The Nervous System - The Neuron PDF

# The Nervous System - The Neuron ## 1.1 Glia Cells - Glia (neuroglia) are the other major component of the nervous system. - Glia have many different functions but they do not transmit information like neurons - Glia are smaller and slightly more numerous than neurons. - Several types of glia ex...

# The Nervous System - The Neuron ## 1.1 Glia Cells - Glia (neuroglia) are the other major component of the nervous system. - Glia have many different functions but they do not transmit information like neurons - Glia are smaller and slightly more numerous than neurons. - Several types of glia exist to perform different functions ### Image: Shapes of Various Glia Cells * This image shows different types of glial cells. * The image shows a variety of Glial cells, including: * Astrocyte * Oligodendrocyte * Schwann cells * Microglia ## 1.1 The Blood-Brain Barrier - The Blood-Brain Barrier is the mechanism that keeps most chemicals out of the vertebrate brain. ### Why We Need a Blood-Brain Barrier? - The blood-brain barrier is needed because the brain lacks the type of immune system present in the rest of the body. - Because neurons generally cannot be replicated and replaced, the barrier is in place to minimize the risk of irreparable brain damage. - A wall is formed that keeps out most viruses, bacteria, and harmful chemicals. - When viruses do enter, like the rabies virus, it can infect the brain and lead to death. - A virus that enters your nervous system probably stays with you for life (e.g., chicken pox and shingles). ### How the Blood-Brain Barrier Works? - Endothelial cells form walls of the capillaries. These cells are tightly joined in the brain, blocking most molecules from passing. In the rest of the body, the endothelial cells are separated by gaps. - Small uncharged molecules (e.g., oxygen and carbon dioxide) and molecules that can dissolve in the fats of the capillary wall can cross passively (without using energy) through the blood-brain barrier. - An active transport system (a protein-mediated process that uses energy) exists to pump necessary chemicals, such as glucose, through the blood-brain barrier. - The blood-brain barrier is essential for health. - For example, in Alzheimer’s disease, the endothelial cells lining the brain’s blood vessels shrink and harmful chemicals can enter the brain. - The blood-brain barrier poses difficulty in medicine because it keeps out many useful medications that may be used to treat diseases like brain cancer. ## 1.2 The Nerve Impulse - Resting Potential ### The Resting Potential of the Neuron - The membrane of a neuron maintains an electrical gradient (also called polarization- a difference in electrical charge between the inside and outside of the cell). - In the absence of any outside disturbance (i.e., at rest), the membrane maintains an electrical polarization (i.e., a difference in electrical charge between two locations) that is slightly more negative on the inside relative to the outside. - This difference in electrical potential or voltage is known as the resting potential. - The resting potential is measured by very thin microelectrodes. - A typical resting membrane potential is -70 millivolts (mV). This may vary from one neuron to another. ### Forces Acting on Sodium and Potassium Ions: - The neuron membrane is selectively permeable, which allows some molecules to pass freely (e.g., water, carbon dioxide, oxygen) while restricting others. - Most large molecules and ions cannot cross the membrane. A few important ions cross through protein channels. - During the resting potential, chloride channels (or gates) remain open along the membrane, which allows ions to pass through. - Potassium channels are mostly closed, causing potassium to cross the membrane slowly. - Sodium gates remain closed, restricting the passage of sodium ions. ### Sodium-Potassium Pump: - A protein complex found along the neuron membrane that transports three sodium ions outside of the cell while also drawing two potassium ions into the cell; this is an active transport mechanism (requires energy in the form of ATP to function). - The sodium-potassium pump causes sodium ions to be more than ten times more concentrated outside than inside. - ATP - energy carrying molecule found on cells. ATP captures chemical energy found in food and uses to fuel cellular processes. ### Why a Resting Potential? - The advantage of the resting potential is to allow the neuron to respond quickly to a stimulus. ## 1.2 The Nerve Impulse - Action Potential ### Key terms: - **Action potential**: Messages sent by axons. - **Hyperpolarization** (increased polarization): Occurs when the negative charge inside the axon increases (e.g., -70 mV becomes -80 mV). - **Depolarization** (reduce polarization towards zero): Occurs when the negative charge inside the axon decreases (e.g., -70 mV becomes -55 mV). - **Threshold of excitation**: The level that a depolarization must reach for an action potential to occur. A subthreshold stimulation produces a small response proportional to the amount of current. However, as long as the stimulation is above the threshold, regardless of how far beyond, the stimulation produces a big response. - **The All-or-None Law:** - The all-or-none law means that the amplitude and velocity of an action potential are independent of the intensity of the stimulus that initiated it. - If the threshold is met or exceeded, an action potential of a specific magnitude will occur; if the threshold is not met, an action potential will not occur. ## The Molecular Basis of the Action Potential - **Principles of the action potential:** - At the start, sodium ions are mostly outside the neuron and potassium ions are mostly inside. - When the membrane depolarizes, sodium and potassium channels in the membrane open. - At the peak of the action potential, the sodium channels close. - **Voltage-gated channels** regulate channels of sodium and potassium. Their permeability depends on the voltage difference across the membrane. - When the action potential reaches its peak, voltage-activated sodium gates close, and voltage-gated potassium channels open, allowing potassium ions to flow outside of the membrane due to their high concentration inside the neuron as opposed to outside. In addition, the electrical gradient is now pushing the potassium to flow outward. - A temporary **hyperpolarization** (membrane potential below the resting potential) occurs before the membrane returns to its normal resting potential (this is due to potassium gates opening wider than usual, allowing potassium to continue to exit past the resting potential). - After the action potential, the neuron has more sodium and fewer potassium ions for a short period (this is soon adjusted by the sodium-potassium pumps to the neuron’s original concentration gradient). - The sodium-potassium pump does not restore the resting membrane potential; diffusion does. Less than 1 percent of the total sodium ions present cross the membrane during an action potential. - **Local anesthetic drugs** (e.g., Novocain, Xylocaine, etc.) block the occurrence of action potentials by blocking voltage-activated sodium gates (preventing sodium from entering a membrane). - Action potentials only occur in axons; dendrites do not have voltage-dependent channels. - In most neurons, the absolute refractory period is about 1 ms and the relative refractory period is another 2-4 ms. ### Propagation of the Action Potential - The action potential begins at the axon hillock (a swelling located where the axon exits the cell body). - The action potential is regenerated due to sodium ions moving down the axon, depolarizing adjacent areas of the membrane. - **Propagation of the action potential**: Transmission (movement) of an action potential down an axon. The action potential moves down the axon by regenerating itself at successive points on the axon.