Neural transmission and processing (study notes)

Neural transmission and processing

• Neural transmission refers to the electrochemical transfer of information from one brain cell to another.
• The human brain is composed of between $80 ext{ to } 120{,}000{,}000{,}000$ brain cells or neurons.
• The process has two main components:
  • An electrical component: the action potential.
  • A chemical component that occurs at synapses: synaptic transmission.
• The action potential originates from ionic chemical variations between the inner and outer cell walls of the soma (cell body) of a neuron.
• Action potentials are transmitted from the soma via axons, which are analogous to wires or cables within electrical circuits.
• To transmit a signal from one neuron to another, an additional stage is required: synaptic transmission.
• Synapses are tiny gaps in the communication pathway that connects one neuron to another.
• During synaptic transmission, the action potential in the presynaptic neuron triggers the release of neurotransmitters.
• Neurotransmitters travel across the synapse and chemically bind with the postsynaptic neuron.
• This chemical binding may induce a net excitation or a net inhibition in the postsynaptic neuron.
• This electrochemical process of action potential followed by synaptic transmission is the process of information transfer between one brain cell and another.
• It is the fundamental currency of all brain function, which constitutes all thought, all experience, and all behavior.
• This description reflects the immense complexity and connectivity of the human brain.
• So far, we have described neural transmission between just two neurons.
• For any given neuron, there exist many hundreds or tens of thousands of connections to other neurons.
• To illustrate the potential scale, if all connective tissue (axons and dendrites) linking neurons in a single brain were placed end to end, it would subtend over $8.5 \times 10^5 \text{ km}$.
• This vast connectivity underlies the brain’s incredible computational capacity and functional richness.

Action potentials and synaptic transmission (mechanism)

• Action potentials are the primary electrical component of neural signaling.
• They originate from ionic variations across the neuron's cell membranes, particularly at the soma (cell body).
• Once generated, action potentials travel along the axon to reach the synapse.
• Axons function analogously to wires or cables in an electrical circuit, transmitting the electrical signal over distance.
• When the action potential reaches the synaptic terminal, it initiates synaptic transmission.
• Synapses are the tiny gaps where communication between neurons occurs.
• In synaptic transmission, the presynaptic neuron releases neurotransmitters into the synaptic cleft.
• Neurotransmitters diffuse across the gap and bind to receptors on the postsynaptic neuron.
• Binding can lead to excitation (increasing the likelihood of an action potential in the postsynaptic neuron) or inhibition (decreasing the likelihood).
• The combined sequence—action potential generation, propagation along the axon, neurotransmitter release, synaptic diffusion, and postsynaptic binding—constitutes the fundamental information transfer between brain cells.

Significance and scope

• The electrochemical process of neural transmission is the basis for all brain function, including thought, experience, and behavior.
• Although we often describe transmission between two neurons, each neuron typically forms hundreds to thousands of connections with other neurons, creating a highly interconnected network.
• The length-scale example (placing axons and dendrites end to end) highlights the brain’s extensive connectivity and its implications for information processing.

Key numerical references (for quick recall)

• Neurons in the brain: $80 ext{ to } 120{,}000{,}000{,}000$
• Total connective tissue length (if laid end to end): $8.5 \times 10^5 \text{ km}$