Along the axon there are several cells named Schwann Cells. These produce layers of fat called myelin and insulate the membranes within it from electrical impulses, meaning it isn't affected by nervous impulses.
Between schwann cells are small gaps called nodes of Ranvier, which do allow nervous impulses to propagate. Nervous impulses jump past the myelinated areas to the nodes of Ranvier, which makes impulses a lot faster.
At the end of each nerve cell there is a synapse which is a junction between nerve cells. On presynaptic neurones there are synaptic vesicles that contain neurotransmitters.On postsynaptic neurones there are neuroreceptors that have an active site complimentary to the transmitter.
When an Action Potential reached the presynaptic neurone, it causes calcium ions (in this example) to diffuse into the cell membrane via the concentration gradient, making synaptic vesicles fuse with the synaptic membrane and causing the vesicles to release the neurotransmitters via exocytosis.
The neurotransmitters then diffuse across the synaptic cleft and attach to their complementary receptors, causing ion channels to open and start another action potential.
Normally the potential difference within a cell membrane is negative, and contains a lot of K+ ions. Outside the membrane it is 0V by definition and contains many Na+ ions.
Action potentials are created when a stimulus is detected. Stimulation causes sodium ion channels to open, allowing Na+ ions into the cell membrane via diffusion from an area of high concentration to an area of low concentration, depolarising the membrane to 0V, and then reverse polarising it to +30mV.
Next, potassium ion channels open and allowing K+ ions to exit the cell and repolarise the membrane back to -70mV. There is a delay following this which enables the concentration gradients to restore the cell membrane back to the original potentials, and changes the permeability of the ion channels in the surrounding cells, carrying along the action potential. This is one mechanism that causes nervous impulses to only travel in one diection.
The All or Nothing principle states that to send a nervous impulse the stimulus must meet a certain threshold potential for it to continue across synapses. To meet the threshold summation can take place.
Temporal Summation : This is when one presynaptic neurone sends many rapid but small bundles of neurotransmitters to one postsynaptic neurone, or nervous potentials.
Spatial Summation : This is when two or more presynaptic neurone sends neurotransmitters to one postsynaptic neurone, or nervous potentials..
Summation is also called synaptic integration.
The Oestrous Cycle
FSH - Follicle stimulating hormone. This stimulates the growth of graafian follicles. It is made in the pituitary gland and results in Oestrogen to be produced.
Oestrogen - Produced in the graafian follicles. Inhibits the release of FSH and stimulates the uterus to build up a lining. It stimulates the release of LH.
LH - Leutenizing hormone. This stimulates the ovaries to release the ovum via ovulation. It also stimulates the follicle to become a corpus luteum which secretes progesterone.
Progesterone - Stimulates the uterus to complete the development of the blood lining so it is ready to receive an embryo. Progesterone inhibits the release of LH and FSH so both progesterone and oestrogen stop being released.
After around 10 days the corpus luteum disintegrates, menstruation occurs and progesterone levels decrease. FSH is no longer being inhibited and so increases in concentration, starting the cycle again.
Transcription is the process in which DNA is replicated using a template strand and mRNA.
At the start of each gene there is a promoter. RNA polymerase attaches to the promoter and temporarily breaks the hydrogen bonds, and allows the strands to act as a template as RNA polymerase binds complimentary nucleotides together with covalent bonds.
As RNA polymerase works it way down the DNA the sections that have already been used as a template eventually bond back together. The new strand usually only consists of the nucleotides used for that one gene.
Splicing occurs so that all introns (non-coding triplet codes) are removed so that protein synthesis doesn't end midway.
The mRNA then diffuses out of the nucleus via the nuclear pore to attach to a ribosome for protein synthesis.