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Role of oxygen (or energy) in nerve impulse transmission - Author Amit Kulshresth      (Nov 22, 2005)

Nerve cells require oxygen and glucose to transmit impulse, in this article I will discuss what is the role of each constituent into propagation of impulse. A look at the structure and composition of neuron and neuronal transmission mechanism will help in understanding the concept.

neurons

What are neurons?
The nervous system in the human body is made up of billions of nerve cells, or neurons, organized in various networks. The majority of these neurons are located in the brain, brain stem and spinal cord comprising the central nervous system (CNS).

Each nerve cell or neuron has a central portion containing the nucleus ( cell body) and one or more extensions protruding from the cell body called axons and dendrites.

mitochondria in neurons

  • A cell membrane surrounds neurons.
  • Neurons have a nucleus that contains genes.
  • They also contain cytoplasm, mitochondria and other "organelles".
  • They carry out basic cellular processes such as protein synthesis and energy production.
  • They have specialized extensions called dendrites and axons. Dendrites bring information to the cell body and axons take information away from the cell body.
  • Neurons communicate with each other through an electrochemical process.
  • They contain some specialized structures like synapses and chemicals like neurotransmitters
  • Dendrites receive nerve impulses from other neurons or from sensory receptors.
  • The axon carries the nerve impulses (action potential) away from the cell body to another neuron or to a muscle. Axons can be very long-up to a meter in the case of a neuron that reaches from the big toe to the base of the spinal cord.


Neurons need energy to function :
Like all the other cells in the human body, neurons need energy to function. They consume an especially large amount of energy. The brain is only 2% of the human body's weight, but consumes about 20% of energy. Its consumption is about 25 watts.

Neurons consume oxygen and glucose for fuel. The more challenging the brain's task is, the more fuel it consumes. This energy is produced in the mitochondria. There are hundreds or thousands of mitochondria in neurons. They use oxygen to extract energy from glucose and fats and to produce molecules of the energy-storage compound adenosine triphosphate (ATP). These ATP molecules are then used to fuel the various chemical reactions that take place within the neuron.

firing of neurons

Proof of energy consumption : Using fMRI (functional Magnetic Resonance Imaging), it can be detected that active neurons consume oxygen . The blood supply of the brain is regulated to give active neural assemblies more energy whilst inactive assemblies receive less energy. This dynamic regulation of the blood flow (called haemodynamic) is the principal on which fMRI is based.
When neurons fire, they consume oxygen. Nearby capillaries dilate in response to the oxygen deficit, thereby infusing the region with oxygen rich blood. This slightly increases the proportion of hemoglobin that has been stripped of oxygen relative to the amount of hemoglobin carrying oxygen. The magnetic properties of hemoglobin-molecules in red blood cells that transport oxygen to the brain-differ when they are carrying oxygen. Optically hemoglobin without oxygen reflects less red light than hemoglobin with oxygen, active areas look slightly darker.
Thus neuronal activity affects the ratio of oxygenated and deoxygenated hemoglobin in the brain.
FMRI can track this increase in blood flow and generate increasingly intense images of heightened brain function. This implies that activated neurons do increase their consumption of oxygen.

stimulus through nerves
Neuronal Transmission - The impulse needs energy :
Neurones send message electrochemically. Neural transmission relies on the rapid and precise delivery of chemical signals along neurons and across the spaces between them.

When a neuron is not sending a signal, it is at 'rest'. Its membrane is responsible for the different events that occur in a neuron. A strong stimulus like touch, pressure, stretching, sound waves, motion to the dendrite of a neuron initiates a nerve impulse. The impulse begins as an exchange of ions across a localized area of the plasma membrane.

The stimulus makes the nerve cell secrete a hormone, changing the permeability of outer cell membrane of the axon to positive ions that allow electrically-charged ions to move in and out of the cell . The influx of positive ions reduces the electrical potential across one segment of the membrane (depolarization). This reversal of polarity causes the same change in the neighboring area of the membrane, which causes a reversal of polarity further along, and so on. This impulse travels along the axon as a self-generating chain reaction. This is what really occurs when a nerve is described as "firing." Nerves do not conduct electricity or any other form of energy.

action potential

The impulse propagates along the length of the neuron as a cascade of ionic exchanges. It does not lose energy as it travels because it is continually regenerated at each new site along the membrane. Maintaining these ionic gradients is an energy-consuming process that requires a constant supply of glucose and oxygen to the neuron. Lack of glucose and oxygen deplete the cellular energy stores required to maintain electrical potentials and ion gradients. The energy of a nerve Impulse has been calculated to be 5 x 10-15 joules on an average, it may vary because of nature of sheath. As the supply of oxygen decreases the energy of the impulse also decreases. Lack of glucose and oxygen deplete the cellular energy stores required to maintain electrical potentials and ion gradients. The oxygen or energy requirement can be judged from an experiment performed at University of Rochester

The resting rate of oxygen consumption of the excised sciatic nerve of the frog is 1.23 c.mm. of oxygen per gm. of nerve per minute during stimulation with an induction coil with 100 make and 100 break shocks per second there was an excess oxygen consumption amounting on the average to 0.32 c.mm. of oxygen per gm. of nerve per minute of stimulation, or a 26 per cent increase over the resting rate thus establishing the importance of energy in neuronal transmission.(source :www.jgp.org/cgi/content/abstract/10/5/767)

For understanding more about neuronal transmission, view our animation, click here


 
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