Nervous System 1 to 10
Please describe a monosynaptic stretch reflex.
Draw & label the action potential in a nerve cell.
Discuss the factors that affect conduction.
- Myelinated vs demyelinated
- Saltatory vs non-saltatory
- Direction of the conduction
What are the ion fluxes that occur during the action potential?
- Fast Na+ influx
- Slow K+ efflux
Once it is released, how is the effect terminated?
Draw a nerve action potential.
What are the ion fluxes that occur during an action potential?
Fast sodium influx; slow potassium efflux
Describe the sequence of events in transmission of a motor nerve impulse to a muscle.
- Motor neuron action potential;
- end-plate potential;
- Acetylcholine release;
- Ach binding to nicotinic receptors;
- muscle end-plate potential.
How does the muscle become depolarised?
T tubules and release of Ca2+ from sarcoplasmic reticulum
Describe the withdrawal reflex.
Noxious stimulus to skin or sub cut
Response of flexor muscle contraction and extensor relaxation
Result in withdrawal of limb from stimulus
Cross extensor response
3 out of 5
What is meant by the term polysynaptic reflex?
One or more interneurons and interposed between the afferent and efferent neurons
What are the effects of a polysynaptic reflex?
Prolonged effect as different time for stimulus to reach effector
Reverberation circuit as some interneurons turn back on themselves further prolonging the effect.
Describe the sequence of events in contraction and relaxation of skeletal muscle.
Steps in contraction
Discharge of motor neuron.
Release of transmitter (acetylcholine) at motor end-plate.
Binding of acetylcholine to nicotinic acetylcholine receptors.
Increased Na+ and K+ conductance in end-plate membrane.
Generation of end-plate potential.
Generation of action potential in muscle fibers.
Inward spread of depolarization 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 myosin and sliding of thin on thick filaments, producing movement.
Steps in relaxation
Ca2+ pumped back into sarcoplasmic reticulum.
Release of Ca2+ from troponin.
Cessation of interaction between actin and myosin.
What is summation of contractions?
- The electrical response of a muscle fibre to repeated stimulation
- Contractile mechanism does not have a refractory period, so repeated stimulation before relaxation has occurred produces additional activation and a response added to the contraction already present
- With rapidly repeated stimulation, individual responses fuse into one continuous contraction (tetanus; tetanic contraction).
- Complete tetanus: no relaxation between stimuli; tension developed ~ 4 times that of an individual twitch contraction
- Incomplete tetanus: periods of incomplete relaxation between summated stimuli
What happens to acetylcholine when released into a synapse?
- Binds to post-synaptic cholinergic receptors
- Catabolism by acetylcholinesterase at the postsynaptic membrane
- Reuptake of choline – No acetylcholine reuptake
- Catabolism by pseudocholinesterase in the circulation
3/5 to pass
Describe the differences between the two types of acetylcholine receptors.
- Divided on basis of pharmacological properties into muscarinic and nicotinic
– needed to pass
- Muscarinic–actions mimicked by muscarine and blocked by atropine. Found in smooth muscle, glands and brain. G- protein coupled to adenylyl cyclase and/or phospholipase.
– 2 sites to pass
- Nicotinic–actions mimicked by nicotine. Found in neuromuscular junction, autonomic ganglia and the central nervous system. Ligand-gated sodium ion channels.
– 2 sites to pass
Describe the resting membrane potential of a cell.
- There is difference in electronic charge across a cell membrane. The inside is negative compared to the outside. Resting MP results from separation of positive and negative charges across a cell membrane. Neuron average RMP -70 mV.
What conditions are required to create a resting membrane potential?
- Lipid bilayer, unequal distribution of ions, membrane must be permeable to ions, concentration gradient.
In a neuron what ions are involved and how is the concentration gradient produced?
- Na and K. Na is primarily extracellular and K intracellular. Passive movement of ions occur via selective ion channels. Na-K ATPase actively move ions against their electrochemical gradient.
What is clonus?
- Regular, repetitive, rhythmic contractions of a muscle subjected to sudden, sustained stretch.
Why does ankle clonus occur with upper motor neuron lesions?
- 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.
What are the components of the stretch reflex?
- Sensor, afferent nerve, Monosynaptic at spinal level, efferent nerve, effector