Nervous System 21 to 30
What are upper motor neurons?
- Upper motor neurons usually refer to corticospinal neurons that innervate spinal motor neurons (also include brain stem neurons that control spinal motor neurons)
What clinical features are seen when they are injured?
- Damage initially causes muscles to become weak and flaccid but eventually leads to spasticity, hypertonia, hyperactive stretch reflexes and an abnormal plantar extensor reflex (upwards)
- 2 of bold findings
What is the physiological basis to clonus?
- Loss of descending cortical input to inhibitory neurons called Renshaw cells, and therefore loss of inhibition of antagonists, resulting in repetitive sequential contractions of ankle flexors and extensors.
List the long term complications of spinal cord injury.
- Protein/muscle degradation
- Renal stones (calcium)
- Urinary tract infection
- 2 to pass
Define the resting membrane potential of a neuron.
- Potential difference across cell at rest, as a result of separation of positive and negative electronic charges across cell membrane
- Inside negative relative to outside of cell
- Normal RMP of neuron = – 70 mv
Explain how resting membrane potential is created.
Prompt: Why is RMP negative on the inside of a cell?
- Main ions involved Na+ & K+
- Na+ K+ ATPase pump creates electrochemical gradient by pumping out 3 Na+ for every 2 K+ pumped in
- Na+ & K+ diffuse down concentration gradient across permeable cell membrane (K+ diffuses from inside to outside of cell; opposite for Na+)
- Cell membrane more permeable to K+ at rest –> that’s why RMP is close to equilibrium potential for K+
- RMP represents an equilibrium state; driving force for ions down concentration gradient = driving forces down electrical gradient
Why is a cell more excitable in hyperkalaemia?
- RMP moves closer to threshold potential for eliciting action potential (becomes less negative on the inside of cell
Draw a skeletal muscle action potential.
- Correct shape, axes, resting membrane potentials and durations (+/- 25%)
What is the sequence of events in the contraction of a skeletal muscle fibre, starting at the motor end-plate?
- Discharge of motor neuron
- Release of transmitter (acetylcholine) at motor endplate
- Binding of ACh to NIcotinic ACh receptors
- Increased NA+ & K+ conductance in end plate membrane
- Generation of end plate potential
- Generation of action potential in muscle fibers
- Inward spread of depolarisation along T tubules
- Release of Ca2+ from terminal cisterns of sarcoplasmic reticulum and diffusion to thick and thin filaments
- Binding of Ca2+ to troponin C, uncovering myosin-binding sites on actin
- Formation of cross-linkages between actin and mysoin and sliding of thin on thick filaments, producing movement
- 5 of 10 to pass
What is the sequence of events in the relaxation of a skeletal muscle fibre?
- Ca2+ pumped back into sarcoplasmic reticulum
- Release of Ca2+ from troponin
- Cessation of interaction between actin and mysoin
- Bold to pass
What factors affect cerebral blood flow?
- MAP at brain level
- MVP at brain level
- Viscosity of the blood
- Local constriction/dilatation of cerebral arterioles
- 3 of 5 to pass
What is the mechanism of the Cushing response?
- Increase in ICP results in decreased CBF
- Ischaemia of VMA
- SNS output increases
- Increased systemic BP
- Stimulation of baroreceptors
- Stimulation of vagal outflow
- Decreased HR and RR
- Explains concept
What is the Monro-Kellie doctrine?
- The volume of blood (75 mL), CSF (75 mL) and brain (1400g) in cranium must be relatively constant
- Negative effects on these therefore if additional intracranial volume (ie: SDH/EDH) occurs
- Explains concept