lecture 17 Flashcards
Rule #1: Most organs receive input from the autonomic nervous system - not all organs receive input from both e.g. many blood vessels - SNS only Rule #2: the autonomic nervous system consists of two neurons connected in series - the neuron innervating the target occurs outside the central nervous system Rule #3: the axons of sympathetic preganglionic neurons are short. - the ganglionic axons project long distances to contact the target tissue Rule #4: the axons of parasympathetic preg
Sodium and chloride are osmotically active, what does this mean?
- they will attract water
Which direction does water move across a semi-permeable membrane?
- from the low solute concentration to the high solute concentration
How can the osmotically driven movement of water be problematic?
for cells as it also exerts pressure
Why do solute concentrations need to be carefully controlled?
- they contribute to factors such as membrane potentials and neuronal cell excitability
Which sensors of osmolality are key for osmotic homeostasis?
- whilst there are peripheral sensors of osmolality, the central sensors that detect extracellular osmolality are key for osmotic homeostasis
- the main group are in the OVLT, but some other cell groups including those in the PVN and SON are osmosensitive too (though not understood why as they are behind the blood-brain barrier)
What do OVLT neurons do in response to increased ECF osmolality? How?
- increase their action potential firing
- with increased ECF osmolality, water leaves the cell toward the area of higher solute concentration
- the cell shrinks
- this causes the conductance (the measure of whether membrane channels are open) to increase
- the opposite occurs with decreased extracellular osmolality
What mediates the change in activity?
- the change in cell stretch
To what is the change in conductance due?
- opening of a cation channel
- this allows sodium/calcium to enter the cell and depolarise it to threshold
- it fires action potentials
What is the key channel that appears to mediate the firing of action potentials in the OVLT cells?
- a member of the transient receptor potential family – TRPV1
How must TRPV1 be connected to the cellular skeleton?
- it must be connected to the cellular skeleton such that it is opened when the cell shrinks and closed when the cell swells
Where do outputs from OVLT cells go?
- ACC: allows you to perceive that you are dehydrated –> thirst
- potentially a path to ACC via thalamus
- PVN
- SON
- these last two are critical for water retention: act via the kidney to remove water from the urine (i.e. save it from being excreted)
- these are the magnocellular cells in the hypothalamus that secrete vasopressin
What are magnocellular cells?
- neurosecretory cells located in the hypothalamus whose axons travel down the pituitary stalk and whose ‘synapse’ is on a blood vessel
- make vasopressin
- package it in vesicles which stay in the pre-synaptic terminals to be released when the cell is excited
- make quite a lot
- release a hormone as their transmitter, and therefore need to release quite a lot
- vasopressin has a number of effects but in this context it is important for the maintenance of osmolality homeostasis
- vasopressin, aka ADH
- cell bodies in paraventricular nucleus
- axons travel down and around through superoptic nucleus to get to pituitary stalk
What is the biochem of vasopressin?
- made with a signal peptide
- a large binding protein is made with it
- 9 amino acids - Gly in the 10 position (which is removed) is necessary for the amidation of the Gly residue in the 9 position of AVP (arginine vasopressin)
What does vasopressin do when it gets to the kidney?
- interacts with its receptor: g-protein coupled receptor (metabotropic receptor)
- V1 and V2 receptor
- V2 is the important receptor in the kidney
- activation of adenyl cyclase produces cAMP
- this via a number of steps has two main effects:
1. go to the nucleus and increase transcription of AQP2, increased synthesis, put into vesicles and inserted into the membrane
2. promote insertion of AQP2 into the membrane
What are aquaporins?
- pores in the membrane that allow greater movement of water across the membrane