homeostasis and neurones

the endocrine system is made up of a series of glands that secrete chemicals known as hormones 

hormones are transported in the blood and taken to a target organ 

this can activate a response by activating enzymes others do this by affecting genes for example steroids 

the endocrine system plays a major role in homeostasis which is maintaining a constant internal environment 

endocrine glands I need to know and functions 

• pancreas = controls blood sugar levels by releasing insulin wich lowers blood sugar levels and glucagon which raises blood sugar levels.
• adrenal gland = responsible for the release of adrenalin  
• pituitary gland = connection between the nervous and endocrine systems releases Anti-diuretic hormone which allows control of water potential of the blood. 
• finally the hypothalamus of the brain stimulates the releases of many of these hormones  

hormones take far longer to take effect than a nervous system response

however the effects of hormones are often long lasting 

hormone mechanism for adrenalin 

the hormone gets to the target organ through the blood vessels 

the hormone then binds to a receptor on the surface of the cell 

this causes an enzyme connected to the receptor adrenal cyclase, to change shape and become active 

the adrenal cyclase turns ATP into cyclic AMP

the cyclic AMP goes on to activate a second enzyme this is the enzyme that will cause an response  

hormones take far longer to take effect than a nervous system response

however the effects of hormones are often long lasting 

hormone mechanism for adrenalin 

the hormone gets to the target organ through the blood vessels 

the hormone then binds to a receptor on the surface of the cell 

this causes an enzyme connected to the receptor adrenal cyclase, to change shape and become active 

the adrenal cyclase turns ATP into cyclic AMP

the cyclic AMP goes on to activate a second enzyme this is the enzyme that will cause an response  

Theirs are two different types of animal when it comes to controlling body temperature 

endotherms = warm blooded animals they control their body temperature at a set level this allows internal processes to carry on unimpeded their levels of activity fluctuate less 

ectotherms = cold blooded their body temperature fluctuates with environmental temperature. as such their enzyme action is dependent on the environment   

how temperature is regulated in mammals   

the hypothalamus is the thermoregulatory center in the brain detects a cahnge in the core blody tempture by the blood that flows through the brain

it causes a response

such as vasoconstriction(too cold) or vasodiolation(too hot), shivering or sweating  

• the level of glucose needs to be carefuly controled as a low glucose blood level means their is not enough ennergy for metabolic processes 
• to much glucose will lower the water potntail of the blood causeing water to move from the organs to the blood stream causeing majour damage 
• blood glucose levels are reguated by a negtive feedback loop allwoing a constant enviorment to be maintained

if their is a rise in blood glucose

• cells in islets of landerhands in the pancreus detect the high glucose blood levels
• insulin is secreted by beta cells
• the insulin causes muscel and liver cells to uptake glucose and convert it into glycogen via glycogensis  

if glucose blood levels are too low 

• cells in the iselts of landerhans
• glucogon is screted by alpha cells 
• this causes cells to convert glcogen into glucose which is then relased via glycogenolysis 

• the level of glucose needs to be carefuly controled as a low glucose blood level means their is not enough ennergy for metabolic processes 
• to much glucose will lower the water potntail of the blood causeing water to move from the organs to the blood stream causeing majour damage 
• blood glucose levels are reguated by a negtive feedback loop allwoing a constant enviorment to be maintained

if their is a rise in blood glucose

• cells in islets of landerhands in the pancreus detect the high glucose blood levels
• insulin is secreted by beta cells
• the insulin causes muscel and liver cells to uptake glucose and convert it into glycogen via glycogensis  

if glucose blood levels are too low 

• cells in the iselts of landerhans
• glucogon is screted by alpha cells 
• this causes cells to convert glcogen into glucose which is then relased via glycogenolysis 

type one 

• pancreas cannot produce enough insulin or receptors to insulin not working/damaged
• this means after a meal their blood glucose concentration remains high
• kidneys are healthy, but there is so much glucose in the blood it is in the urine
• symptoms include dehydration, weight loss and lethargy
• this can be controlled with insulin injections and a carefully controlled diet

type two 

• this occurs in later life 
• insulin still works, but receptors cannot respond properly 
• glucose uptake is limited 
• this can only relay be controlled by a careful diet 

the liver plays a vital function to do with excretion it provides one of the main waste products of excretion users 

deamination occurs in the live this is where excess amino acids are broken down from proteins. 

they are broken down like so 

2NH2CH(R)COOH + O2=======>2 R-C=OCOOH + 2NH3

the ammonia produced by this reaction is very toxic 

so it is converted into urea by the ornithine cycle 

two ammonia and carbon dioxide react to make water and urea 

the kidney is split into three sections 

• the outermost section is called the renal cortex 
• the middle section is called the renal medulla 
• the innermost section is called the renal pelvis

the renal artery takes blood into the kidney where all metabolic waste products are filtered out of the kidney. 

any useful substances filtered out in the kidneys are completely  reabsorbed

the thing that actually filters the blood in the kidney is its functional unit the nephron their are hundereds of thousand of nephrons in the kidnay that all filter the blood.

 how blood is filtered 

blood arrives at the glomerulus the blood is under high pressure due to the contractions of the heart, the arterioles of the afferent arteriole being wider then the venule leaving and the high resistance of the glomerular capillaries and the inner wall of the bowman capsule

the blood undergoes ultra filtration where all particles under 69,000 mm are filtered into the proximal convoluted tube

this makes the glomerular filtrate made up of urea, glucose, water and amino acids 

selective reabsorption takes place in the proximal convoluted tube thanks to its brush border this is where important substances like water glucose and amino acids are reabsorbed 

100% of glucose is reasorbed and up to 80% of water

the filtrate then travels through the loop of Henle where some more water is reabsorbed 

then it goes through the distal convoluted tube to the collecting duct this is where anti diuretic hormone acts allowing for further water reabsorption to happen here 

this leads to the urethra where it is then excreted  

the glomerulus lies very close to the membrane of the bowman's capsule 

the glomerulus has many tiny pores

podocytes support the basement membrane

the basement membrane acts like a sieve to the high-pressure blood only allowing small molecules through it 

selective reabsorption happens in the PCT due to its brush border this is made of epithelial cells with many microvilli 

• this gives the PCT an increased amount of surface area
• glucose is reabsorbed using carrier proteins which pull it into the cell it is then passed into the blood vessels surrounding the PCT 
• by the time the filtrate reaches the end of the PCT 100% of the glucose has been reabsorbed 
• ions are reabsorbed by ion channels in the pct 
• water is also largely reasoned here via osmosis this is where most of the water is reabsorbed 

the amount of water in the urine is carefully controlled by the countercurrent multiplication of the loop of Henle as well as the collecting duct. 
the loop of Henle works like so 
sodium and chloride ions are actively transported from the ascending limb of the loop of handle to the media surrounding it lowering its water potential
the ascending limb is impermeable to water and only ions can leave from the ascending limb 
as the filtrate enters the descending limb water  leaves the filtrate going from high water potential to low after potential down the water potential gradient as it passes ions from the medulla enter the filtrate as the filtrate passes through the ascending limb the cycle starts over

this is controlled by a negative feedback loop 

low water potential in the blood. 

osmoreceptors detect a fall in the levels of water in the blood the hypothalamus produces ADH

ADH is then released into the blood by the pituitary glands 

it then travels through the blood stream to the collecting ducts 

this causes vesicles with aquaporins to move towards the membrane of the collecting duct fusing with it 

this allows water to be reabsorbed

all neurones carry these features 

• a cell body this is where the nucleus is 
• communicate via processes from the cell body 
• processes that carry away from the cell body are called axons
• processes that carry towards the cell body are called dendrons and many of them are called dendrites 

both sensory neurones and motor neurones are examples of these 

they are able to transmit an action potential much quicker using saltatory conduction 

characteristics of myelinated neurones 

• the axon or dendron is surrounded by an insulating myelin sheath 
• this is created by Schwann cells that wrap around the axon 
• between the cells theirs are gaps called the nodes for Ranvier 

how saltatory conduction works 

the action potential can only depolarise at the gaps in the myelin sheath as the local circuits are elongated  the sodium ions diffuse from one node to the next as such the action potential appears to jump from one node to the next 

the impulse that passes down an neurone is called an action potential 

the neurone is at rest and is polarised it has a resting potential of -70mv

a stimulus causes an action potential to be generated this causes sodium ion channels to open and potassium ions enter causing the neurone to depolarise

if the stimulus is large enough it will reach the threshold potential and an action potential will be generated this causes the neurone to become positive.

at +50mv the sodium ion channels close and potassium ion channels open allowing potassium to exit the cell 

this is called repolarization the cell starts to become more negative again it will hyper polarise this will cause the potassium ion channels to close and the sodium-potassium pumps will pump sodium out and potassium in to return the neurone to its resting potential 

Theirs is a short refractory period between action potential being fired as the neurone resets itself 

synapses are the points at which two neurones communicate 

these steps happen and allow the action potential to be passed on 

• the action potential reaches the presynaptic knob
• this causes the presynaptic knob to depolarise this makes voltage-gated calcium ion channels open
• calcium ions enter the presynaptic knob these cause vesicles containing neurotransmitters to move towards the membrane of the presynaptic knob
• the vesicle then fuses with the presynaptic membrane releasing neurotransmitters into the synapse via exocytosis  

what happen next 

• the receptors bind to receptors on the postsynaptic membrane 
• this causes sodium ion channels to open 
• this causes sodium ions to enter the postsynaptic membrane 
• depolarising it carrying on the action potential  

• the heart rate and respiration are carefully linked 
• the heart rate is controlled by a part of the brain called the medulla oblongata 
• the heart has its own pacemaker that dictates the heart rate this is the SinoAtrial Node
• this is controlled by the medulla oblongata that has two levels connected to the SAN that dictates the pace these are called the accelerator and vegas nerves 
• the accelerator nerve causes and increase in the frequency of the heart's contractions 
• the vegas nerve causes a decrease in the frequency of contractions fo the heart

what tells the medulla oblongata whether to increase the heart rate?

• movement of the limbs is detected b the stretch receptors sending a message that more oxygen may be needed causing an increase in heart rate 
• when exersizing lots of CO2 is produced this reacts with water causing the blood pH to lower slightly this is detected by chemoreceptors in the coroted atery the aorta and the brain this cuses a acion potnetial to be  sent b the medulla oblogata that increses the heart rate 
• blood pressure is monitored by stretch receptors in the walls of the carried sinus if blood pressure rises to high tis will be detected then causes the vegas nerve to activate nad lower the blood pressure