Chapter Summary:

Chapter 48 covers a variety of concepts from neuron structures to how neurons communicate with other cells at synapses. Neurons are nerve cells that transfer information within the body and the communication by neurons consists of two distinct types of signals; long-distance electrical signals and short-distance chemical signals. With this, neurons transfer many types of information ranging from heart rates, brain signals that allow hand- eye coordination, memories and much more.



Vocabulary:


48.1:

Sensory Neurons: a nerve cell that conducts impulses from a sense organ to the central nervous system.

Interneuron: any neuron having its cell body, axon, and dendrites entirely within the central nervous system, especially one that conveys impulses between a motor neuron and a sensory neuron.

Motor Neurons: a nerve cell that conducts impulses to a muscle, gland, or other effector.

Central Nervous System (CNS): the part of the nervous system comprising the brain and spinal cord.

Peripheral Nervous System (PNS): the portion of the nervous system lying outside the brain and spinal cord.

Cell Body: the compact area of a nerve cell that constitutes the nucleus and surrounding cytoplasm, excluding the axons and dendrites

Dendrites: branched extensions of a neuron that receive signals from other neurons

Axon: the part that attaches (appendage) of the neuron that transmits impulses away from the cell body.

Axon Hillock: the part on a nerve-cell body from which an axon arises

Synapse: a region where nerve impulses are transmitted and received

Synaptic Terminal: a bulb at the end of the axon where neurotransmitter molecules are stored and released

Neurotransmitters: any of several chemical substances, such as epinephrine or acetylcholine that transmit nerve impulses across a synapse to another nerve, muscle, or gland.

Presynaptic Cell: the transmitting cell at the synapse

Postsynaptic Cell: the target cell at a synapse

Glia: the delicate web of connective tissue that surrounds and supports nerve cells
48.2:

Membrane Potential: the difference between the inside of a cell and the fluid beyond or outside the membrane

Resting Potential: when the inside of a cell is more negative than the outside; the membrane potential of a cell that is not exhibiting the activity resulting from a stimulus

Ion Channels: a cell membrane channel that is selectively permeable to certain ions

Equilibrium Potential: the magnitude of a cell membrane’s energy at stability
48.3:

Gated Ion Channels: gated channel for a specific ion; protein channel that opens or closes in response to a particular stimulus and it can alter a membranes potential

Hyperpolarization: to increase the difference in electric potential across a cell membrane

Depolarization: to decrease the difference in electric potential across a cell membrane

Voltage-Gated Ion Channels: a special ion channel that its opening and closing depends on the changes in the membrane potential

Action Potential: the rapid change in a membrane potential. It can be caused by certain openings and closings of voltage-gated ion channels in sodium and potassium.

Threshold: the amount of potential an active cell membrane must reach for action potential to apply.

Refractory Period: the short time after action potential when the voltage-gate sodium channels do not open because the neuron does not respond to any other stimulus

Myelin Sheath: a wrapping of myelin around certain nerve axons, serving as an electrical insulator that speeds nerve impulses to muscles and other effectors.

Oligodendrocytes: a glial cell that forms myelin sheaths around the axons of neurons that are in the central nervous system.

Schwann Cells: a cell of the peripheral nervous system that wraps around a nerve fiber in a jelly-roll fashion, forming the myelin sheath.

Nodes of Ranvier: small gaps in the myelin sheath of axons

Saltatory Conduction: a form of nerve impulse conduction in which the impulse jumps from one Ranvier's node to the next, rather than traveling the entire length of the nerve fiber.

48.4:

Synaptic Vesicles: a membranous sac that hold neurotransmitter molecules at the tip of an axon.

Synaptic Cleft: the small gap between an axon terminal and any of the cell membranes in the immediate vicinity.

Excitatory Postsynaptic Potentials (EPSPs): a temporary depolarization of postsynaptic membrane potential caused by the flow of positively charged ions into the postsynaptic cell as a result of an opening of ligand-sensitive channels

Inhibitory Postsynaptic Potentials (IPSPs): an electrical change in the membrane of a postsynaptic neuron and it is caused by the binding of a
neurotransmitter from a presynaptic cell to a postsynaptic receptor.

Temporal Summation: when the membrane potential of the postsynaptic cell in a chemical synapse is determined by the combination of EPSP or IPSP’s effects when produced rapidly.

Spatial Summation: when the membrane potential of the postsynaptic cell determined by the combination of EPSP or IPSP’s effects and it is produced randomly by different synapse

Acetylcholine: a neurotransmitter that binds to receptors and changes the permeability of the postsynaptic membrane to certain ions.

Biogenetic Amines: a neurotransmitter that that comes from an amino acid

Serotonin: a neurotransmitter that works in the central nervous system

Dopamine: a neurotransmitter that acts like catecholamine (a compound that acts as a neurotransmitter or hormone; drug) i.e. epinephrine

Epinephrine: a catecholamine that gives a flight or fight response in the body when short term stresses are present. It releases adrenaline

Norepinephrine: noradrenalin that acts the same way as epinephrine; hormone.

Gamma-amino butyric Acid (GABA): an amino acid that works in the vertebrae’s of the central nervous system as a neurotransmitter to the central nervous system

Glutamate: an amino acid that acts as a neurotransmitter in the central nervous system

Neuropeptides: short chains of amino acids that act ad neurotransmitters.

Substance P: a neuropeptide that produces the feeling of pain.

Endorphins: hormones produced in the brain that shows pain.


The Neuron


The neuron consists of a cell body, which contains the nucleus and other organelles, and two types of cytoplasmic extensions called dendrites and axons. Dendrites are sensory; they receive incoming messages from other cells and carry the electrical signal to the cell body. A neuron can have hundreds of dendrites. a neuron has only one axon, which can be several feet long in larger mammals. Axons transmit an impulse from the cell body outward to another cell. Mnay axons are wrapped in a fatty myelin sheath that is formed by Schwann cells. The picture below depicts a neuron.

external image Neurons.gif

There are three types of neurons.
  • Sensory Neurons- receive an initial stimulus from a sense organ, such as the eyes and ears, from an another neuron.
  • A motor neuron stimulates effectors (Muscles or glands). a motor neuron, for example, can stimulate a digestive gland to release a digestive enzyme or to stimulate a muscle to contract.
  • The interneuron or association neuron resides within the spinal cord and brain, receives sensory stimuli, and transfers the information directly to a motor neuron or to the brain for processing.


Resting Potential


All living cells exhibit a membrane potential (to view more information on membrane potential click here), a difference in electrical charge between the cytoplasm (negative ions) and extracellular fluid (positive ions). Physiologists measure this difference in membrane potential using microelectrodes connected to a voltmeter. This potential should be between -50 mV and -100 mV. The negative sign indicates that the inside of the cell is negative relative to the pitside of the cell. A neuron in its unstimulated or polarized state (resting potential) has a membrane potential of about -70 mV. The sodium- potassium pump maintains the polarization by actively pumping ions that leak across the membrane. In order for the nerve to fire, a stimulus must be strong enough to overcome the resting threshold or resting potential. The larger the membrane potential, the stronger the stimulus must be to cause the nerve to fire. The picture below shows an example of resting potential in a neuron.
external image 070a63d0.gif

Gated Channels

Neurons have gated-ion channels that open or close in response to a stimulus and play an essential role in transmission of electrical impulses. These channels allow only one kind of ion, such as sodium or potassium, to flow through them. If a stimulus triggers a sodium ion-gated channel to open, sodium flows into the cytoplasm, resulting in a decrease in polarization to about -60 mV. The membrane becomes somewhat depolarized, so it is easier for the nerve to fire. In contrast, if a potassium ion-gated channel is stimulated, the membrane potential increases and the membrane becomes hyperpolarized, to about -75 mV, so that it is harder for the neuron to fire. below is a sodium ion-gated channel
Sodium ion-gated channel
Sodium ion-gated channel


and below is a potassium ion-gated channel

external image Ion_Channel.jpg

Action Potential

An action potential, or impulse, can be generated only in the axon of a neuron. When an axon is stimulated sufficiently to overcome the threshold, the permeability of a region of the membrane suddenly changes and the impulse can pass. Sodium channels open and sodium floods into the cell, down the concentration gradient. In response, potassium channels open and potassium floods out of the cell. This rapid movement of ions or wave of depolarization reverses the polarity of the membrane and is called an action potential. The action potential is localized and lats a very short time. The sodium-potassium pump restores the membrane to its original polarized condition by pumping sodium and potassium ions back to their original position. This period of re-polarization, which lasts a few milliseconds, is called the refractory period (to view more on the refractory period click here), during which the neuron cannot respond to another stimulus. The refractory period ensures that an impulse moves along an axon in one direction only since the impulse can move only to a region where the membrane is polarized.

Action Potential showed below:
external image 12_38_2action_potential_properties.jpg

The action potential is like a row of dominoes falling in order after the first one is knocked over. The first action potential generates a second action potential, which generates a third, and so on. The impulse moves along the axon propagating itself without losing any strength. If the axon is myelinated, the impulse travels faster because it leaps from one node of Ranvier to another, in a saltatory fashion.

The action potential is an all-or-none event; either the stimulus is strong enough to cause an action potential or it is not. The body distinguishes between a strong stimulus and a weak one by the frequency of action potentials. A strong stimulus sets up more action potentials than a weak one does.


The Synapse

Although an impulse travels along an axon electrically, it crosses a synapse chemically. The cytoplasm at the terminal branch of the presynaptic neuron contains many vesicles, each containing thousands of molecules of neurotransmitter. Depolarization of the presynaptic membrane causes Ca++ ions to rush into the terminal branch through calcium-gated channels. This sudden rise in Ca++ levels stimulates the vesicles to fuse with the presynaptic membrane and release the neurotransmitter by exocytosis (for more information on exocytosis click here) into the synaptic cleft. Once in the synapse, the neurotransmitter bonds with receptors on the postsynaptic side, altering the membrane potential of the postsynaptic cell. Depending on the type of receptors and the ion channels they control, the postsynaptic cell will be excited. In either case, shortly after the neurotransmitter is released into the synapse, it is destroyed by an enzyme called esterase and recycled by the presynaptic neuron. One common neurotransmitter is acetylcholine. Others are serotonin, epinephrine, and dopamine. In addition, many cells release gas molecules in response to chemical signals. The neurotransmitter acetylcholine stimulates some cells to release the gas nitric oxide (NO), which, in turn, stimulates other cells.

Terminal Branch of neuron at synapse:

external image synani1.gif




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