Neurology and Neuroscience Flashcards
What are the components of the CNS and the PNS?
The central nervous system (CNS) consists of the two cerebral hemispheres, the brainstem, the cerebellum and the spinal cord. The peripheral nervous system (PNS) consists of the nerve fibres originating from the CNS.
Outline the structure of the cerebral hemispheres
The cerebral hemispheres (also known as the telencephalon) have a distinctive convoluted surface appearance where the ridges are called gyri (singular gyrus) and the valleys are called sulci (singular sulcus).
What are the 4 functionally distinct regions/lobes comprising the cerebral hemispheres?
1) Frontal: Responsible for executive functions such as personality
2) Parietal: Contains the somatic sensory cortex responsible for processing tactile information
3) Temporal: Contains important structures such as the hippocampus (short term memory), the amygdala (behaviour) and Wernicke’s area (auditory perception & speech)
4) Occipital: Processing of visual information
What are the components of the brainstem?
The brainstem consists of the midbrain, pons and the medulla in descending order. These structures have a multitude of important functions (e.g. control of respiration & heart rate) and are the target or the source of all the cranial nerves.
Outline the location and function of the cerebellum
The cerebellum is located towards the dorsal region of the CNS and is attached to the brainstem. It has an important role in motor coordination, balance and posture.
Outline the location and function of the spinal cord
The spinal cord extends down from the medulla and acts as a conduit for neural transmission but can coordinate some reflex actions.
What is a mature neuron?
A mature neuron is a non-dividing excitable cell, with a heterogeneous morphology, whose main function is to receive and transmit information in the form of electrical signals. Although neurons come in a variety of different shapes and size, they do share a number of common features.
Outline the morphology of the 4 types of neurons
1) Unipolar: 1 axonal projection
2) Psuedo-unipolar: Single axonal projection that divides into two
3) Bipolar: 2 axonal projections from the cell body
4) Multipolar: Numerous axonal 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
What are the functions of the soma, axon and dendrites?
1) Soma (cell body, perikaryon)
> Contains nucleus, & ribosomes
> Contains neurofilaments which are important in maintains the structure structure, as well as useful for transportation
2) Axon
> Long process (aka nerve fibre) - originates from soma at axon hillock
> Can branch off into ‘collaterals’
> Usually covered in myelin
3) Dendrites
> Highly branched cell body - NOT covered in myelin
> Receive signals from other neurons
Other than neurons, what other cells are found in the CNS?
1) Astrocytes: the most abundant cell type in the mammalian brain. They function as structural cells and are known to play an important role in cell repair, synapse formation, neuronal maturation and plasticity.
2) Oligodendrocytes & Schwann cells: oligodendrocytes are the myelin producing cells of the CNS, whilst Schwann cells perform the same function in the PNS. Each oligodendrocyte cell body sends out numerous projections that form internodes of myelin covering the axons of neurons. Whilst each oligodendrocyte is capable of myelinating a number of axons a Schwann cell only myelinates a single axonal segment.
3) Microglia & Ependyma: microglial cells are specialised cells that are similar to macrophages and they perform immune functions in the CNS. Ependymal cells are epithelial cells that line the fluid filled ventricles regulating the production and movement of cerebrospinal fluid.
What is the resting membrane potential (RMP)?
This is an ionic imbalance between the extracellular fluid and the intracellular fluid of a neuron with an unequal distribution of the major physiological ions. These concentrations are determined by the activities of a variety of membrane bound channels and transporters. The resting membrane potential due almost entirely to the movement of K+ ions out of the cell.
How are the major physiological ions distributed in RMP?
1) Sodium (Na+): > Intracellular conc. 5-15 mM > Extracellular conc. 140-155 mM 2) Potassium (K+): > Intracellular conc. 140-160 mM > Extracellular conc. 2-5 mM 3) (Ca2+): > Intracellular conc. ~0.0001 mM > Extracellular conc. 1-2 mM 4) Chloride (Cl-): > Intracellular conc. 5-10 mM > Extracellular conc. 70-140 mM 5) Organic Phosphates (-): > Intracellular conc. 130 mM > Extracellular conc. 3 mM
What is the electromotive force (emf)?
The relative concentrations of the major physiological ions is one of the factors that gives the cell membrane an electromotive force (emf), a potential difference between the inside and the outside of the cell. As the cell membranes are impermeable to the major physiological ions, transportation is regulated by channels & pumps.
How is the RMP of cells calculated?
Conventionally the outside of the cell is referred to as the zero reference point and has a voltage of 0mV, the inside of the cell (in particular the area immediately adjacent to the cell membrane) has a negative membrane potential of around -50 to -90 mV in neurones. Thus neurones are said to have a resting membrane potential (RMP) of around -70 mV.
What is an action potential?
If, the membrane potential becomes more negative, the cell is hyperpolarised. If the membrane potential becomes more positive the cell is depolarised. When a cell is sufficiently depolarised an action potential is generated, where there is a brief depolarisation spike in the membrane potential (to around +10 mV) before returning back to the RMP. This action potential is transmitted along the membrane and axon by means of cable transmission and it is the ability to propagate action potentials, which makes neurons ‘excitable’.
Outline the role of voltage-gated ion channels in action potentials
At resting membrane potential (RMP), voltage-gated Na+ channels (VGSCs) and voltage-gated K+ channels (VGKCs) are closed. Membrane depolarisation leads to a change in the ionic channel configuration, allowing VGSCs to open, causing an Na+ influx and further depolarisation. VGKCs open at a slower rate than VGSCs and lead to repolarisation, due to the efflux of K+ from the cell membrane.
Outline the function of Na+-K+-ATPase (pump)
Action potentials leave a Na+ & K+ imbalance that needs to be restored. This is accomplished using Na+-K+-ATPase (pump), which restores the ion gradients. It does this in 3 steps.
1) Resting configuration: Na+ enters vestibule and upon phosphorylation, Na+ ions are transported through the protein pump.
2) Active configuration: An exchange of energy, using ATP, allows Na+ to be removed from cell and K+ enters the vestibule.
3) Pump returns to resting configuration: This allows K+ to be transported back into the cell.
What is saltatory conduction?
The action potential spreads along the axon by ‘cable transmission’. The myelin sheath on axons prevents the action potential from spreading, due to its high resistance & low capacitance. The Nodes of Ranvier are small gaps of myelin spread intermittently along axons. The action potential can ‘jump’ between nodes by saltatory conduction. However, action potentials are unable to ‘jump’ across the gap at the axon terminal.
What are synapses?
These are the small gaps that exist between two neurones are known as synapses. The synapse itself is a junction consisting of a pre-synaptic nerve terminal (e.g. the axon terminal), which is separated from the postsynaptic cell (e.g. the dendrite of another neurone) by an extracellular space known as the synaptic cleft. Since the electrical signal cannot jump over the synaptic cleft it is converted into a chemical signal to cross the synapse and then back into an electrical signal on the post-synaptic cell.
Outline the process of neurotransmission at the synapse
1) Propagation of the action potential (AP):
> AP is propagated by VGSCs opening
> Na+ influx leads membrane depolarisation and AP ‘moves along’ the neuron
> VGKC opening allows K+ efflux and repolarisation
2) Neurotransmitter (NT) release from vesicles:
> AP opens voltage-gated Ca2+ channels at presynaptic terminal, leading to Ca21 efflux
> Ca2+ binds to vesicles containing the neurotransmitter
> The vesicles bind to the presynaptic membrane and release the neurotransmitter into the synaptic cleft by exocytosis
3) Activation of postsynaptic receptors:
> NT binds to receptors on post-synaptic membrane
> Receptors modulate post-synaptic activity
4) Neurotransmitter reuptake:
> NT dissociates from receptor and can be
1) Metabolised by enzymes (e.g. cholinesterase for acetylcholine) in synaptic cleft
2) Recycled by transporter proteins
Outline communication between neurons
The communication between neurons can be either autocrine or paracrine. Both types of communication involve the use of neurotransmitters.
Outline the 3 types of synapses
1) Axodendritic synapse: connection between presynaptic terminal and the neuronal dendrite
2) Axosomatic synapse: connection between presynaptic terminal and the neuronal soma
3) Axoaxonic synapse: connection between presynaptic terminal and the neuronal axon
What is the neuromuscular junction?
This is a specialised structure incorporating axon terminal & muscle membrane allowing unidirectional paracrine chemical communication between peripheral nerve & muscle.
Outline the process of paracrine communication in the neuromuscular junction
1) Action potential propagated along axon (Na+ & K+), leads to Ca2+ entry at presynaptic terminal.
2) Ca2+ entry causes acetylcholine (ACh) release into synaptic cleft.
2) ACh binds to nicotinic ACh receptors (nAChR) on skeletal muscle and leads to a change in end-plate potential (EPP).
4) A miniature EPP indicates quantal ACh release.