Paper 2 Flashcards
(37 cards)
Explain the weakness experienced by the patient, and the relative roles of upper and lower neurones
in the motor pathway based on the clinical signs outlined in the case. (3 marks) In addition outline
the motor pathway. You may use a well-labelled diagram of the motor pathway in your answer.
Polyneuropathy - symmetrical, multiple nerves
Weakness = lower motor neurone. UMN = inhibition of LMN, loss of tonic LMN firing.
Diagram showing motor pathway from cortex to muscle, including decussation and synapses. (7) Lateral and anterior corticospinal tracts
Give an overview of what is meant by “secondary neurulation” in an embryo. You may use a well
labelled diagram as your answer.
Formation of the neural tube inferior to the second sacral level by secondary neurulation. Mesoderm
invading this region during gastrulation condenses into a solid rod called the caudal eminence, which
later develops a lumen. At the end of the sixth week, this structure fuses with the neural tube.
Describe the formation (3 marks) of the neural crest cells and name four (4) cells (2 marks) and/or
structures that develop from these cells.
from neural plate border, signals induce border cells to become specialised neural crest cells
Examples: cranial and sensory ganglia, adrenal medulla, melanocytes, brachial arch cartilages, cranial mesoderm and CT
The properties of that peripheral receptor together with their associated primary sensory afferent
nerve endings encode the quality of a stimulus.
The quality of a stimulus refers to the type of energy transmitted by a stimulus and is encoded by the
identity of the activated peripheral receptor. Different axonal endings respond to restricted sets of sensory
stimuli (selectivity). Selectivity is explained by the morphological specializations of receptor endings and
properties of the ionotropic channels in receptor membranes. This selectivity is conveyed and preserved
in parallel pathways in the CNS.
Stimulus amplitude affects the response of receptor potentials and the frequency of action
potentials once a stimulus is applied to that receptor.
The quality of a stimulus refers to the type of energy transmitted by a stimulus and is encoded by the
identity of the activated peripheral receptor. Different axonal endings respond to restricted sets of sensory
stimuli (selectivity). Selectivity is explained by the morphological specializations of receptor endings and
properties of the ionotropic channels in receptor membranes. This selectivity is conveyed and preserved
in parallel pathways in the CNS.
Define “receptive field” for a sensory neuron.
The receptive field of a neuron is the area on a sensory surface (such as skin or tissue), that a
stimulus must reach to activate that neuron.
Explain why the topographic representation (homunculus) of the body surface in the
somatosensory cortex is an altered map of actual anatomic space.
The neural maps of the body surface are distorted because of the disproportionate representation
of different areas of the body (e.g., the hand and face areas) in the postcentral gyrus. That is, as
the neurons representing the hand and face have small receptive fields, a greater number of
neurons are required to represent the hand and face. This distortion reflects that of the DCN
(dorsal column nuclei), e.g., the skin of the back has a small representation because of low
afferent density and because of the high convergence (and large receptive fields) in DCN
neurons where many afferents converge onto a single DCN neuron, whereas the skin on the
fingertip has a high afferent density and only a few afferents converge on a single DCN neuron.
Thus, many DCN neurons are required to represent a given area of the skin on the fingertip. (5
marks)
State the purpose of this feature. Homunculus
Certain body regions that are especially important for function (hands, face) are over-
represented; i.e., receive disproportionate cortical allocation within the body map. (1 mark)
Define a “motor unit”.
Answer: an alpha motor neuron and the muscle fibres it innervates.
Explain how motor neurons use a rate code to signal the amount of force to be exerted by a muscle.
An increase in the rate of action potentials fired by the motor neuron causes an increase in the amount
of force that the motor unit generates. When the motor neuron fires a single action potential, the muscle
twitches slightly, and then relaxes back to its resting state. If the motor neuron fires after the muscle
has returned to baseline, then the magnitude of the next muscle twitch will be the same as the first
twitch. However, if the rate of firing of the motor neuron increases, such that a second action potential
occurs before the muscle has relaxed back to baseline, then the second action potential produces a
greater amount of force than the first (i.e., the strength of the muscle contraction summates). With
increasing firing rates, the summation grows stronger, up to a limit, and when successive action
potentials no longer produce a summation of muscle contraction (because the muscle is at its
maximum state of contraction), the muscle is in a state called tetanus.
Describe the process of Wallerian degeneration and list the typical histological features that
accompany it.
(Degeneration of severed axons after injury. Proximal: chromatolysis, swelling of cell body. Distal:
degradation/phagocytosis of disconnected axon and associated myelin sheath by macrophages and
Schwann cells)
Outline the role of Schwann cells in the regeneration of peripheral nerves.
(Remove debris, provide growth promoting substrate and guidance for regenerating axon, promote
neuron survival by secreting neurotrophic factors, re-myelinate)
Describe the role of the three (3) major types of glia/supporting cells in the failure of axon
regeneration in the central nervous system.
(Oligodendrocytes: release/upregulation of neurite growth inhibitory proteins; astrocytes: glial scar
formation and upregulation of axon growth inhibitory proteoglycans; microglia: may further damage
and kill stressed neurons)
Describe the features of a CAT scan or/special investigations that would lead you to suggest a
patient is suffering from a metastasis to the brain.
i] primary tumour elsewhere : lung, colon, breast, renal, chorio
ii] multiple sites
iii] region of MCA territory [straightest course]
iv] located at grey/white matter interface
v] ring enhancing lesion : indicate its components
i] marked surrounding oedema [dark]
ii] circular area of contrast uptake [white]
iii] central necrosis [dark]
Insulin resistance may be associated with changes in muscle, adipose tissue, the vasculature, the liver
and even the brain. Briefly describe these changes for each system
Resistance to the suppressive effects of insulin on endogenous glucose production and the stimulatory
effects of insulin on peripheral glucose uptake and glycogen synthesis.
Decreased suppression of adipose tissue lipolysis by insulin, resulting in elevated circulating non-
esterified fatty acid (NEFA) levels.
Decreased vascular endothelial cell sensitivity to various actions of insulin, which include stimulating the
expression and activation of eNOS with consequent vasodilatation.
Insulin impacts on capillary recruitment to increase nutritive tissue perfusion, insulin resistance may
inhibit capillary recruitment.
Brain insulin action is required for physiological glucose homeostasis. Targeted impairments in insulin
receptors either in all neurons or specifically in the hypothalamus rapidly cause hyperphagia and diet-
dependent obesity
Consider the following metabolic “events” and the respective role of insulin and glucagon, for each.
Describe the role of insulin and glucagon for each metabolic event, tissue or process or indicate “no
effect”.
Fatty acid uptake and release in fat
Insulin: Stimulates synthesis of triglycerides (TG) from free fatty acids (FFA); inhibits release of FFA from TG.
Glucagon: Stimulates release of FFA from TG.
Liver glycogen
Insulin: Increases synthesis and thereby glucose uptake and and storage.
Glucagon: Stimulates glycogenolysis and glucose release.
Liver gluconeogenesis
Insulin: Inhibits, saves amino acids.
Glucagon: Stimulates, hepatic glucose synthesized and released.
Describe the oral and injectable polio vaccines available today, and mention differences in immunity
acquired through the use of each.
Answer: Oral polio vaccine = live attenuated; 3 strains (1,2,3), with 2 eradicated, and 1+3 left in the
vaccine; IgA / mucosal / gut immunity also. IPV = inactivated, injected, 3 strains, humoral immunity
only.
State when each of the polio vaccines is given in the public healthcare sector in South Africa.
Answer: OPV = birth + 6 weeks. IPV = 10 weeks, 14 weeks, 18 months.
Briefly explain why the CSF glucose needs to be interpreted with a plasma glucose measurement.
CSF glucose is in equilibrium with blood glucose concentration [1] over the past 4 hours [½];
interpretation therefore requires knowledge of concurrent blood glucose [½].
Name one test used to evaluate blood-brain barrier permeability.
CSF:plasma albumin ratio; or IgG index
List four (4) biochemical abnormalities that occur in tumour lysis syndrome.
Hyperphosphataemia
Hyperkalaemia
Hyperuricaemia
Hypocalaemia
Metabolic acidosis
List the names of the encepalopathic diseases that can be caused by measles through these different
pathophysiologic mechanisms. Explain how each is brought about, indicating for each whether or
not virus can be cultured from the brain
*Acute measles post-infectious encephalitis
*Measles inclusion body encephalitis
*Subacute sclerosing panencephalitis (SSPE)
List the four (4) most common types of support cells in the CNS and name their principal function
(Astrocytes, metabolic support; oligodendrocytes, myelination; microglia, immune defence; ependyma,
interface between CSF and brain tissue/movement of CSF)
Name the anatomical structure that contains the cell bodies (somata) of primary somatosensory
neurons, and state how these neurons are classified in terms of their morphology
(Spinal/dorsal root ganglion; pseudo-unipolar)
