<div epub:type="pagebreak">43</div>

<figure>
	<img src="img01.jpg" alt="…"/>
	<figcaption>
		Figure 2.2: Neurons transmit information to 
		other neurons. Information passes from the 
		axon of the presynaptic neuron to the 
		dendrites of the postsynaptic neuron.
	</figcaption>
</figure>

<p>
	The site where a presynaptic terminal ends in 
	close proximity to a receiving dendrite is 
	called the <dfn>synapse</dfn>. 
	The cell that sends out information is called the 
	<dfn>presynaptic</dfn> neuron, and the cell
	that receives the information is called the 
	<dfn>postsynaptic</dfn> neuron. It is 
	important to note that the synapse is not a 
	physical connection between the two 
	neurons; there is no cytoplasmic continuity 
	between the two neurons. The intercellular space 
	between the presynaptic and postsynaptic neurons 
	is called the <dfn>synaptic space</dfn> or
	<dfn>synaptic cleft</dfn>. An average
	neuron forms approximately 1,000 synapses with
	other neurons. It has been estimated that there
	are more synapses in the human brain than there
	are stars in our galaxy. Furthermore, synaptic
	connections are not static. Neurons form new
	synapses or strengthen synaptic connections in
	response to life experiences. This dynamic
	change in neuronal connections is the basis of
	learning.
</p>

<figure>
	<img src="img02.png" alt="…"/>
	<figcaption>
		Figure 2.3: The synapse is the site where 
		chemical signals pass between neurons. 
		Neurotransmitters are released from the 
		presynaptic neuron terminals into the 
		extracellular space, the synaptic cleft or 
		synaptic space. The released neurotransmitter
		molecules can then bind to specific receptors 
		on the postsynaptic neuron to elicit a 
		response. Excess neurotransmitter can then be 
		reabsorbed into the presynaptic neuron through 
		the action of specific reuptake molecules 
		called transporters. This process ensures that 
		the signal is terminated when appropriate.
	</figcaption>
</figure>

<div role="doc-pagebreak">44</div>

<p>
	The brain contains another class of cells called 
	glia. There are as many as 10 to 50 times more 
	glial cells than neurons in the central nervous 
	system. Glial cells are categorized as microglia 
	or macroglia. Microglia are phagocytic cells that 
	are mobilized after injury, infection, or 
	disease. They are derived from macrophages and
	are unrelated to other cell types in the nervous 
	system. The three types of macroglia are 
	oligodendrocytes, astrocytes, and Schwann cells.
	The oligodendrocytes and Schwann cells form the 
	myelin sheaths that insulate axons and enhance 
	conduction of electrical signals along the axons.
</p>

<p>
	Scientists know less about the functions of glial 
	cells than they do about the functions of
	neurons. Glial cells fulfill a variety of 
	functions including as
</p>

<ul>
	<li>
		support elements in the nervous system, 
		providing structure and separating and 
		insulating groups of neurons;
	</li>
	<li>
		oligodendrocytes in the central nervous 
		system and Schwann cells in the peripheral 
		nervous system, which form myelin, the sheath 
		that wraps around certain axons;
	</li>
	<li>
		scavengers that remove debris after injury 
		or neuronal death;
	</li>
	<li>
		helpers in regulating the potassium ion 
		(K+) concentration in the 
		extracellular space and taking up and 
		removing chemical neurotransmitters from 
		the extracellular space after synaptic 
		transmission;
	</li>
	<li>
		guides for the migration of neurons and for 
		the outgrowth of axons during development; and
	</li>
	<li>
		inducers of the formation of impermeable tight 
		junctions in endothelial cells that line the 
		capillaries and venules of the brain to form 
		the blood-brain barrier.<a role="doc-noteref"
		href="#fn03">3</a>
	</li>	
</ul>

<aside epub:type="sidebar">
	<h3>The Blood-Brain Barrier</h3>
	
	<p>
		The blood-brain barrier protects the neurons 
		and glial cells in the brain from substances 
		that could harm them. Endothelial cells that 
		form the capillaries and venules make this 
		barrier, forming impermeable tight junctions.
	</p> 
	
	<p>
		Astrocytes surround the endothelial cells 
		and induce them to form these junctions. 
		Unlike blood vessels in other parts of the 
		body that are relatively leaky to a variety 
		of molecules, the blood-brain barrier keeps 
		many substances, including toxins, away from 
		the neurons and glia.
	</p>
	
	<p>
		Most drugs do not get into the brain. Only 
		drugs that are fat soluble can penetrate the 
		blood-brain barrier. These include drugs of 
		abuse as well as drugs that treat mental 
		and neurological illness. The blood-brain 
		barrier is important for maintaining the 
		environment of neurons in the brain, but it 
		also presents challenges for scientists who 
		are investigating new treatments for brain 
		disorders. If a medication cannot get into 
		the brain, it cannot be effective. 
		Researchers attempt to circumvent the 
		problems in different ways. Some techniques 
		alter the structure of the drug to make it 
		more lipid soluble. Other strategies attach 
		potential therapeutic agents to molecules 
		that pass through the blood-brain barrier,
		while others attempt to open the blood-brain 
		barrier.<a epub:type="noteref"
		href="#fn04">4</a>
	</p>
</aside>