The Electrical Nature of Nerves Homeostasis 2: The Electrical Nature of Nerves

Electrical Nature of Nerves Neurons use electrical signals to communicate with other neurons, muscles and glands. When microelectrodes are placed on either side of the membrane of an inactive neuron, measurements from a voltmeter indicate an electrical potential difference of -70mV (millivolts) The charge of the inside of the neuron cell is negative in relation to the outside This charge separation across the membrane is known as the membrane potential.

Electrical Nature of Nerves

Sodium Potassium Pump A system that uses ATP in order to keep the electrical potential difference across the membrane. For every three sodium ions transported out of the cell, two potassium ions are transported into the cell. An overall positive charge is going to accumulate on the outside of the cell membrane and a negative charge on the inside. This ensures the resting potential of the neuron is at -70mV so that the cell is ready for an impulse to happen.

The Sodium Potassium Pump

Action Potential A nerve cell is polarized because it has a negative charge. It will depolarize during an action potential because the inside becomes less negative. An action potential is the movement of an electrical impulse along the membrane of a nerve cell’s axon. Action potentials are an all-or-none phenomenon where the strength of the action potential is always the same as long as there is enough depolarization to trigger the potential. (usually around -50mV and is called the threshold potential).

Action Potential Across the Membrane

Steps of an Action Potential Action potential triggered when threshold potential is met. Voltage gated sodium channels open and make membrane more permeable to sodium ions and they rush into the cell, making it depolarize. Now the membrane potential is +40mV Sodium channels close, potassium channels open, potassium moves down the concentration gradient out of the cell, which makes the membrane repolarize and actually becomes even hyperpolarized to about -90mV. Potassium channels close and the sodium-potassium pump continues to work so that the resting potential is restored. The next few milliseconds the membrane cannot be stimulated again as the membrane goes through a refractory period.

Myelinated Nerve Impulse At regular intervals along the axon, the nerve has nodes of Ranvier that are parts of exposed nerves between glial cells of the myelin sheath. These nodes contain many voltage-gated sodium channels. When the sodium ions move into the cell, the charge travels through the cell to the next node. This occurs at each node along the axon until it reaches the end of the neuron. Because the action potentials are forced to jump from one node to the next due to the myelin sheath, the conduction of an impulse along a myelinated neuron is called saltatory conduction (saltatory means to jump in latin) Saltatory conduction: 120 m/s, unmyelinated: 0.5m/s

Myelinated Propagation

Synapse A synapse is a junction between two neurons or between a neuron and an effector (muscle or gland) A neuromuscular junction is a synapse between a motor neuron and a muscle cell. Neurons are not directly connected. They have a small gap between them called the synaptic cleft.

Signal Transmission Across a Synapse When an impulse reaches the far end (called the synaptic terminal), the impulse must travel from the presynaptic neuron to the postsynaptic neuron. Chemical messengers called neurotransmitters carry the neural signal from one neuron to the next neuron or effector.

Neurotransmitters are found in the presynaptic terminal inside synaptic vesicles. Once the impulse reaches the synaptic terminal the synaptic vesicles move towards and fuse with the presynaptic membrane. Neurotransmitters are released into the synaptic cleft. Neurotransmitters bind to receptor proteins and affect the postsynaptic neuron. An enzyme comes in and breaks up the neurotransmitter and its components will be reabsorbed by the presynaptic neuron.

Synapse

Neurotransmitters Can have either an excitatory or an inhibitory effect on the postsynaptic membrane. If the effect is excitatory, the receptor proteins will allow positive ions, such as sodium to flow into the postsynaptic neuron and the membrane will depolarize. If the neurotransmitter is inhibitory, the receptor will trigger potassium ions to open, allowing potassium ions to flow out, hyperpolarizing the membrane.

Examples of Neurotransmitters Acetylcholine: crosses a neuromuscular junction and excites the muscle causing depolarization and contraction of the muscle. Dopamine: affects brain synapses in control of body movements, linked to sensations of pleasure (eating) Serotonin: regulates sensory and temperature perception, involved in mood control Endorphins: act as natural pain killers, emotional areas of the brain. Norepinephrine: complements the actions of the hormone epinephrine that ready the body to respond to danger or stress.

Practice: P. 362 # 6, 7, 9, 10, 11.