Neurons are characterized by their remarkable ability to propagate signals rapidly over large distances. The primary mechanism by which they do this is called an action potential, an electrical pulse/spike that travel down nerve fibers.
Neurons are able to encode and transmit information and behavior by firing sequences of these spikes in various temporal patterns. This first chapter is dedicated to the understanding of neural coding, which will involve the measurement and characterization of how stimulus attributes(light, sound, etc / motor actions, arm movements, etc) are represented by these action potentials.
Most of this study takes the form of enacting some stimulus, and observing the response thereof. This is called Neural Encoding, where the stimulus is the input and the response is the output. An example of this would be to catalog how neurons respond to a wide variety of stimuli, and then construct models that attempt to predict responses to other stimuli.
Neural Decoding on the other hand refers to the opposite, given a response and attempting to understand or reconstruct the stimulus that caused it. However that is discussion for a later chapter.
This chapter will focus on the underlying biological principles on how neurons generate their response, and how such activity can be recorded.
Properties of Neurons
Neurons are essentially specialized cells that are capable of generating an electrical signal given a certain chemical input, or by receiving some other information from another input.
One of the more important specializations seen in the Neuron is that of the dendrite. These receive inputs from other neurons and the axon that carries the neuronal output to other cells. They are characterized by their elaborate “branching” structure gives them the ability to receive inputs from many other neurons through their synaptic connections. In figure 1.1, you can see that for A and C, the cortical pyramidial neurons have thousands of synaptic inputs. and in the special cell called a cerebellar Purkinje cell, it has over a hundread thousand( however this figure does not convey the full extent of the axon network ).
Axons can travel extreme distances from a single neuron to other areas in the brain, and in some cases the entire body. For a typical mouse, it is estimated that there is about 40mm of axon “cable” stemming from the cortical neurons, and about 4mm of total dendritic cable in their branched dendritic trees. An average axon has about 180 synaptic connections with other neurons per mm of length, while the dendritic trees have about 2 synaptic inputs per . The average cell soma has a range in diameter of about 10 to 50 .
Beyond the physical features of the neuronal components, there are associated physiological specializations. Namely, Neurons use a variety of membrane spanning Ion Channels that allow for ions ( mostly sodium , calcium, potassium , and chloride ) to move in and out of the cell. Put simply, these channels are like gates that open and close to allow ions to flow in and out of the cell. The movement of these ions across the membrane is what generates the electrical signals that neurons use to communicate with each other.
The relevant signal that dictates the behavior of the nervous system is the difference in these electrical potentials, specifically between the interior of a neuron and its surrounding extracellular medium. Under certain resting conditions, this potential (about relative to the surrounding bath, which is understood to be )
