2.1 Nervous System and Action Potential.pdf

[00:00:00] >> [MUSIC] [00:00:10] The nervous system and action potential. These are the objectives for this lecture. The nervous system includes the central nervous system and the peripheral nervous system. The central nervous system includes the spinal cord and the brain. And the peripheral nervou...

[00:00:00] >> [MUSIC] [00:00:10] The nervous system and action potential. These are the objectives for this lecture. The nervous system includes the central nervous system and the peripheral nervous system. The central nervous system includes the spinal cord and the brain. And the peripheral nervous system includes the pairs of nerves leaving the central nervous system including the cranial nerves and the nerves to the body. [00:00:33] The peripheral nerve fibers are usually organized into sensory, motor, and mixed. The mixed nerves usually contain both sensory and motor neurons. Peripheral nervous system can also be organized into afferent and efferent neurons. The afferent neurons include all nerves associated with the transmission of sensory information back to the central nervous system. [00:00:57] And the efferent nervous system includes nerves that regulate movement and motor behavior or kind of that sends signals back into the body. The initial efferent nerves are within the central nervous system and are called upper motor neurons since their connections, cell bodies, and axons lie within the brain and the spinal cord. [00:01:19] Neurons have many different shapes and sizes, depending upon their location and function in the nervous system. The typical neuron consists of a cell body, which contains the nucleus, several short radiating processes called the dendrites, one long process called the axon, which you can see here. Which terminates into twig-like branches or the axon terminal. [00:01:45] The the axon may also have branches or collaterals projecting along its course. The axon together with its covering or sheath, the myelin sheath, forms the nerve fiber. Communication between nerves occurs at synapses and a chemical called a neurotransmitter is released. The functional unit of a muscle contraction is called the motor unit, and it includes the alpha motor nerve and all the muscle fibers it innervates. [00:02:12] The majority, if not all, of the neurons that innervate skeletal muscles are within the alpha size classification, thus they are known as alpha motor neurons. A single muscle can contain many motor units. The number of muscle fibers belonging to a motor unit and the number of motor units in an entire muscle vary. [00:02:32] The nervous system regulates the activity of muscle fibers by sending control signals in the form of action potentials. Conversion of a nerve impulse into the muscle impulse occurs through a complex process. The nerve fiber branches at its end to form a motor and plate, which adheres tightly to the surface of the muscle fiber but does not penetrate the muscle fiber membrane as you can see in this picture. [00:02:55] This connection is a type of synapse referred to as a mio-neural junction commonly referred to as a neuromuscular junction. The motor neuron and that plate contain mitochondria that synthesise a neurotransmitter acetylcholine. Movement of these ions into the muscle cells depolarizes the muscle fibre and triggers a muscle action potential that moves along the muscle fibre by an electrochemical mechanism similar to that of a nerve impulse. [00:03:25] Now let's look at an action potential. The differences in electrical potential exist across the membranes of all living cells. There are fluids inside, or intracellular, and outside or extracellular of each cell. These intracellular and extracellular fluids contain negatively and positively charged particles called ions. The ions are predominantly negative inside the cell and positive outside the cell. [00:03:51] This imbalance of ions from one side of the cell membrane to the other is called a potential difference. You can see that potential difference in this image here. The potential inside a cell membrane is measured relative to the fluid just outside of the membrane. Under resting conditions when no action potential's occurring, the membrane potential is called a resting potential and is negative. [00:04:16] Nerve cells, muscle cells, and sensory receptors maintain a negative resting potential in the range of -60 to -90 millivolts between the inside and outside of their membranes. The neuron innervating a skeletal muscle and the skeletal muscle itself each possesses a membrane characteristics that allow them to react when a stimulus is provided. [00:04:38] This ability to react to a stimulus is called irritability. Once nervous and muscular tissues react to a stimulus, the cell's membrane changes its resting potential and becomes more positive. This process is called depolarization. And you can see in this image where the resting membrane potential is at -70 and then After it reacts to a stimulus, it depolarizes and becomes more positive. [00:05:07] When the nerve or the muscle cell membranes are depolarized they become excitable and transmit the electrochemical impulse along the membranes so that the depolarization moves along the cell's membrane. When this depolarization continues to be transmitted, this impulse is known as an action potential. The action potentials are the electro chemical messages that are propagated in the movement system. [00:05:32] Immediately after depolarization, an active process termed repolarization returns the membrane back down to his resting potential. Neurons send control signals to other neurons or to muscles by releasing small amounts of the chemicals neurotransmitters. The neurotransmitters are released at the synapse. The chemical synapse between the two neurons may either be excitatory or inhibitory. [00:05:56] The excitatory synapses cause that depolarization. Inhibitory synapse results in a hyperpolarization or a more negative potential in the postsynaptic membrane. This inhibition increases the voltage requirement, so it's more difficult to create an action potential. These are the references.