The dendrites are covered with synapses formed by the ends of axons from other neurons. Illustration by Lydia V. The brain is what it is because of the structural and functional properties of interconnected neurons. The mammalian brain contains between million and billion neurons, depending on the species. Each mammalian neuron consists of a cell body , dendrites , and an axon.
When neurons receive or send messages, they transmit electrical impulses along their axons, which can range in length from a tiny fraction of an inch or centimeter to three feet about one meter or more.
Many axons are covered with a layered myelin sheath, which accelerates the transmission of electrical signals along the axon. This sheath is made by specialized cells called glia. In the brain, the glia that make the sheath are called oligodendrocytes, and in the peripheral nervous system, they are known as Schwann cells. The brain contains at least ten times more glia than neurons. Glia perform many jobs. Researchers have known for a while that glia transport nutrients to neurons, clean up brain debris, digest parts of dead neurons, and help hold neurons in place.
The activation of adenylyl cyclase can result in the manufacture of hundreds or even thousands of cAMP molecules. These cAMP molecules activate the enzyme protein kinase A PKA , which then phosphorylates multiple protein substrates by attaching phosphate groups to them. Each step in the cascade further amplifies the initial signal, and the phosphorylation reactions mediate both short- and long-term responses in the cell Figure 2. How does cAMP stop signaling?
It is degraded by the enzyme phosphodiesterase. Other examples of second messengers include diacylglycerol DAG and inositol 1,4,5-triphosphate IP3 , which are both produced by the enzyme phospholipase , also a membrane protein. Figure 2: An example of a signal transduction cascade involving cyclic AMP The binding of adrenaline to an adrenergic receptor initiates a cascade of reactions inside the cell.
The signal transduction cascade begins when adenylyl cyclase, a membrane- bound enzyme, is activated by G-protein molecules associated with the adrenergic receptor.
Adenylyl cyclase creates multiple cyclic AMP molecules, which fan out and activate protein kinases PKA, in this example. Protein kinases can enter the nucleus and affect transcription.
Figure Detail. Within proteins, the amino acids serine, threonine, and tyrosine are especially common sites for phosphorylation. These phosphorylation reactions control the activity of many enzymes involved in intracellular signaling pathways. Specifically, the addition of phosphate groups causes a conformational change in the enzymes, which can either activate or inhibit the enzyme activity.
Then, when appropriate, protein phosphatases remove the phosphate groups from the enzymes, thereby reversing the effect on enzymatic activity. Phosphorylation allows for intricate control of protein function. Phosphate groups can be added to multiple sites in a single protein, and a single protein may in turn be the substrate for multiple kinases and phosphatases.
At any one time, a cell is receiving and responding to numerous signals, and multiple signal transduction pathways are operating in its cytoplasm. Many points of intersection exist among these pathways. For instance, a single second messenger or protein kinase might play a role in more than one pathway.
Through this network of signaling pathways, the cell is constantly integrating all the information it receives from its external environment. This page appears in the following eBook. Aa Aa Aa. Cell Signaling. In order to respond to changes in their immediate environment, cells must be able to receive and process signals that originate outside their borders. Individual cells often receive many signals simultaneously, and they then integrate the information they receive into a unified action plan.
But cells aren't just targets. They also send out messages to other cells both near and far. How Do Cells Recognize Signals? Figure 1: An example of ion channel activation. An acetylcholine receptor green forms a gated ion channel in the plasma membrane. How Do Cells Respond to Signals? It diffuses from this neuron across a junction and excites the next neuron. Over half of all the nerve cells in your nervous system do not transmit any impulses.
These supporting nerve cells are located between and around your neurons to insulate, protect and nourish them. Home Explore the BBC. This page has been archived and is no longer updated. Find out more about page archiving. Prehistoric Life. The mind The body. Nervous system.
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