pass through the membrane)
High extracellular concentration of Na+, Cl- and Ca2+, and high intracellular concentration of K+, high concentration gradients for all the ions (especially for Ca2+)
Difference / concentration gradients form membrane potential - concentrated towards the inside of the cell
The more negative the value of the MP, the less excitable, so the less likely it is to generate AP (usually around -70 mV) - and vice versa, closer to -40mV is more excitable and so more likely to generate AP
Other factors contribute to MP, not solely difference in charges inside and outside the cell e.g. production of proteins produces a negative charge
Which ion has the highest concentration gradient across the membrane of a neuron?
Calcium
An imbalance of these ions can cause which conditions?
Hypo or hyper natremia - fall / rise in sodium concentration in the blood
Hypo or hyper kalemia - fall / rise in potassium concentration in the blood
Why is there a concentration gradient despite both being positive ions?
It is all relative - Na/K pump, pumps out … look at notes
What is an action potential and how does it work at the molecular level?
At RMP : voltage gated Na+ and VG K+ channels are both closed
Change in membrane potential = increase in the mV,
VG Na+ channel open
Na+ influx, K+ efflux
but basically A level
Membrane depolaristaion Repolarisation - potassium efflux
Why are the channels called voltage gated?
Open and close at different voltages
Do these channels open and close sequentially on purpose?
No, it just so happens Na+ channel open faster and close slightly slower than K+ channels, but both are stimulated at the same time
How is the balance for the RMP restored after an AP?
The pumps (Na+/K+ ATPase) restore the ion gradients
At resting configuration, it allows the sodium to leave the cell - when Na+ enters the pump, the pump is phosphorylated
This causes it to change to an active configuration, which allows for the movement of the Na+ ions out of the cell
Afterwards, it returns to its resting configuration, which allows for the potassium to go into the cell
3Na+ pumped out, 2 K+ pumped in
Why is it important the restoration rapid?
So another AP / stimulus can be received
Why do the pumps / channels use so much energy?
Consistently restoring balance and causing depolarisations / repolarisations
What is saltatory conduction?
AP that spreads along the axon is called ‘cable transmission’
In saltatory conduction, the myelin sheath covers parts of the axon acting as an insulator - prevents AP spreading due to high resistance and low capacitance
The gaps between the myelin are called the nodes of Ranvier, where the AP jumps in between
Why is saltatory conduction better than smooth conduction?
Why is it not possible for the whole axon to be myelinated?
More efficient and faster - more unnecessary APs take place in smooth conduction, but the jumping between the nodes or Ranvier save time
The AP cannot jump that far
What happens once the AP has reached the end of the axon?
Reaches the synapse
The electrical signal changes to a chemical signal
How does synaptic transmission work?
something something basically A level
How is the synaptic transmission controlled?
Enzymes to break down the neurotransmitters etc.
What is the post synaptic cell?
The cell that receives the neurptransmitters
What are the 2 main types of transmissions / communications?
Paracrine - acts on the post synaptic receptor
Or autocrine - acts on itself, important for negative feedback
What are the 2 main types of post synaptic cells?
- Can be another neuron
- Can be a neuromuscular junction (NMJ), and so terminates on the muscular membrane
What are the 3 different types of contact for the post synaptic neuron?
- Axodendritic synpase - terminates on the dendrite of the neuron
- Axosomatic synpase - terminates on the cell body
- Axoaxonic synapse - directly on the axon of another neuron and regulates the release of neutransmitter (NT) from that neuron
What is the NMJ?
Connects to the skeletal muscle membrane - encorporates the axon terminal and the muscular membrane (sarcolemma)
Allows undirectional communication - from the presynaptic cell to the muscle only
Paracrine communication
How does transmission work at the NMJ?
Basically A level again ACh - acetylcholine
binds to nicotinic ACh receptor
How does it cause the skeletal muscle to depolarise?
The MP is called end plate potential (EPP) in the NMJ in the sarcolemma
The neurotransmission causes an increase in the EPP, which depolarises the membrane
What is required for the muscle to become depolarised ?
Need summation of many mini EPPs
What is the sarcolemma?
Muscular membrane with ACh receptors
How does the depolarisation translate to a muscle contraction?
The depolarisation of the muscular membrane causes the generation of an AP
T-tubules are extensions that project into the inner components of the muscle cells, including the sarcoplasmic reticulum (which surrounds each myofibril) and stores Ca2+
As the AP travels through the T-tubules and reaches the sarcoplasmic reticulum, it causes the SR to release Ca2+
The Ca2+ then take part in muscle contraction i.e. the sliding filament theory
What are myofribrils?
Contractile units - involved in sliding filament theory
What are some of the disorders of the NMJ?
- Botulism
- Myasthenia Gravis (MG)
- Lambert-Eaton Myastenic Syndrome (LEMS)
What is Botulism?
Botulinum toxin, released from a bacterium (Clostridium botulinum bacterium)
Acts on a specific area of the membrane, prevents acetyl choline release
Normally vesicle fusing and ACh release requires many proteins, however, the botulinum toxin binds to these proteins so ACh is not released from the vesicles
This toxin is used in botox, prevents muscle relaxation, so muscles are contracted = smooth skin
What is MG?
Myasthenia Gravis is an autoimmune condition
The body produces Abs targetted against the ACh receptors fouond on the muscular membrane
Abs cause the break down of these receptors, therefore making it difficult to generate and propagate a signal to move muscles as there aren’t enough receptors to receive the ACh molecules
Causes fatigable weakness - difficult to contract muscles
What is LEMS?
Lambert-Eaton Myastenic Syndrome
Also an autoimmune condition - Abs produced against the voltage gated calcium channels
Prevents calcium entering the pre-synaptic terminal, therefore causing fatigable weakness, difficulty contracting muscles
