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4. Conversion of signals

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Conversion of electrical signals into chemical signals When an action potential  reaches nerve terminals, the signal must be relayed to target cells. This transmission occurs at synapses (the gap between two neurons). For the message to be transmitted from one neuron to another (also known as from the pre-synaptic neuron to the post-synaptic neuron ), the electrical signal is converted into a chemical signal in the form of a small signalling molecule known as a neurotransmitter .  Neurotransmitters are stored in nerve terminals in membrane bounded synaptic vesicles, and are released by exocytosis from the nerve ending when an action potential reaches the terminal. Depolarization of the nerve terminal plasma membrane opens voltage gated calcium channels . Since the concentration of  Ca 2 +  ions outside the cell is greater than that in the cytosol,  Ca 2 +  ions rush into the nerve terminal through the open channels. The increase in the concentrati...
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3. Action potential

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An action potential is a travelling wave of electrical excitation or a nerve impulse that can carry a message without the signal weakening from one end of a neuron to another. An action potential is triggered by a sudden local depolarization of the plasma membrane, i.e., by a shift in the membrane potential. A stimulus that causes sufficient large depolarization to surpass a certain threshold value causes voltage gated  Na +  channels to open, thus allowing  Na +  ions to enter the cell down their electrochemical gradient. ©  Molecular Devices The influx of positive charge depolarizes the membrane further, causing the membrane potential to become less negative. More voltage gated sodium  channels open, causing more Na+ ions to enter and therefore leading to further depolarization. The membrane potential becomes less and less negative, until it becomes positive.  The membrane potential reaches to a peak of about +40 mV, which is close t...
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2. Neurons

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Nerve cells, or neurons, receive, conduct, and transmit signals. They carry signals inwards, from sense organs to the central nervous system (CNS), which consists of the brain and the spinal cord. In the CNS, neurons signal from one to another through networks of enormous complexity, to analyse, interpret, and respond to the signals. Neurons can be extremely elongated. From the CNS, neurons extend processes outward to convey signals for action to muscles and glands.   Based on their roles, the neurons found in the human nervous system can be divided into three classes:  1)  sensory neurons - receive impulses and transmit them from the sense organs to the CNS. 2)  interneurons - connect sensory and motor neurons and interpret the impulse; only in the brain and spinal cord. 3)  motor neurons - carry impulses from the CNS to muscles or glands. Neurons, like other cells, have a cell body.  Various  processes  extend from the cell body. The...
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1. Membrane Potential

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Imagine taking two electrodes and placing one on the outside of the plasma membrane of a living cell, and the other on the inside. If you did this, you would measure an ele ctrical potential difference between the electrodes, i.e. the voltage. This electrical potential difference is called the  membrane potential . © Lumen Learning The inside of a cell contains a high concentration of anions, in the form of phosphate ions and negatively charged proteins. This is balanced by potassium ions ( K + ), which are the predominant positive ion inside the cell. Sodium-potassium pumps present in the plasma membrane actively pump  K +  ions into the cell, and  Na +  out. This results in a large concentration difference for  K +  across the membrane. It is to be noted that there are  K +  channels known as  K +  leak channels in the plasma membrane, allowing  K +  ions to move freely. These channels randomly flicke...
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