A recently discovered channel family termed TRPV (short for transient receptor potential vanilloid ) responds to osmotic stress by allowing calcium influx. The increased calcium concentration inside the cell regulates volume by activating calcium-dependent potassium channels. Defective TRPV channels are associated with cystic fibrosis.
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.
In most neurons, all the inputs and their interactions result in a net flow of current into the cell body, or soma, and the initial segment of the axon. If the voltage produced by this current is below threshold, it doesn’t produce any spikes. Above this threshold, the rate of action potentials, or spikes, is generally proportional to the net excitatory (depolarizing) current. The spikes are sent down the axon where, at the axon terminals, neurotransmitter is released. Neurons may release neurotransmitter on neurons a few tens of micrometers away or a meter or more away.
Moving our limbs: Motor neurons
Besides going to other neurons, the information that neurons transmit can also go to our muscles. All vertebrate neuromuscular transmission in vertebrates uses the neurotransmitter acetylcholine. The evolution of the nervous system is closely tied to the ability to move — animals have neurons, but plants don’t. The nervous system allows us to move based on the environment (sensation) and on previous experience (learning). The nervous system also enables muscles in different parts of the body to work in a coordinated way to perform complex behaviors. For example, putting one foot in front of the other and walking — which most people do every day — is actually a highly complex behavior. Think of all the muscles that need to work together just so you can take a few steps!
When Things Go Wrong: Genetics and Neurological Illness
Genetics is absolutely amazing: A random shuffle of genes from two parents get thrown together, producing a working nervous system
