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5.1.3 Neuronal Communication (Generating an action potential: ( membrane…
5.1.3 Neuronal Communication
Mammalian sensory receptors
convert different types of stimuli into nerve impulses
sensory receptors: cells/sensory nerve endings that respond to a stimulus in the internal or external environment of an organisms and can create action potentials
Pacinian corpuscle: a pressure sensor found in the skin
oval shaped structure consisting of a series of concentric rings of connective tissue wrapped around the end of a nerve cell
pressure on skin deforms the rings of connective tissue which pushes against nerve endings
when pressure is constant, they stop responding
pressure causes deformation of membrane; becomes leaky so sodium ion channels open, and sodium ions diffuse into the sell producing a generator potential
transducer: a cell that converts one form of energy to another; in this case an electrical impulse
detect changes in different forms of energy e.g : table from book
if a stimulus is constant, it will not continue causing a response
Neurones at rest
lipid bilayers are not permeable to charged particles so must pass through a protein channel to cross the membrane
sodium potassium pump uses ATP to pump three sodium ions out of the cell and two potassium ions into the cell; creates a concentration gradient
action potential: a brief reversal of the potential across the membrane of a neurone causing a peak of +40mV compared to resting potential of -60mV
resting potential: the potential difference across the membrane while the neurone is at rest
gated sodium ion channels are kept closed
negatively charged anions in the cell enhance the negative potential of the cell
Neurones:
myelinated neurone:
has an individual layer of myelin around it; myelin sheath
one third of neurones
Schwann cells wrapped tightly around neurone
nodes of Ranvier: gaps in myelin sheath at regular 1-3mm intervals
myelin sheath prevents movement of ions across membranes; ion can only move at nodes of Ranvier
action potential jumps from one node to the next; rapid conduction
are usually longer than non-myelinated neurones
non-myelinated neurone:
has no individual layers of myelin
several neurones covered by one Schwann cell
action potential moves as a wave
transmission is slower
tend to be shorter than myelinated neurones
structure:
long to transmit action potential over long distance
plasma membrane has many gated ion channels
sodium potassium pump to generate ATP
maintain potential difference across plasma membrane
cell body contains nucleus, mitochondria and ribosomes
many dendrites to connect to other neurones; carry impulses towards cell body
axon carries impulses away from cell body
surrounded by fatty layer to insulate the cell from electrical activity in other neurones; composed to Schwann cells
differences:
sensory neurone: carry an action potential from the sensory receptor to the CNS; have a long dendron and a short axon
motor neurone: carries action potential from the CNS to the effector; long axon
relay neurone: join sensory neurones to motor neurones; short dendrites and axon
Generating an action potential:
membrane is at its resting state at -60mV
sodium ion channels open and some sodium ions diffuse into the cells, down the concentration gradient
membrane depolarises and reaches threshold level of -50mV
positive feedback causes voltage gated sodium ion channels to open and many sodium ions flood into the cell
cell becomes positively charges at +40mV
sodium ion channels close and potassium channels open
potassium ions diffuse out of the cell and the inside of the cell becomes negative; repolarisation
potential difference overshoots slightly, making the cell hyperpolarised
original potential difference is restored and cell is in resting state
positive feedback: a mechanism that increases a change taking the system further away from optimum; a small depolarisation of the membrane causes a change that increases depolarisation further
refractory period: short period of time after an action potential has been generated when concentration of ions inside and outside must be restored; ensures action potentials only travel in one direction; impossible to stimulate another action potential during this time
Transmission of an action potential:
when action potential occurs, sodium ion channels open
open sodium ion channels allow sodium ions to diffuse into the neurone; concentration of sodium ions inside the neurones increases where the sodium ion channels are open
sodium ions diffuse sideways along the neurone, down the concentration gradient ; movement of charged particles is a local current
local current causes slight depolarisation further along the neurone; opens voltage gated sodium ion channels
open channels allow rapid influx of sodium ions causing full depolarisation (action potential) further along the neurone
action potential travels in the same direction until it reaches the end of a neurone; not reverse direction as the concentration of sodium ions behind the action potential is still high
saltatory conduction
ionic movements that create an action potential cannot occur over much of the length of the neurone as there is an insulating myelin sheath; ions cannot diffuse through the fatty layer
ionic movements only at the nodes of Ranvier; gaps between the Schwann cells
in myelinated neurones, local currents are elongated and sodium ions diffuse along the neurone from one node to the next
speeds up the transmission of action potentials as it jumps across the nodes of Ranvier
myelinated neurones conduct action potentials much more quickly than non-myelinated neurones
frequency of transmission:
all action potentials have the same intensity; produces a depolarisation of +40mV
higher frequency of action potentials means more intense stimulus
when stimulus is at higher intensity, more sodium channels open in the sensory receptors; produces more generator potentials; more frequent action potentials in sensory neurone; more frequent action potentials entering the CNS
Sammer Sheikh