Biology: Neuroscience Flashcards
Nervous System
Includes all of the neural tissues in the body. Neural tissue, which supporting blood vessels and connective tissue, are the components of the nervous system, such as the brain and spinal cord, as well as complex sensory organs like the eye and ear. The two major divisions of the nervous system are central and peripheral nervous systems. In general, the nervous system enables organisms to receive and respond to stimuli from their external and internal environments.
Neuron
Neurons are the functional units of the nervous system. A neuron converts stimuli into electrochemical signals that are conducted through the nervous system. The nervous system responds to stimuli more rapidly than the endocrine system.
Neuron Structure: General
It’s an elongated cell consisting of several dendrites, a cell body, and a single axon.
Neuron: Dendrites
Dendrites are cytoplasmic extensions that receive information and transmit it toward the cell body.
Neuron: Cell Body
The cell body (soma) contains the nucleus and controls the metabolic activity of the neuron.
Neuron: Axon
The axon is a long cellular process that transmits impulses away from the cell body.
Most mammalian axons are sheathed by an insulating substance known as myelin, which allows axons to conduct impulses faster. Myelin is produced by cells known as glial cells. (Oligodendrocytes produce myelin in the central nervous system, and Schwann cells produce myelin in the peripheral nervous system.)
The gaps between segments of myelin are called nodes of Ranvier.
The axons end as swellings known as synaptic terminals (sometimes also called synaptic buttons or knobs). Neurotransmitters are released from these terminals into the synapse (or synaptic cleft), which is the gap between the axon terminals of one cell and the dendrites of the next cell.
Cells In Central Nervous System
Astrocytes, Oligodendrocytes, Ependymal Cells
Astrocytes
Maintain the integrity of the blood brain barrier, regulate nutrient and dissolved gas concentrations, and absorb and recycle neurotransmitters.
Oligodendrocytes
Myelinate CNS axons as well as provide structural framework for the CNS. Microglia - Remove cellular debris and pathogens.
Ependymal Cells
Line the brain ventricles and aid in the production, circulation, and monitoring of cerebral spinal fluid.
Cells in Peripheral Nervous System
Satellite Cells, Schwann Cells
Satellite Cells
surround the neuron cell bodies in the ganglia.
Schwann Cells
Enclose the axons in the PNS and aid in the myelination of some peripheral axons.
Norepinephrine and Acetylcholin
- Two neurotransmitters in the nervous system.
- When norepinephrine is synthesized, its immediate precursor is dopamine. The synthesis of norepinephrine begins in the axoplasm of the terminal nerve endings of adrenergic fibers. However, its synthesis is completed inside the vesicles of these fibers.
- The basic steps in the synthesis of norepinephrine are as follows. Tyrosine is converted to DOPA through the process of hydroxylation, and then DOPA undergoes decarboxylation to become dopamine. Dopamine is then transported into the vesicles of the adrenergic fibers, where it undergoes hydroxylation to become norepinephrine. In the adrenal medulla, norepinephrine is transformed into epinephrine though the process of mthylation.
- Choline is combined with acetyl-CoA to become acetylcholine.
Action Potential
Neurons are specialized to receive signals from sensory receptors or from other neurons in the body and transfer this info along the length of the axon. Impulses, known as action potentials, travel the length of the axon and invade the nerve terminal, thereby causing the release of neurotransmitter into the synapse.
Resting Potential
When a neuron is at rest, the potential difference between the extracellular space and the intracellular space is called resting potential.
How Resting Potential Is Maintained
- Even at rest, a neuron is polarized. This potential difference is the result of an unequal distribution of ions between the inside and outside of the cell.
- A typical resting membrane potential is -70millivolts (mV), which means that the inside of the neuron is more negative than the outside. This difference is caused by selective ionic permeability of the neuronal cell membrane and is maintained by the active transport of ions by the Na+/K+ pump (also called the Na+/K+ ATPase).
- The concentration of K+ is higher inside the neuron than outside; the concentration of Na+ is higher outside than inside. Additionally, negatively charged proteins are trapped inside the cell. The resting potential is creased because the neuron is selectively permeable to K+, so K+ diffuses down its concentration gradient, leaving a net negative change inside. (Neurons are impermeable to Na+, so the cell remains polarized.)
- Because the transmission of action potential leads to the disruption of the ionic gradients, the gradients must be restored by the Na+/K+ pump. This pump, using ATP energy, transports 3 Na+ out for every 2 K+ it transports into the cell.
Action Potential Process
- The nerve cell body receives both excitatory and inhibitory impulses from other cells. If the cell becomes sufficiently excited or depolarized (i.e., the inside of the cell becomes less negative), an action potential is generated. The min. threshold membrane potential (usually around -50 mV) is the level at which an action potential is initiated.
- Depolarization occurs during phase I, repolarization occurs during phase II, and hyperpolarization occurs during phase III. Ion channels located in the nerve cell membrane open in response to these changes in voltage and are called voltage-gated ion channels.
- An action potential begins when voltage-gated Na+ channels open in response to depolarization, allowing Na+ to rush down its electrochemical gradient into the cell, causing a rapid further depolarization of that segment of the cell.
- The voltage gated Na+ channels then close, and voltage-gated K+ channels open, allowing K+ to rush out down its electrochemical gradient. This returns the cell to a more negative potential, a process known as repolarization. In fact, the neuron may shoot past the resting potential and become even more negative inside than normal; this is called hyperpolarization.
Refractory Period
Immediately after an action potential, it may be very difficult or impossible to initiate another action potential; this period of time is called the refractory period.
All-or-None Response
The action potential process is often described as an all-or-none response. Whenever the threshold membrane potential is reached, an action potential with a consistent size and duration is produced. The nerve fires maximally, or not at all. Stimulus intensity is coded by the frequency of action potentials.
Action Potential Bidirectional?
Although axons can theoretically propagate action potentials bidirectionally, info transfer will occur only in one direction: from dendrite to synaptic terminal. This is cause synapses operate only in one direction and because refractory periods make the backward travel of action potentials impossible.
Action Potential Speed
Different axons can propagate action potentials at different speeds. The greater the diameter of the axon and the more heavily it is myelinated, the faster the impulses will travel. Myelin increases the conduction velocity by insulating segments of the axon, so that the membrane is permeable to ions only in the nodes of Ranvier. In this way, the action potential “jumps” from node to node.
Synapse
The gap between the axon terminal of one neuron (called the presynaptic neuron because it is before the synapse) and the dendrites of another neuron (postsynaptic neuron).
Effector Cells
Neurons may also communicate with postsynaptic cells other than neurons, such as cells, in muscles or glands; these are called effector cells.
