Regulating Water and Cell Volume
The plasma membranes of neurons, as well as other cells in the body, are stable, but not rigid like the cell walls of plants. This means that neurons are flexible and can be deformed by mechanical pressure. Neurons exist within a matrix of glial cells and an extracellular matrix that holds the tissue together.
Observing osmotic pressure
What is osmotic pressure, and why does it matter to cells? Well, you’ve probably heard the classic lines, “Water, water, everywhere; nor any drop to drink.” Consider how people stranded in the middle of the ocean can’t survive by drinking the seawater all around them. High concentrations of salts (primarily sodium chloride) are dissolved in seawater. If seawater is added to the already highly concentrated extracellular fluid around our cells, the result is that water will tend to move from the higher concentration inside our cells to the lower concentration (relative to the concentration of ions) outside.
The plasma membranes of cells are not totally impermeable to water, because, among other reasons, membranes have water-permeable channels called aquaporins. This causes a net loss of water inside our cells so that they become hyperconcentrated with ions and die. Neurons, like other cells, have a variety of mechanisms for dealing with concentration balances between their cytoplasm and the extracellular fluid.
Responding to osmotic challenges
Osmotic imbalances are detected by a variety of sensors in cells, which respond in many ways such as with secretion, enzyme production, or changes in gene expression. Mechanosensitive channels in cell membranes respond to mechanical stress such as stretching of the membrane. These channels open or close to allow bulk flow of ions through the membrane to balance between different concentrations inside versus outside. Protein pump molecules may selectively bring in or expel soluble ions in response to different total ionic concentrations inside versus outside the cell.
Moving water with aquaporins
Although phospholipid plasma membranes have some limited permeability to water, we know that in many neurons water channels called aquaporins control much of this movement. Aquaporin channels in the membrane tend to equilibrate water between the cytoplasm and extracellular fluid. In addition, aquaporin channels can be up or down regulated over longer time scales. Aquaporin channel deficiencies are involved in some types of diabetes.
Knowing the Neuron: Not Just Another Cell
At the most fundamental level, neurons differ from other cells because they express different portions of the DNA sequence, leading to the production of different proteins. These proteins result in two major types of differences between neurons and other cells:
Structure
Membrane receptors and ion channels
Noticing neuron anatomy
The most obvious feature of most neurons is their extensive dendritic branching patterns. Most neurons have several primary dendritic branches that leave the cell body. These typically divide numerous times (out to the tenth order or more) forming a tree-like structure often called the dendritic tree or dendritic arborization. Figure 2-4 shows two cells from a rabbit retina with very different arborizations.
Figure 2-4: Two neurons from a rabbit retina: a starburst amacrine cell (left), and a directionally selective ganglion cell (right).
The dendritic arborization is where neurons receive most of their synaptic input — typically hundreds or thousands of synapses. These synapses are almost always a mixture of excitatory and inhibitory inputs. In many cases, the excitatory inputs are on dendritic spines, which are little mushroom-like appendages on the dendrites.
In any region of the nervous system are usually a finite number of distinct dendritic neuronal forms, between 10 and 30. Neurons are anatomically classified by the types of branching patterns using parameters such as total size, branching density, and the location of branches relative to other neurons. If you use a microscope to look at unstained tissue sections, these branching structures are usually not evident. Stains are necessary to reveal the structure of individual neurons.
Neurons also have an axon, their typical output structure. Usually a single process leaves the cell body and branches before forming synapses with other neurons. The axon conducts action potentials, millisecond-long electrical pulses that move from the cell body to the synaptic terminals of the axon, where they cause neurotransmitter to be released onto postsynaptic neurons.
Understanding what neurons do
Neurons enable sensation, communication, and movement in animals. They do this using their membrane receptors, ion channels, and exocytosis mechanisms. Reception and exocytosis themselves are both mediated by gated ion channels.
Taking in information: Receptors
Different types of receptors detect various kinds of energy from the environment. Here are a few examples:
Photoreceptors capture photons of light.
Auditory hair cells respond to acoustic energy, or sounds.
Mechanoreceptors are located in the skin and respond to touch.
Olfactory and taste receptors respond to molecules interacting with receptors in our noses and tongues.
Transforming information: Interneurons
After neurons receive information from either the environment or other neurons, they process that information using electrotonic potentials and ion concentrations that interact within the neuron. Neurons then transmit information to other neurons or to muscles or glands.