Electrochemical Activity

• Activity in the neurons involves the movement of the following ions:

  • Sodium (Na +):

    • Found more on the outside of the neuron.

  • Chloride (Cl - ):

    • Found more on the outside of the neuron.

  • Potassium (K +):

    • Found more on the inside of the neuron.

  • large proteins ( A-):

    • Found more on the inside of the neuron.

• The interiors of a neuron contain intercellular fluid.

• The exterior of the neuron contains extracellular fluid.

Neuron Membrane

Electrochemical Activity

• The semipermeable membrane:

  • Acts like a filter allowing the passage of some ions and preventing the passage of others.

• During the resting state of the neuron, more positively charged potassium ions (K+) are actively moved to the interior of the neuron, and more positively charged sodium ions (Na+), and negatively charged chloride ions (Cl-) are actively moved to the exterior of the neuron.

  • The large proteins (A-) are part of the cell membrane and thus don’t move across it.

    • Proteins are trapped, creating a negative charge which then causes the inside to be negatively charged relative to the outside when in a resting state/resting potential.

Membrane Potential

• membrane potential:

  • When the inside of a neuron has a negative charge compared with the outside.

• Resting potential

  • The difference between the inside and outside of a neuron when it is not sending a signal is typically around -70mV.

• The forces that maintain resting potential:

  • An uneven distribution of ions in both intracellular and extracellular parts of the neuron.

  • Two forces that cause this include the following:

    • Diffusion/concentration gradient

      • The movement from an area of high concentration down to an area of low concentration. (Na+ pulled in, Cl- pulled in, K+ pushed out.)

    • Electrostatic force

      • The interior of the neuron is negatively charged relative to the outside, which means positive ions are pulled in, and negative ions are pushed out. (Na+ pulled in, Cl- pushed out, K+ pulled in.)

• In general, always remember that Na+K+ Pump.

The Sodium-Potassium Pump

• The sodium-potassium pump does and is the reason for the following:

  • A process for actively pumping Na+ out of neurons and pumping K+ in.

  • Maintains polarized state/keeps equilibrium from forming.

  • Keeps the polarized state necessary to get nerve impulses.

    • If there weren’t a maintained polarized state, we would not get nerve impulses.

• If there is a poisoning of the pump, there won’t be any nerve impulses.

  • If you eat a puffer fish that has its toxin still inside it (Tetrodotoxin), your nerve impulses would be blocked, and the result can be fatal.

Depolarization, Hyperpolarization & Repolarization

• Depolarization:

  • When Na+ enters the neuron & it makes less of a negative charge on the inside, making it more likely to reach the threshold and achieve action potential/fire.

• Hyperpolarization:

  • A state that is more negative than the resting state which makes it less likely to achieve action potential.

• Repolarization:

  • The return of the resting state.

Activity Inside Dendrites

• Na+ moves inside dendrites, and the inside of the dendrites become less negatively charged, also known as depolarized/depolarization.

• As the dendrite moves down, the amount of Na+ moving in is decreased, which is known as decremental.

• If the dendrites have enough Na+ moving in to achieve -55mV at the axon hillock, it will reach the threshold and achieve action potential/nerve impulse.

The Threshold of a Neuron

• The threshold of a neuron occurs at the axon hillock.

• Although different neurons have different thresholds, the most common threshold is often -55mV.

Action Potential (AP)

• Also known as nerve impulse.

• Occurs inside the axon.

• A brief wave of positive electrical charges moves down the axon.

• A wave of positive ions that are moving in is called Depolarization.

  • Inside becomes positive relative to the outside (inside at +30 mV or +40 mV)

• Action Potential (AP) propagates down the axon/follows a domino effect

  • sodium moves in at one part of the axon, causing the voltage-dependent gate to open in the next part of the axon, Na+ moves in, and so on down the axon.

• Non-decremental

  • Does not decrease in intensity as it moves down the axon; the same amount of Na+ that is moving in is the same as it moves down.

• When Action potential (AP) gets to the terminal buttons, it causes neurotransmitters to be released into the cleft.

Postsynaptic Potentials

• Neurotransmitters plug into receptor sites on the postsynaptic membrane and can cause one of the following:

  • EPSP: Excitatory postsynaptic potential:

    • Excitatory: increases the likelihood of the postsynaptic
      neuron firing.

    • Na+ moves in, Making it less negative inside.

    • Causes depolarization.

  • IPSP: Inhibitory postsynaptic potential:

    • Inhibitory: reduces the likelihood of the postsynaptic
      neuron from firing.

    • Cl- moves in: makes a more negative charge on the
      inside.

    • Causes hyperpolarization.

Refractory Periods

• Refractory periods are also known as times when the neuron can’t fire.

  • Absolute:

    • Doesn’t fire, no matter how strong the stimulus is.

  • Relative:

    • Takes more substantial amounts of stimulus but can achieve action potential/ fire if enough stimulation occurs.

• Absolute refractory period:

  • 1-2 milliseconds during an action potential when another action potential cannot be produced

• Relative refractory period:

  • The short time when the neuron membrane is hyperpolarized (more negative than resting potential.)

  • Difficult but not impossible to generate another action potential, but will need a powerful stimulus.

Graded Potentials Vs. Action Potentials

• The graded potential is also known as activity inside dendrites.

  • Graded potential affects whether there will be enough activity at the axon hillock to reach the threshold & fire neurons.

• Many messages come into dendrites from other neurons.

• Whether there is enough activity to reach the threshold is determined by the additive effects of the messages called “Summation.”

Summation

• There are two types of Summation, those being:

  • Temporal: 2 EPSPs come close together in time at the same place on dendrites or a cell body.

  • Spatial: 2 EPSPs arriving at different places on dendrites or a cell body and combine.

Fibers & Cell Bodies

• Nerves:

  • Groups of fibers in the PNS (axons).

• Tracts:

  • Groups of fibers in the CNS (axons).

• Ganglion:

  • (Ganglia) cell bodies in the PNS.

• Nucleus:

  • (Nuclei) cell bodies in the CNS.

• Gray matter:

  • cell bodies

• White matter:

  • Axons (myelinated)

• Don’t confuse nerves with neurons; nerves are bundles of axons in the peripheral nervous system and are much larger structures than a neuron.

Types of Nerves: Reflex Arc

• Afferent or Sensory Nerves:

  • Bundles of sensory fibers that carry sensory information into the spinal cord.

• Motor Efferent Nerves:

  • Bundles of motor fibers that carry motor commands & messages from the spinal cord to muscles.

• Mixed Nerves:

  • Bundles of sensory & motor fibers.

• Example

  • Touch something hot: Sensory & Motor fibers
    travel together in mixed nerves up the arm.

    • As it gets to the spinal cord, Sensory nerves branch
      off and enter the spinal cord through the back (dorsal
      root.)

      • Want to pull your hand away: Motor nerves carry
        message out to muscles; come out from the front
        (ventral root.)

Other Structures & Types of Cells in the Nervous System

• Meninges:

  • Coverings of the brain & spinal cord.

• Meningitis:

  • Infection of meninges

• Central canal:

  • Runs the length of the spinal cord filled with CSF (cerebrospinal fluid.)

• Ventricles:

  • Fluid-filled cavities/chambers in the brain; continuous with central canal Filled with CSF.

• Hydrocephalus:

  • The buildup of fluid in ventricles; Ventricles enlarge, putting pressure on the brain

    • Treatment: shunt to drain fluid.

• Functions of the Glial Cells include:

  • Provide nutrients

  • Create myelin

  • Remove dead neurons

  • Repair

  • Protect blood-brain barrier