Course Topics

An invertebrate neuron in culture.

Neurons are Electrical and Chemical Devices

• Like many cells in the body, the concentration of moveable charged particles, or ions, is different inside and outside of the neuron.
• In the neuron's membrane are small holes, ion channels, that can allow ions to flow in or out, and that can be controlled by voltage or chemicals (gated).
• The ability of an ion channel to allow an ion to flow through it is known as the channel's permeability to that ion.
• At rest, the permeabilities of the ion channels that are not gated are primarily responsible for setting a potential difference, or voltage, across the neuron's membrane, known as the resting potential.
• The set of ion channels within a neuron's membrane gives it a unique electrical "personality".
• Activation of neurons occurs by altering their resting electrical properties.
• At connections between neurons, chemicals may be released to communicate to other neurons; complex chemical pathways may be used to amplify and transmit signals within the neuron.
• Chemicals may also affect how intensely a neuron responds to other signals.

Neuronal Components

• The cell body, or soma, contains the nucleus, which synthesizes the proteins that are essential for the neuron's function.
• Most neurons have a long and very thick cylindrical process, the axon, that allows them to send signals over long distances.
• Inputs to the neuron come through fine branching processes known as dendrites.
• Outputs from the neuron generally occur at specialized endings known as synapses, where chemicals (neurotransmitters) are released to transmit messages from one neuron to the next.
• Some synaptic connections are electrical rather than chemical.

Communicating Quickly Over Long Distances: The Action Potential

• The axon generates an electrical signal that can propagate over long distances, the action potential (also referred to as a spike).
• The speed of action potentials determines how quickly an animal can take in information and react to it.
• Action potential speed also determines how quickly different parts of the brain can communicate with one another.
• Two kinds of ion channels play major roles in the action potential: voltage-gated sodium channels, and voltage-gated potassium channels.
• The cycle of ion movements that generates an action potential can be described mathematically.
• Not all neurons have axons, and so not all neurons generate action potentials.

Ion Channels and the "Personality" of a Neuron

• Each neuron may have different kinds of ion channels in its membrane.
• Different ion channels may permeate different ions (e.g., sodium, potassium, chloride, or calcium).
• Ion channels may be gated by the voltage across the membrane or by calcium levels within the neuron.
• Ion channels may be distributed differently in different parts of the neuron membrane depending on the function of that part of the neuron.
• The set of ion channels in a neuron's membrane determines its electrical personality:
  • It may be silent unless it receives input.
  • It may be active and fire spontaneously at a steady rate even in the absence of input.
  • It may spontaneously fire bursts of spikes interspersed with periods of quiescence.
  • It may fire irregularly.
  • In response to a steady input, it may initially fire many action potentials, and then become quiet.
  • In response to a steady input, it may not fire action potentials until after a delay.

Neuronal Shapes Affect Their Function

• As axons reach their targets, they often form many fine branches, where they form output synapses.
• Inputs to neurons occur on fine branches called dendrites.
• Changes in the diameter of a neuron can change how quickly an action potential propagates.
• Such changes can also affect how well any electrical signal moves from one part of the neuron to another.
• These changes in response to electrical input allow neurons to summate their many inputs in complicated ways that vary in space and in time, so that neuronal shape can cause complex spatio-temporal integration.
• It is possible to mathematically describe the complex neuronal responses to electrical input due to changes in shape.

How Neurons Talk to One Another: The Synapse

• The neuron sending a message is referred to as the presynaptic neuron, the neuron "before" the synapse.
• The neuron receiving a message is referred to as the postsynaptic neuron, the neuron "after" the synapse.
• The gap between the neurons is the synapse itself.
• At a chemical synapse, the presynaptic terminal releases neurotransmitters.
• At a chemical synapse, the postsynaptic terminal has receptors that bind the neurotransmitter.
• The receptors are connected, directly or indirectly, to ion channels.
• Changing the state of ion channels in the postsynaptic membrane alters its voltage, and may induce it to fire an action potential (an excitatory input), or make it less likely to fire an action potential (an inhibitory input).
• Treatments that change the probability of transmitter release, or the strength of the response of the receptors, can alter the effectiveness of neural transmission, and this can be characterized mathematically.
• Understanding neurotransmitters and receptors can lead to new drugs and treatments for diseases.
• Drugs that have structures similar to natural neurotransmitters can block their effects (antagonists) or mimic their effects (agonists).
• Chemical synapses can be used to amplify or modify signals between nerve cells.
• At electrical synapses, channels between neurons at the synapse allow currents to flow directly between them.

Modifying the Message: Neuromodulation and Plasticity

• Chemicals released by neurons may affect only one other neuron, or may affect much larger groups of neurons.
• Similarly, hormones released by glands may affect groups of neurons.
• Thus, chemical communication can range from fairly private (like a phone conversation in your room) to only moderately private (like a phone conversation on a bus), to fairly public (like a radio show).
• Chemical communication can be excitatory or inhibitory, or it can change the responsiveness of neurons to other signals, i.e., it can act as a neuromodulator.
• Neuromodulation is ubiquitous in the nervous system. An example is the release of chemicals within the brain that make you sleepy, and less likely to take in new information, or very awake, so that you notice everything.
• Over time, as chemical synapses are used, they may become stronger or weaker, i.e., they show plasticity.
• Plasticity is the major mechanism by which we learn and remember things, so understanding plasticity can help us learn and remember better.

Neural Circuitry and Systems

• Neurons connect together to form circuits.
• Neurons take on specialized forms as part of circuits.
• In a circuit, neurons can often be divided into sensory neurons, motor neurons, or interneurons.
• Sensory neurons transduce some environmental signal (e.g., light, heat, odors, sound, vibration) into action potentials that propagate into the nervous system.
• Motor neurons transduce neural signals into activation of muscles that allow organisms to move.
• Interneurons are interposed between sensory and motor neurons, and provide a myriad of parallel pathways for activity to propagate within the neural circuit.
• A simple example of a neural circuit is a reflex response, such as the very rapid withdrawal of your hand if you touch a hot stove.
• Circuits do not passively wait for sensory input to generate outputs; spontaneous activity ensures that neuronal circuits can initiate activity (and movement) in the absence of sensory input.
• Neural circuits are massively parallel, and the number of synapses from input to output is usually very small (they are "shallow"), allowing very rapid responses, compression and elimination of irrelevant information.
• Large collections of interacting circuits are often referred to as neural systems,
• Many neural systems have consistent anatomical architectures, such as the cerebellum or the cerebral cortex.
• In addition, many neural circuits and systems can be characterized more abstractly and mathematically by their patterns of physiological activity, or their neural dynamics.
• Understanding the nervous system requires a deep understanding of its ability to generate behavior that allows animals to survive and reproduce, because evolution does not select isolated neural circuits or systems.
• Ultimately, understanding the nervous system may require understanding how it is coupled to the body, the sensory system, and the environment.
• Understanding the brain will lead to an understanding of higher order mental functions, including consciousness, language, mathematics, music, and art.