HNN Topic 17 - General Neurology Flashcards
List the sequence of events which occur when an action potential is fired
- Membrane potential reaches threshold value - fires action potential (depolarisation)
- Repolarisation
- Hyperpolarisation
What is the effect of an excitatory postsynaptic potential?
Brings the postsynaptic membrane potential closer to threshold (depolarisation), therefore increasing the probability of the postsynaptic cell producing an action potential
Where are the parasympathetic preganglionic neurons located?
- Brainstem - leave with 4 cranial nerves
- Sacral spinal cord, leave in spinal nerves S2-4
Which ions are most important in determining the resting membrane potential?
Sodium, potassium and chlorine
What causes multiple sclerosis?
- Autoimmune, antibodies attack myelin
- Discrete lesions affecting myelin of many CNS axons
- Formation of scars (‘sclerosis’) delays/blocks AP - seen as ‘plaques’ in white matter of brain, spinal cord
How does Myasthenia Gravis disrupts cholinergic synaptic transmission?
- Interferes with cholinergic signalling at the neuromuscular junction
- Inflammatory disease, antibodies bind to ACh receptors in postsynaptic membrane at motor end plate
- Severe muscle weakness particularly affects eyelids, eyes, face, throat and limb muscles
What is the effect of an inhibitory postsynaptic potential?
Moves the postsynaptic membrane potential further away from threshold (hyper-polarisation), therefore reducing the probability of the postsynaptic cell producing an action potential
What is the role of glial cells?
Support neurons
Define divergence and convergence in the nervous system
Methods of spread of information in the nervous system
- Divergence - one neuron, signal spreads to many targets
- Convergence - many signals affect one neuron
What is the cause of Guillain-Barre syndrome?
Inflammatory disease - inflammatory cells destroy myelin sheath
Where are the sympathetic preganglionic neurons located?
Thoracic and upper lumbar spinal cord, leave in spinal nerves T1-L3
Explain the clinical relevance of voltage-gated sodium channels?
- Local anaesthetics - Lidocaine (NaV antagonist)
- Pain treatment - target NaV on nociceptive terminals
- Epilepsy - NaV antagonist
Describe the normal resting membrane potential of cells
- More positive ions outside, more negative ions inside
- Measured in relation to outside, RMP is negative
- Typically -60mV to -70mV
How can cerebral function be investigated?
- Electroencephalography (EEG) - record electrical activity of brain, investigate cognitive response to stimulus
- Positron emission tomography (PET) - measures blood flow via small dose of radiation in the bloodstream, locate brain activity while performing a task
- Functional magnetic resonance imaging (fMRI) - measures blood flow
- Transcranial magnetic stimulation (TMS) - uses electromagne to stimulate brain activity, causing depolarisation or interrupted firing, interrupts brain activity while performing a task
Describe the sequence of events which occurs in firing an action potential in terms of movement of ions
- Sodium channels (voltage gated) open, sodium ion influx, depolarisation
- Rapid inactivation of voltage gated sodium channels, repolarisation
- Slow opening voltage-gated potassium channels open, hyperpolarisation
- Sodium-potassium pump rebalances potassium and sodium ions
What causes depolarisation and repolarisation of membranes?
- Depolarisation - increased permeability of membrane to sodium ions
- Repolarisation (+ hyperpolarisation) - increased permeability to potassium ions
How is an action potential propagated along an axon?
- Opening of NaV during AP depolarises axon on either side, reaches threshold, opening of NaV, trigger zone = axon hillock
- Wave depolarisation travels one way along axon
- Destination of current depends on axon diameter + number of open pores in membrane
- Follows path of least resistance - widest diameter and most pores
What are the clinical signs of Guillain-Barre syndrome?
Progressive motor weakness, motor and sensory loss (face, limbs, trunk, diaphragm)
List the types of glial cells in the PNS
- Satellite cells
- Schwann cells - myelination
What is usually the first symptom of an entrapment neuropathy?
Paraesthesia - tingling
Define pre- and post-ganglionic neurons of the ANS
- Preganglionic - cell body in CNS, axon enters PNS
- Postganglionic - cell body in PNS, axon supplies target organ
- Ganglion - synpase of pre- and post-, collection of neurons which causes a swelling, where postganglionic cell body is located
List the types of glial cells in the CNS
- Ependymal - ventricles
- Oligodendrocytes - myelination
- Astrocytes
- Microglia - neuropathic pain
List disorders associated with defective myelination
- Multiple sclerosis
- Guillain-Barre syndrome
What is the effect of hyperpolarisation of the membrane following the firing of an action potential?
Inhibition of new action potential for a few seconds
Describe the innervation of the limbs
- Nerve fibres travelling to/from limbs leave spinal cord as spinal nerves - 2 per segment
- Pass through plexuses - brachial and lumbosacral, emerge as peripheral nerves
List the types of neurotransmitters
- Cholinergic - acetyl choline
- Biogenic amines - catecholamines (noradrenaline, dopamine), serotonin
- Amina acids - excitatory (glutamine), inhibitory (gamma-aminobutyric acid) - GABA
- Neuropeptides - endogenous opiods
- Miscellaneous - gases (nitric oxide), pruines (adenosine, ATP)
What is the function of the autonomic nervous system?
Regulates eyes, GI tract and glands, heart and BVs, lungs, reproductive and urinary systems, skin
Explain the patterns of sensory loss which can occur
- Dermatomal - in the distribution of a spinal nerve e.g. slipped disc compressing L5, muscles supplied by spinal nerve also potentially affected
- Peripheral nerve injury - sensory changes in area supplied by nerve e.g. median nerve in carpal tunnel syndrome - entrapment neuropathy
- Longest fibres affected = stocking-glove distribution e.g. metabolic disorders such at Vitamin B12 deficiency
Describe the clinical presentation of multiple sclerosis
- Partial vision loss, blurred vision, double vision
- Sensory changes e.g. numbness
- Motor symptoms e.g. weakness, partial paralysis, ataxia
- Fatigue
- Pattern of symptoms can’t be explained by one focal lesion - widespread CNS white matter lesions
What is the function of neurons?
Functional unit of nervous system:
- Excitable
- Conduct action potentials
- Release neurotransmitters
Different classes of neurons with specialised structures have different functions, all neurons have input, conductile region and an output (secretory)
Where are the sympathetic and parasympathetic ganglia located?
- Sympathetic - sympathetic chain next to vertebral column
- Parasympathetic - walls of viscera which they innervate e.g. heart wall
Define action potential
- Brief but dramatic change in membrane potential
- If membrane potential reaches threshold value, fires action potential
What produces the myelin sheath?
Oligodendrocytes in CNS, Schwann cells in PNS
Define the equilibrium potential
Membrane potential when number of ions entering cell = number of ions leaving cell (no net gain/loss)
Calculated using Nernot equation
How does Botulism disrupt cholinergic synpatic transmission?
- Distrupts cholinergic signalling at the neuromuscular junction
- Clostridium bacteria produce botulinum toxin (contaminated food), disrupts exocyotsis preventing ACh release
- Skeletal muscle weakness, paralysis of diaphragm (respiratory failure)
How do ions move across membranes?
- Ion channels - voltage-gated, ligand-gated (neurotransmitter, drug etc.)
- Ion pump - active transport, requires ATP
How is resting membrane potential measured?
Using electrophysiology - intracellular and extracellular electrodes
List the parts of neurons
- Dendrites - input
- Cell body (soma)
- Nucleus
- Axon hillock
- Axon - transmission
- Node of Ranvier
- Myelin
- Synapse
- Pre- and post-synaptic terminals
Describe the sequence of events which occurs during synaptic transmission
- AP enters presynaptic terminal
- Calcium entry through voltage-gated calcium channel
- Docking of synaptic vesicled containing neurotransmitters
- Neurotransmitters released (exocytosis)
- Neurotransmitter binds to and activates receptors on post-synaptic membrane, ions (sodium/chlorine) enter cell
- Depolarisation (excitation) or hyperpolarisation (inhibition)
Describe summation of post-synaptic potentials
- When combined, potentials can depolarise the membrane to reach firing threshold
- Temporal summation - increased frequency of firing
- Spatial summation - firing at multiple sites
What is the function of the nodes of Ranvier?
Gaps in myelin - allows for saltatory conduction (action potential jumps from node to node), faster transmission
