Homeostasis UNIT 3 AOS 2 Flashcards
what is the definition of homeostasis?
maintaining the internal environment of the body at constant levels between narrow limits
what are the two systems that take part in homeostasis?
- nervous system
2. endocrine system
define positive feedback
the response makes the process go faster e.g. when giving childbirth hormone oxytocin released to speed up labour
define negative feedback
the initial stimulus is cancelled out by the response which occurs as a result of the stimulus being present
what is the afferent pathway?
from stimulus to CNS
what is the efferent pathway?
from CNS to response
what is the direction of neuron signals
from dendrite to axon
what is an action potential? what happens so sodium and potassium?
electrical message along the neuron in response to a stimulus and leads to the transmission of an electrical impulse
- sodium goes in, potassium exits at the same time
why are nerve responses so fast?
- myelin sheath insulates message
- most of the sodium and potassium gates are open in the nodes of ranvier. this means that once the AP starts in the dendrites it whizzes down the axon to the next node where a new AP will start
define hormone
a chemical signalling molecule that is produced by ductless endocrine glands and travel somewhere else where they bring about a response
what are the two main types of hormones?
- peptide (AA based)
- steroid (lipid based)
define target cells
cells with special receptors for that particular hormone
what is the order of the stimulus response model?
stimulus, receptor, message, effector, response
what is a reflex arc?
shows how neurons are linked in pathways, illustrates a reflex action which you cannot control
what do dendrites do?
receives signals from other neurons
what are the nodes of ranvier?
the unmyelated spaces between adjacent Schwann cells. impulses jump from one node to the next, allowing for more rapid impulse transmission
what do excitatory synapses do?
sodium ions enter the postsynaptic cell and potassium leaves - there is a net flow of positive charges into the cell, the cell becomes depolarised moving membrane potential closer to the threshold
what do inhibitory synapses do?
potassium ions leave the postsynaptic cell, membrane potential is pushed to a voltage more negative than resting potential, cell is hyperpolarised, membrane potential further away from threshold, reduces chance of postsynaptic cell generating an action potential
where do steroid hormones bind to the cell? (hydrophobic)
these cross the plasma membrane and bind directly to receptors in the nucleus of cytoplasm
where do peptide hormones bind to the cell? (hydrophilic)
these do not cross the plasma membrane and bind to receptors on the membrane, this binding activates something within the cell
example of steroid hormones?
- sex hormones
example of peptide hormones?
- insulin and glucagon
what is signal transduction?
case of events originating outside the cell leading to a specific cellular response. initiated by cell-surface receptors
what is signal amplification?
the initial signal is made stronger due to a secondary reaction. this will then make the eventual response greater
what are guard cells?
pairs of highly specialised epidermal cells which form stomata and regulate the degree of opening and closing of stomata
what is endocrine signalling?
hormones released by ductless glands into the bloodstream to bond on or in target cells
what is paracrine signalling?
signals that are made but have a local effect. they do not travel large distances like endocrine ones
what is autocrine signalling?
secreting cells acts on itself by secreting hormone into extracellular fluid, releasing signals for its own receptors
what is a first messenger?
extracellular signal molecule that binds to the membrane receptors
what is a secondary messenger?
they amplify the cell response. initiate the intracellular signal transduction cascades
what is apoptosis?
- programmed cell death
- autocrine signalling
- signal transduction and amplification
how does apoptosis happen?
a cell will respond to a signal molecule which trigger a series of responses within the cell creating a destroyer protein the chops up the cell
how does abscisic acid respond to low water levels?
- abscisic acid is made when the roots detect low water levels
- this binds with receptors on the guard cells and allow potassium to diffuse out
- guard cells become plasmolyse and the stomata will close
how does abscisic acid respond to high water levels?
- no abscisic acid is released
- potassium is pumped into the guard cells via active transport
- guard cells become turgid, and stomata open
- water leaves via evaporation
what are neurohormones?
- a cross between a neurotransmitter and a hormone
- they act like hormones but are made by specialised nerve cells
- usually happens in the brains
e. g. ADH and oxytocin
what are pheromones?
chemical signals released by animals in very small quantities for communication between their own species. often used to attract the opposite sex
what does glucagon do?
raises blood sugar (peptide hormone)
what does insulin do?
lowers blood sugar (peptide hormone)
