COMMUNICATION AND HOMEOSTASIS Flashcards
Sensory neurones
Transmit nerve impulses from receptors to the CNS
Short dendrites
One long dendron
One short axon
Motor neurones
Transmit nerve impulses from the CNS to the effectors
Many short dendrites
One long axon
Relay neurone
Transmit nerve impulses between sensory neurones and motor neurones
Many short dendrites
Many short axons
Pacinian corpuscles
Mechanoreceptors
Skin
Sensory neurone wrapped in lots of layers of connective tissue called lamellae
When stimulated the lamellae deforms and presses on the sensory nerve ending causing a deformation of stretch mediated sodium channels along the sensory neurones cell membrane
The sodium ion channels open and sodium ions diffuse into the cell creating a generator potential
If the generator potential reaches the threshold potential an action potential is triggered
Neurone cell membranes
When at resting state the outside of the membrane is positively charged compared to the inside and the membrane is polarised
Resting potential is -70mV and is maintained by the sodium potassium pumps and potassium ion channels
The sodium potassium pumps use active transport to move 3 sodium ions out the neurone for every 2 potassium ions moved in
The membrane isn’t permeable to sodium ions so they can’t diffuse back in which creates a sodium ion electrochemical gradient as there are more sodium ions outside the cell than inside
The sodium potassium pumps also move potassium ions in to the neurone, the membrane is permeable to potassium ions so they diffuse back out through potassium ion channels, making the outside of the cell positively charged compared to the inside
Potassium ion channels use facilitated diffusion to allow potassium ions out of the neurone
Action potential
STIMULUS
excited neurone cell membrane which opens sodium ion channels
Membrane becomes more permeable to sodium so sodium ions diffuse into the neurone down the sodium ion electrochemical gradient
The inside of the neurone becomes less negative
DEPOLARISATION
If the potential difference reaches the threshold, voltage gated sodium ion channels open so more sodium ions diffuse into the neurone (positive feedback)
REPOLARISATION
At a potential difference of +30mV the sodium ion channels close and potassium voltage gated channels open
Potassium ions diffuse out the neurone down the potassium ion concentration gradient which makes the membrane get back to its resting potential (negative feedback)
HYPERPOLARISATION
Potassium ion channels are slow to close so there is a alight overshoot where too many potassium ions diffuse out the neurone
The potentials difference becomes more negative than the resting potential
RESTING POTENTIAL
Ion channels reset
The sodium potassium pumps returns the membrane to its resting potential until it’s excited by another stimulus
Wave of depolarisation
When an action potential happens some sodium ions that enter the neurone diffuse sideways cause the sodium ion channels in the next region to open and sodium ions diffuse into that part
The wave moves away from the parts of the membrane in the refractory period because these parts can’t fire an action potential
Myelinated neurones
Contains a myelin sheath
Made of Schwann cells
Schwann cells are patches of bare membrane called the nodes of Ranvier
Sodium ion channels are concentrated at the nodes
Depolarisation only happens at the nodes
The neurones cytoplasm conducts enough electrical charge to depolarise the next node, the impulse jumps from node to node and this is called saltatory conduction
Synapses
The presynaptic neurone has a swelling called the synaptic knob, this contains synaptic vesicles which are filled with neurotransmitters, there are many different neurotransmitters.
Synapses using ACh are called cholinergic synapses
An action potential arrives at the synaptic knob of the presynaptic neurone, the action potential stimulates calcium voltage gated ion channels in the presynaptic neurone to open and calcium ions diffuse into the synaptic knob
The influx of calcium ions into the synaptic knob causes the synaptic vesicles to move to the presynaptic membrane, they then fuse with it and release the neurotransmitter into the synaptic cleft by exocytosis
The neurotransmitter diffuses across the synaptic cleft and binds to specific receptors in the postsynaptic membrane which makes sodium ion channels in the postsynaptic neurone to open
This influx of sodium ions causes depolarisation and an action potential on the postsynaptic membrane is generated if the threshold is reached
The neurotransmitter is removed from the synaptic cleft so the response doesn’t continue
Synaptic importances
Synaptic divergence- allows information to be dispersed
Synaptic convergence- when one neurone connects to different parts of the body and the information can be made stronger
Summation- where the effect of the neurotransmitters can be combined
Spatial summation- neurones converge
Temporal summation- 2 of more nerve impulses arrive in a quick succession from the same presynaptic neurone which makes an action potential more likely because more neurotransmitter is released into the synaptic cleft
Receptors for neurotransmitters are only on the postsynaptic membranes so synapses ensures impulses travel in one direction
Hormonal system
Made of endocrine glands and hormones
Hormones are secreted when the endocrine gland is stimulated
Glands are stimulated by a change in concentration of a substance or by other electrical impulses
Hormones are secreted directly into the blood and are taken around the body in the circulatory system
They diffuse out of the blood into different parts of the body but each hormone will only bind to specific receptors for that hormone, on some membranes are target cells
Hormones trigger a response in the target cells
Endocrine gland
Group of cells specialised in secreting hormones
Second messengers
When a hormone binds to its receptor it carries the message for the first part of the way from the endocrine gland to the receptor in the target cells
When a hormone binds to its receptor it activates an enzyme in the cell membrane
This enzyme catalyses the production of a molecule inside the cell called a signalling molecule which signals to the other parts of the cell changing how it works
The signalling molecule is the second messenger because it carries the chemical message the second part of the way from the receptor to the other parts of the cell
Second messengers activate a chain reaction inside the cell
Adrenal glands
Endocrine glands found just above the kidney
Each adrenal gland has an outer cortex and inner medulla
CORTEX
Secretes steroid hormones when you’re stressed
Stimulates the breakdown of proteins and fats into glucose which increases the amount of energy available so the brain and muscles can respond to a situation
Increasing the blood volume and pressure by increasing the uptake of sodium ions and water by the kidneys
Suppresses the immune system
MEDULLA
Secretes catecholamine hormones (adrenaline and noradrenaline) when you’re stressed they act to make more energy available in the short term
Increases heart and breathing rate
Causes cells to break down glycogen into glucose (glycogenolysis)
Constructs some blood vessels so that the blood is diverted to the brain and muscles
The pancreas
Gland below the stomach Endocrine function Areas of the pancreas that contain endocrine tissues are called the Islets of Langerhans Found in clusters around blood capillaries Secrete hormones directly into the blood Made of alpha and beta cells Alpha cells secrete the hormone glucagon Beta cells secrete the hormones insulin
