2.1

• The study of the nerve impulse looks at how neurons communicate information
• Axons do not use electrical conduction because the body's composition causes impulses to decay over distance, making long-distance communication unreliable.
• Impulse conduction in an axon is suited for information transfer

Resting Potential of a Neuron

• Disturbances to the resting potential develop messages in a neuron
• Resting potential is the electrical potential across a neuron's membrane when it isn't actively transmitting a message.
• All parts of a neuron are covered by a membrane about 8 nanometers thick
• The membrane is composed of two layers of phospholipid molecules with embedded protein molecules
• Some proteins form channels that allow certain chemicals (ions) to pass through the membrane at a controlled rate
• The membrane maintains an electrical gradient, also known as polarization, when at rest
• Polarization is a difference in electrical charge between the inside and outside of the cell
• The inside of the neuron has a negative charge in relation to the outside of the cell
• The negative charge is mainly due to negatively charged proteins inside the neuron
• The sodium-potassium pump transports three sodium ions (Na+) out of the cell and draws two potassium ions (K+) into it
• The sodium-potassium pump is a protein complex
• The sodium-potassium is an active transport process requiring energy
• Sodium ions are more than 10 times more concentrated outside the membrane than inside
• Potassium ions are more concentrated inside than outside due to the sodium-potassium pump
• The membrane prevents the sodium ions from leaking back in because of the selective permeability of the sodium-potassium pump
• Potassium ions slowly leak out, carrying a positive charge and increasing the electrical gradient across the membrane
• Two forces act on sodium tending to push it into the cell when the neuron is at rest: electrical gradient and concentration gradient
• The electrical gradient consists of sodium being positively charged, the inside of the cell being negatively charged, and opposite charges attracting.
• The concentration gradient consists of a higher concentration of sodium outside the cell than inside, that tends to diffuse into the cell to equalize the concentration
• The electrical gradient tends to move potassium ions into the negatively charged cell when at rest
• The concentration gradient tends to move potassium ions out of the cell when at rest
• There is a net tendency for potassium to exit the cell, because these two forces on potassium almost balance out
• The resting potential of a neuron is typically around -70 millivolts (mV).
• The neuron is prepared to respond rapidly to a stimulus by the resting potential
• The cell is ready for a vigorous response upon stimulation because of the concentration gradient

Action Potential

• Action potentials are messages sent by axons when the resting potential is disturbed
• A neuron's potential is measured using a microelectrode
• The inside of the axon shows a negative potential when measured at rest
• Hyperpolarization occurs when a negative charge is applied, increasing the negative charge inside the neuron
• The charge returns to the resting level when the stimulation ends
• Depolarization occurs when a current is applied to reduce the polarization toward zero
• Subthreshold stimulation produces a small, local response that quickly decays
• Stimulation beyond the threshold of excitation (around -55 mV for many neurons) produces a massive depolarization of the membrane, known as the action potential
• During the action potential:
• The membrane opens its sodium channels, allowing sodium ions to flow rapidly into the cell
• The electrical potential shoots up far beyond zero to a reversed polarity, such as +40 mV
• The all-or-none law states that for a given neuron, all action potentials are equal in amplitude (intensity) and velocity
• The stimulus has to reach the threshold, regardless of the intensity of the stimulus that initiated it
• A stronger stimulus doesn't produce a bigger or faster action potential
• The all-or-none law does not apply to dendrites because they do not have action potentials; they have graded potentials

Molecular Basis of the Action Potential

• The action potential relies on three key principles:
• Sodium ions are mostly outside the neuron, and potassium ions are mostly inside
• Sodium and potassium channels in the membrane open when the membrane is depolarized.
• These are voltage-gated channels, meaning their permeability depends on the voltage difference across the membrane
• The sodium channels close at the peak of the action potential
• Sodium channels (or gates) are cylindrical proteins in the membrane that open to allow sodium ions to cross
• They are fully closed at the resting potential
• Depolarization causes them to open, allowing sodium to flow into the neuron, driven by both the electrical and concentration gradients
• Potassium channels also open when the membrane depolarizes, but initially, this has little effect because the concentration and electrical gradients for potassium are almost balanced
• Sodium channels snap shut at the peak of the action potential.
• Both the concentration gradient and the electrical gradient drive potassium ions out of the cell through the still-open potassium channels.
• The membrane returns toward its original level of polarization because of the outflow of positive potassium ions
• Enough potassium ions leave, causing a temporary hyperpolarization because potassium channels remain open briefly after sodium channels close
• The membrane is resistant to starting another action potential during the refractory period following the action potential
• Absolute refractory period: the membrane cannot produce another action potential regardless of the stimulation, this is because the sodium channels are closed and cannot be reopened
• Relative refractory period: A stronger than usual stimulus is needed to initiate an action potential because the potassium channels are still open, and the membrane is hyperpolarized.
• The sodium-potassium pump continues to work to restore the original distribution of ions after an action potential, but this process takes time
• Local anesthetic drugs like Novocain and Xylocaine block action potentials by attaching to the sodium channels and preventing sodium ions from entering the axon.

Propagation of the Action Potential

• Propagation of the action potential describes the transmission of an action potential down an axon without loss of strength over distance
• That spot becomes temporarily positively charged when sodium ions enter a point on the axon during an action potential
• The positive ions depolarize the next area of the membrane and cause it to reach its threshold and open its voltage-gated sodium channels
• The action potential is regenerated at each point along the axon, traveling from the axon hillock to the presynaptic terminals
• The action potential always starts in an axon and propagates without loss from start to finish
• The action potential also back-propagates into the cell body and dendrites, which register the electrical event
• Back-propagation is important for making dendrites more susceptible to structural changes involved in learning

Myelin Sheath and Saltatory Conduction

• Action potentials travel at different velocities depending on the axon
• The velocity is less than 1 meter/second in the thinnest unmyelinated axons
• Increasing the diameter of the axon increases the conduction velocity (up to about 10 m/s)
• Myelin is an insulating material composed of fats and proteins that covers vertebrate axons, increasing the speed of conduction even further
• Myelin is formed by oligodendrocytes in the central nervous system and Schwann cells in the peripheral nervous system
• The myelin sheath is interrupted periodically by short unmyelinated sections of axon called nodes of Ranvier, which are about 1 micrometer wide
• Sodium channels are absent between these nodes
• When an action potential occurs at a node, sodium ions enter and diffuse along the axon, pushing a chain of positive charge to the next node
• Local current flow rapidly depolarizes the next node to its threshold, where a new action potential is regenerated.
• "Jumping" of the action potential from node to node is called saltatory conduction
• Saltatory conduction is faster than the continuous regeneration of the action potential along the entire length of an unmyelinated axon
• Saltatory conduction conserves energy because sodium ions enter the axon only at the nodes, reducing the work of the sodium-potassium pump
• Electrical charge flows in both directions during an action potential
• An action potential near the center of an axon doesn't reinvade areas because those areas are still in their refractory period

Local Neurons

• Local neurons are small neurons that have no axon
• They exchange information only with their closest neighbors
• They do not follow the all-or-none law because they lack an axon
• When a local neuron receives information, it has a graded potential, a membrane potential that varies in magnitude in proportion to the intensity of the stimulus.
• This change in membrane potential is conducted to adjacent areas of the cell, gradually decaying as it travels
• These areas then contact other neurons, which they can excite or inhibit

Neurons and Messages

• The physiological mechanisms discussed, like action potentials and ion channels, are the building blocks for understanding the connections between neurons, called synapses
• Synapses are the decision makers of the brain, as they determine whether a message is passed on to the next neuron
• The input to these decision makers is the on/off messages transmitted down axons in the form of action potentials
• All the complexities of human experience originate from these chemical processes
• Meaningful thought and activity require activating some neurons and inhibiting others
• People use all of their brain
• The popular belief that people use only 10 percent of their brain is a myth