Cells of the Nervous system and Neuromuscular junction Flashcards
Anatomy of CNS
Consists of :
- Cerebral hemispheres
- Brainstem
- Cerebellum
- Spinal cord
Neurons in CNS
- Non-dividing excitable cells
2. Share common features: Soma, Axon and Dendrites
Neuroglia
- Astrocytes (support cells)
- Oligodendrocytes (myelin)
- Microglia
- Ependyma
Cerebral hemispheres
- Highly convoluted surface of ridges; gyri and valleys ; sulci
- 4 functional regions: frontal, parietal, temporal, occipital
Brainstem
- Consists of: Midbrain, Pons, Medulla (in descending order)
- Target or source of all cranial nerves and has numerous important functions
Cerebellum
- Hindbrain structures attached to brainstem
- Important role in motor coordination, balance and posture
Spinal cord
Extends down from medulla
- Conduit for neural transmission
- Coordinates some reflex actions
Neurons morphology
- Unipolar : 1 axonal projection
- Pseudo-unipolar : Single axonal projection that divides into two
- Bipolar : 2 projections from cell body
- Multipolar : Numerous projections from cell body
- Pyramidal cells : ‘pyramid’ shaped cell body
- Purkinje cells : GABA neurons found in the cerebellum
- Golgi cells : GABA neurons found in the cerebellum
Soma
(cell body, perikaryon)
- Contains nucleus and ribosomes
- Neurofilaments -> structure and transport
Axon
- Long processes (aka nerve fibre) -> originates from some at axon hillock
- Can branch off into ‘collaterals’
- Usually covered in myelin sheath
Dendrites
- Highly branched cell body - NOT covered in myelin
- Receive signals from other neurons
Astrocytes
Most abundant cell type within CNS
Able to proliferate
Functions:
- Structural cells : blood-brain barrier
- Cell repair : synthesise neurotrophic factors
- Homeostasis : neurotransmitter removal and reuptake
Oligodendrocytes
- Variable morphology and function
- Numerous projections that form internodes of myelin
- One oligodendrocytes -> myelinates many axons
Schwann cells
- Produce myelin for peripheral nerves
- One Schwann cells -> myelinates one axon segment
Oligodendrocytes vs. Schwann cells
Oligodendrocytes are in the BRAIN whereas Schwann cells are found in the peripheral NS
Microglial cells
- Specialised cells : similar to macrophages
- Perform immune functions in CNS
Ependymal cells
- Epithelial cells : Line fluid filled ventricles
- Regulate production and movement of cerebrospinal fluid
CSF production and movement
Ependymal cells produce CSF
CSF is produced in the ventricles (by choroid plexus) It flows through the ventricular system and into the subarachnoid space. Small structures (arachnoid granulations, located along the superior sagittal sinus) permit the drainage of CSF from the sub-arachnoid space into the venous blood
Resting membrane potential
- 4 major physiological ions : K+, Na+, Cl-, Ca2+
- Cell membranes: impermeable to these ions; transportation by channels and pumps
- This causes an uneven ion distribution:
1. High Extracellular Na+ and Cl-
2. Low Extracellular K+
3. High conc. gradient for Ca2+ - Difference in concentration creates a potential difference across the membrane
- Neuronal cells : Negative charge inside compared to outside; RMP of between -40 to -90 mV
- Positive and negatice charges are concentrated around the membrane
Action potential
Na+ and K+ have important roles in generation of neuronal action potential
At RMP
- Voltage-gated Na+ channels (VGSCs) and voltage-gated K+ channels (VGKCs) are closed
1) Membrane depolarisation - opening of VGSC -> Na+ influx -> further depolarisation
2) VGKCs open at a slower rate and causes -> efflux of K+ from cell -> membrane repolarisation
Restoring RMP
AP leaves Na+ and K+ imbalance -> need to be restored
Na+-K+-ATPase (pump) restores ion gradients
1) Resting configuration - Na+ enters the vestibule and 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
Around half of our energy is used for this process of restoring the K+/Na+ balance
Saltatory conduction
- AP spreads along the axon by ‘cable transmission’
- Myelin prevents AP spreading because; it has high resistance and 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
4 steps of neurotransmission
- Propagation of the action potential (AP)
- Neurotransmitter (NT) releases from vesicles
- Activation of postsynaptic receptors
- Neurotransmitters uptake
- Propagation of AP
- AP is propagated by VGSCs opening
- Na+ influx -> membrane depolarisation -> AP ‘moves along’ neuron
- VGKC opening -> K+ efflux -> repolarisation
- Neurotransmitter 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 post-synaptic membrane
- Receptors modulate post-synaptic activity
- Neurotransmitter uptake
- NT dissociate from receptor and can be:
- Metabolised by enzymes in synaptic cleft
- Recycled by transporter proteins
Communication between nerve cells
Autocrine and paracrine: Neurotransmitter release
Post-synaptic cell - 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
Nerve and muscle cells
Specialised structure incorporating axon terminal and muscle membrane allowing unidirectional chemical communication between peripheral nerve and muscle
Communication between nerve and effector cells : Paracrine; neurotransmitter release
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
*EPP is same as membrane potential; Need a summation of EPPs to get an action potential
Sarcolemma
- The skeletal muscle membrane: nAChR activation -> depolarisation - action potential
- AP travels through T-tubules
T-tubules
Continuous with sarcolemma and closely connected to sarcoplasmic reticulum
Sarcoplasmic reticulum
Location: surrounds myofibrils - contractile units of muscle
Function: Ca2+ storage; Ca2+ release following sarcolemma depolarisation
Effect: Ca2+; myofibril contraction and muscle contraction
Botulism
Botulinum toxin (BTx): irreversible disrupts stimulation-induced ACh release from presynaptic nerve terminal
Myasthenia Gravis
Autoimmune disorder: antibodies directed against ACh receptors
Cause fatigable weakness (i.e becomes more pronounced with repetitive use)
Lambert-Eaton myastenic syndrome
Autoimmune disorder: antibodies directed against VGCC
