NERVE CELLS & NERVE IMPULSES: (WK1):

the nervous system

CELLS OF THE NS:

  • comprised of two types: 1) neurons      2) glia

  • human brain contains approx 100 billion individual neurons

  • behaviour depends on the communication between neurons

  • three subdivisions:

    • dendrites (gather info from other neurons)

    • cell body or soma (core region, contains nucleus & integrates the information)

    • axon (carries info to be passed onto other cells)

TYPES OF NEURONS:

  • sensory neurons

    • brings info to the CNS

  • interneurons (association neurons)

    • associate sensory and motor neurons activity within the CNS

  • motor neurons

    • sends signals from the brain and spinal cord to muscles

how neurons communicate

EXCITATION AND INHIBITION:

  • each neuron receives thousands of excitatory and inhibitory signals every second

  • neurons ‘sum’ these signals and respond accordingly: 1 or 0

    • 1 = sending message to downstream neurons

    • 0 = not sending any messages downstream

EQUILIBRIUM:

  • efflux of chloride ions down the chloride concentration gradient is counteracted by the influx (inward flow) of chloride ions down the chloride voltage gradient

  • equilibrium is reached when the concentration gradient of chloride ions is balanced by the voltage gradient of chloride/sodium ions 

  • equilibrium = no NET movement

RESTING MEMBRANE POTENTIAL:

  • electrical charge across the cell membrane in the absence of stimulation

  • a store of negative energy on the intracellular side relative to the extracellular side

  • the inside of the membrane at rest is -70 millivolts relative to the extracellular 

  • four charged particles take part in producing the resting potential

    • sodium (Na+) and chloride (Cl-)

      • higher concentration outside cell

    • potassium (K+) and large proteins (A-)

      • higher concentration inside cells

  • resting is NOT no-moving 

  • large A- molecules cannot leave cell - making the inside negative

  • passive K+ channels allow K+ to move into and out of cell more freely but sodium channels keep out Na+ ions

  • Na+-K+ pumps extrude Na+ from intracellular fluid and inject K+

HOW NEURONS INTEGRATE INFORMATION:

  • graded potential:

  • hyperpolarisation 

    • increase in electrical charge across a membrane (more negative)

    • due to inward flow of chloride ions of outward slow of potassium ions

  • depolarisation

    • decrease in electrical charge across a membrane (more positive)

    • due to inward slow of sodium

GRADED POTENTIAL:

ACTION POTENTIAL:

  • action potential

    • large, brief reversal in polarity of an axon

    • lasts approx 1 millisecond

  • threshold potential

    • voltage on a neural membrane at which an action potential is triggered

    • opening of Na+ and K+ voltage-sensitive channels

    • approx -40 mV relative to extracellular surround