The Nerve Impulse

Where does the impulse occur>?

from the axon hillock to the presynaptic terminals

What happens

• An electrical message that is transmitted down the axon

  • positive and negative charges

  • needs to be regenerated along the way

    • what you send needs to be true to the source

  • variable in speed

    • important cause of timing

    • important for retina distance recognition

    • compensate for delay

• between neurons is chemical

• the speed ranges from less than 1 meter/second to 100 meters/second

Resting potential of a neuron

the state of the neuron prior to the sending of a nerve impulse

When the neuron is at rest

• the membrane maintains an electrical gradient known as polarisation

  • difference between the electrical charge between the inside and outside of the cell

  • the inside is slightly negative relative to the outside

  • the nerve impulse develops from disturbances of the resting potential

• the membrane is selectively permeable

• sodium, potassium, calcium and chloride pass through the channels in the membrane

• sodium channels are closed

• potassium channels are partially closed allowing slow passage of potassium

• Sodium-potassium pump

Ion channels

Sodium-potassium pump

Membrane

Protein molecules around and channel for stuff to get through

Sodium-potassium pump

• the sodium-potassium pump is a protein complex

  • continually pumps three sodium ions out of the cell brings two potassium ions into the cell

  • helps to maintain the concentration gradient

    • outside of the cell sodium ions

    • inside the cell potassium ions → slightly negative

Concetation gradients

the difference between the concentration of sodium and potassium inside the cell relative to outside

Electrical gradient

the difference in the electrical potential of the inside cell relative to outside

Potassiums gradients

Potassium is inside the cell so its electrical gradient is telling it to stay in the negative, and the concentration gradient is telling it to go out of the cell to get away from the other potassiums

Sodium gradients

Sodium is outside the cell so its electrical gradient is telling it to go inside the cell where it is negative, and the concentration gradient is telling it to go inside the cell to get away from sodium

Sodium and potassium at rest

More sodium outside the cell more potassium inside the cell

Action potential

• could push in either direction

• the resting potential remains stable until the neuron is stimulated

  • hyperpolarisation

  • Depolarisation

  • the threshold of excitation

Action

• rapid depolarisation of the neuron

• the threshold of excitation varies from one neuron to another but is consistent for each neuron

Hyperpolarisation

• a bigger difference between the inside and the outside cell

• increasing the polarisation or the difference between the electrical charge of two places

• 70- -80

Depolarisation

• decreasing the difference or put it into positive numbers

• decreasing the polarisation towards/beyond zero

• what can cause action potential

• needs to be strong enough to hit threshold

The threshold of excitation

a level above which any stimulation produces a large depolarisation

Voltage-gated channels

• Membrane channels whose permeability depends upon the voltage difference across the membrane

  • sodium and potassium channels

• when sodium channels are opened positively charged sodium ions rush in and a nerve impulse occurs

• genetically coded to open at that cells threshold of excitation

  • sodium rushes into the cell

Returning to resting state

• After action potential occurs, sodium channels are quickly closed

• the neuron is returned to its resting state by opening the potassium channels

  • potassium ions flow out due to the concentration gradient and take with them their positive charge

• The sodium-potassium pump later restores the original distribution of ions

• astrocytes defuse the toxic potassium

Restoring the sodium potassium pump

• takes time restoring to the original state

• an unusually rapid series of action potentials can lead to a build up of sodium within the neuron

  • can be toxic to a cell, but only in rare instances such as stroke and after the use of certain drugs

All or none law

• the amplitude and velocity of an action potential are independent of the intensity of the stimulus that initiated it

• action potentials are equal in intensity and speed within a given neuron

• action potentials vary from one neuron to another in terms of amplitude velocity and shape

• has to reach that threshold of excitation to have that curve

• light switch

  • hit with enough force to turn it on intensity is always the same

Refractory period

• after an action potential, a neuron has a refractory period during which time the neuron resists the production of another action potential

• absolute refractory period

  • the membrane cannot produce an action potential (inactive sodium gates)

• Relative refractory period

  • stronger than usual stimulus required to trigger action potential (temporary hyperpolarisation)

  • +10 zap to get to threshold at true rest

  • need a +20 to than get to threshold

Propagation of an action potential

• sodium ions moving in a tube at different directions

• enough might get to the next slot in the axon

• as it spreads it dissipates in concertation

• goes backwards as well

• enough gets to the next spot to cause the thresholds and then action potential again and more sodium rushes in

• All or one law

  • the curve stays the same all along

  • how the message does not lose its strength

• Refractory periods

  • action potential only goes that way because of this

The Myelin Sheath

• are interrupted by short

• insulating material

  • stay in their and diffuse in there and stops it from leaking out

• At each node of Ranvier, the action potential is regenerated by a chain of positively charged ions pushed along from the previous segment

Saltatory conduction

• The jumping of the action potential from node to node

  • Provides rapid conduction of impulses

  • conserves energy for the cell

  • insulated axon that allows from longer jumps to be made

• Multiple sclerosis: disease in which the myelin sheath is destroyed

  • associate with poor muscle coordination and sometimes visual impairments

Local neurons

• Short axons. exchange information with only close neighbours, and do not produce action potential

• not the all or none law

• when stimulated, produce graded potentials - membrane potentials that vary in magnitude and do not follow the all or none law

• depolarise or hyperpolarise in proportion to the stimulation