Dynamic behaviour of human neuron
Neurons or nerve cells are electrically excitable cells in the nervous system. The main tasks of a nervous system are collecting information, processing information and eliciting a response to the information. In order to ful ll these functions, a huge number of linked neurons are essential. Alone the human brain,being one of the two part in central nervous system, has about 100 billions
neurons, which are highly connected with each other.
Neuronal cells consists of a cell body, the so-called soma, furthermore of dendrites, axon and the axon foot. The dendrites receive the chemical messages from other neuronal cells. The axon transmits the electro-chemical signal, the action potential, to other neurons. If the action potential reached the axon foot, the electro-chemical message will be again transformed into a chemical message.
We will only focus on the electro-chemical signal, the action potential.
Nerve signals are changes in membrane voltage, also called membrane potential, which are caused by ion movements through the cell membrane. The cell membrane has di erent charges on both sides. Inside from the cell membrane (in the cytoplasm) there are a lot of negative charged proteins and RNA, In the inside there are also a high concentration of potassium ions, which plays a di erent role in generating an action potential. Besides this the potassium ions also have a counter ion function for the negative charged proteins and RNA. But the concentration of negative charged ions overbalances, therefore the inside is negatively charged. Outside from the cell membrane (in the extracellular
uidity) a high concentration of sodium ions and a high concentration of chloride ions can be found, which is lower then the sodium ion concentration. Thus the outside is mainly positive charged.
the resting membrane with is steady state concentration, negatively charged, that means the inside is negative and the outside is positive and the cell membrane has a resting potential of -70mV. That means, that inside has in comparison to the outside a negative potential of -70 mV. But if a stimuli by chemical transmitter or electric signal occurs, than the balance of ions between inside and outside
will be inuenced Consequently the ion concentration steady state will be disturbed. If the
strength of this disturbance is high enough and the membrane voltage exceeds the threshold of -55mV, then an action potential at the axon hillock will be generated and will moves along the axon, The response to a stimulus follows the all-or-none principle. If the membrane potential stays below -55mV nothing will happens. But if the membrane voltage reach the -55mV, an action potential will be red immediately.
neurons, which are highly connected with each other.
Neuronal cells consists of a cell body, the so-called soma, furthermore of dendrites, axon and the axon foot. The dendrites receive the chemical messages from other neuronal cells. The axon transmits the electro-chemical signal, the action potential, to other neurons. If the action potential reached the axon foot, the electro-chemical message will be again transformed into a chemical message.
We will only focus on the electro-chemical signal, the action potential.
Nerve signals are changes in membrane voltage, also called membrane potential, which are caused by ion movements through the cell membrane. The cell membrane has di erent charges on both sides. Inside from the cell membrane (in the cytoplasm) there are a lot of negative charged proteins and RNA, In the inside there are also a high concentration of potassium ions, which plays a di erent role in generating an action potential. Besides this the potassium ions also have a counter ion function for the negative charged proteins and RNA. But the concentration of negative charged ions overbalances, therefore the inside is negatively charged. Outside from the cell membrane (in the extracellular
uidity) a high concentration of sodium ions and a high concentration of chloride ions can be found, which is lower then the sodium ion concentration. Thus the outside is mainly positive charged.
the resting membrane with is steady state concentration, negatively charged, that means the inside is negative and the outside is positive and the cell membrane has a resting potential of -70mV. That means, that inside has in comparison to the outside a negative potential of -70 mV. But if a stimuli by chemical transmitter or electric signal occurs, than the balance of ions between inside and outside
will be inuenced Consequently the ion concentration steady state will be disturbed. If the
strength of this disturbance is high enough and the membrane voltage exceeds the threshold of -55mV, then an action potential at the axon hillock will be generated and will moves along the axon, The response to a stimulus follows the all-or-none principle. If the membrane potential stays below -55mV nothing will happens. But if the membrane voltage reach the -55mV, an action potential will be red immediately.
Phases of action potential
If no stimuli occurs the axon is in steady state with a membrane potential of -70mV, and the
ion conductance for Na+ is very low. Firing of an action potential occurs if the membrane voltage exceeds the threshold of -55mV. In the depolarization phase, when the membrane voltage increase more and more, up to + 50mV,the voltage dependent ion channels for Na+ becomes active and a high Na+ in ux into the inside takes place. After that the repolarization starts, where the Na+ channels close and the also voltage-dependent ion channels for K+ open and the membrane potential falls. Afterwards the hyperpolarization enters. Because there are shortly more K+ outside, so that the outside is much more positive charged then the inside, the membrane voltage decrease to -85mV. The
hyperpolarization happens at the same time of the refractory period, where a new ring of action potential is not possible. Furthermore the Na+/K+ pump restores the membrane potential to -70mV. Finally the resting phase reenters and the neuron is ready for new ring of action potentials.
ion conductance for Na+ is very low. Firing of an action potential occurs if the membrane voltage exceeds the threshold of -55mV. In the depolarization phase, when the membrane voltage increase more and more, up to + 50mV,the voltage dependent ion channels for Na+ becomes active and a high Na+ in ux into the inside takes place. After that the repolarization starts, where the Na+ channels close and the also voltage-dependent ion channels for K+ open and the membrane potential falls. Afterwards the hyperpolarization enters. Because there are shortly more K+ outside, so that the outside is much more positive charged then the inside, the membrane voltage decrease to -85mV. The
hyperpolarization happens at the same time of the refractory period, where a new ring of action potential is not possible. Furthermore the Na+/K+ pump restores the membrane potential to -70mV. Finally the resting phase reenters and the neuron is ready for new ring of action potentials.
Voltage clamp experiment
In this voltage clamp experiment Hodgkin and Huxley measured the membrane voltage Vm by using an intracellular micropipette electrode and an electrode in the extra cellular fluid. They were able to control the Vm , also insert an external current for the purpose of generating an action potential in squid axons. They only could measure potential changes,that means they could not detect influx
or e fflux currents.
or e fflux currents.
Electric analog for cell membrane
The ions inside and outside a neuronal cell are separated through the cell membrane, the membrane behaves here like a capacitor. Discharging the capacitor corresponds to in
ux currents and charging to e ux currents. Because of the diff erent ionic concentrations inside and outside the cell, which are separated through the membrane and the closed ion channels, there will be established an electric potential difference between inside and outside, here called Vm ,the
membrane potential.
ux currents and charging to e ux currents. Because of the diff erent ionic concentrations inside and outside the cell, which are separated through the membrane and the closed ion channels, there will be established an electric potential difference between inside and outside, here called Vm ,the
membrane potential.
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