Cells of the Nervous System and Neuromuscular Junction Flashcards
What are the cells of the nervous system?
Oligodendrocyte
Astrocyte
Microglia
Ependyma
Neuron
What is the morphology of neurones?
Unipolar: 1 axonal projection
Psuedo-unipolar: Single axonal projection that divides into two
Bipolar: 2 projections from cell body
Multipolar: Numerous projections from cell body
What are the types of multipolar neurones?
- Pyramidal cells: ‘pyramid’ shaped cell body
- Purkinje cells: GABA neurons found in the cerebellum
- Golgi cells: GABA neurons found in the cerebellum
What are neurones?
Excitable cells of CNS
Heterogeneous morphology
Non-dividing cells
What are common features of neurones?
Soma (cell body, perikaryon):
Contains nucleus & ribosomes
Neurofilaments-> structure & transport
Axon:
Long process (aka nerve fibre) - originates from soma at axon hillock
Can branch off into ‘collaterals’
Usually covered in myelin
Dendrites: (receives communication from other neurones)
Highly branched cell body - NOT covered in myelin
Receive signals from other neurons
What is the direction of transmission in a neurone?
dendrite to axon
What are oligodendrocytes?
Glial cell - produces myelin in the CNS
What are astrocytes?
Most abundant cell type in CNS
Tend to stay in one place
Removes neurotransmitter and waste
Provides structure
Provides support to neurones
What are microglia?
Neuronal macrophages
Usually move around when activated
What are ependyma?
Epithelial cells lining the ventricles
What cells produce myelin in the PNS?
Schwann cells
What are the 4 major physiological ions?
Potassium, sodium, chloride, calcium
How do they major physiological ions pass membranes?
Membranes are impermeable to these ions
Transportation is regulated by channels and pumps
What do the channels and pumps of these ions cause in terms of ions?
high extracellular Na+ and Cl-
low extracellular K+
High concentration gradient for Ca2+
What does the difference in concentration of ions across a membrane cause?
creates a potential difference across the membrane
What is the charge of neuronal cells?
Negative charge inside compared to outside
RMP of between -40 to -90mV
Positive and negative charges are concentrated around the membrane
Are the channels open or closed at resting membrane potential?
Voltage-gated Na+ channels (VGSCs) & voltage-gated K+ channels (VGKCs) are closed
What happens when there is a membrane depolarisation in terms of Na and K?
1) opening of VGSC -> Na+ influx -> further depolarisation
2) VGKCs opens at a slower rate and causes -> efflux of K+ from cell -> membrane repolarisation
What happens to the balance on ions when there is an action potential and how is it restored?
Ap leaves Na+ and K+ imbalance–> needs to be restored
Na+K+ATPase pump restores the ion gradient
How many Na+ in/ out and how many K+ in/out normally?
3Na+ out
2K+ in
What is the resting configuration and active configuration?
1) Resting configuration - Na+ enters vestibule & upon phosphorylation -> ions are transported through protein
2) Active configuration - Na+ removed from cell -> K+ enters the vestibule
3) Pump returns to resting configuration -> K+ is transported back into the cell
What is saltatory conduction?
Saltatory conduction:
AP spreads along the axon by ‘cable transmission’
Myelin prevents AP spreading because it has - high
resistance & low capacitance
Nodes of Ranvier - Small gaps of myelin intermittently along axon:
AP ‘jumps’ between nodes - saltatory conduction
AP is unable to ‘jump’ across the gap at the axon terminal
What happens with neurotransmission at synapses?
- Propagation of the action potential:
- AP is propagated by VGSCs opening
- Na+ influx -> membrane depolarisation ->AP ‘moves along’ neurone
- VGKC opening -> K+ efflux -> repolarisation - Neurotransmitter (NT) release from vesicles
- AP opens voltage-gated Ca2+ channels at presynaptic terminal
- Ca2+influx -> vesicle exocytosis - Activation of postsynaptic receptors
- NT binds to receptors on postsynaptic membrane
- Receptors modulate post-synaptic activity - Neurotransmitter reuptake
- NT dissociates from receptor and can be:
Metabolised by enzymes in synaptic cleft
Recycled by transporter proteins
What are the types of communication between nerve cells?
Autocrine and paracrine: neurotransmitter release
Describe potential synaptic organisation?
Axodendritic synapse: connection between presynaptic terminal-> neuronal dendrite
Axosomatic synapse: connection between presynaptic terminal -> neuronal soma
Axoaxonic synapse: connection between presynaptic terminal -> neuronal axon
What is the neuromuscular junction?
Specialised structure incorporating axon terminal & muscle membrane allowing unidirectional chemical communication between peripheral nerve & muscle
What is the communication between nerve and effector cells?
paracrine: neurotransmitter release
What happens at the neuromuscular junction?
Action potential propagated along axon (Na+ & K+) -> Ca2+ entry at presynaptic terminal
Ca2+ entry -> acetylcholine (ACh) release into synapse
ACh binds to nicotinic ACh receptors (nAChR) on skeletal muscle —> change in end-plate potential (EPP)
Miniature EPP: quantal ACh release
What is excitation-contraction coupling?
The skeletal muscle membrane: nAChR
activation -> depolarisation – action potential (AP) -> muscle contraction
What happens at the sarcolemma during excitation-contraction coupling?
Sarcolemma:
- The skeletal muscle membrane: nAChR activation -> depolarisation – action potential (AP)
- T-tubules: continuous with sarcolemma & closely connected to sarcoplasmic reticulum
- AP travels through T-tubules
What is the location, function and effect of the sarcoplasmic reticulum?
Location: surrounds myofibrils – contractile units of muscle
Function: Ca2+ storage -> Ca2+ release following sarcolemma depolarisation
Effect: Ca2+ -> myofibril contraction & muscle contraction
What are disorders of the neuromuscular junction? (3)
Botulism
Myasthenia Gravis (MG)
Lambert-Eaton myasthenic syndrome (LEMS)
What is Botulism?
Botulinum toxin (BTx): irreversible disrupts stimulation-induced ACh release from presynaptic nerve terminal
What is myasthenia gravis (MG)?
Autoimmune disorder: antibodies directed against ACh receptor
Cause fatigable weakness (i.e. becomes more pronounced with repetitive use)
What is Lambert-Eaton myasthenic syndrome (LEMS)?
Autoimmune disorder: antibodies directed against VGCC
