BIOL 3170 Neurobiology 9-3-09 Types of Axonal Transport Antrerograde: transport away from the cell body Fast transport Intermediate Slow transport Retrograde: transport towards the cell HYPERLINK "http://multimedia.mcb.harvard.edu/anim_innerlife_hi.html" http://multimedia.mcb.harvard.edu/anim_innerlife_hi.html Parts of the Nervous System-Invertebrate Nervous System Crayfish: A cross section of one of the ganglia shows the cell bodies around the peripheries of the ganglia, the neuropill (all the connections between axons and dendrites) is in the meat of the ganglia, and the axons are in the center. A cross section of a connective shows NO cell bodies, but many axons C. elegans Function of almost every cell is known Can actually go to HYPERLINK "http://www.wormatlas.org" www.wormatlas.org to see what every single cell/neuron has been characterized as. Parts of the Nervous System-Vertebrate Nervous System Spinal Cord and Brian: Central Nervous System Input into CNS can be somatic and autonomic, they are input by afferent fibers The output of the CNS is done through efferent fibers, and again they can be somatic motor nerves and autonomic nerves, which have ganglia with them. (has both sympathetic (short preganglionic fibers, long post) and parasympathetic systems (long preganglionic fibers, short post)) Spinal Cord: Nerves are named from the part of the spinal column they come from Cervical nerves, thoracic nerves, lumbar nerves, sacral nerves, coccygeal nerves. Gray matter is on the inside of the white matter in the spinal cord. Emanating from the dorsal part of the spinal cord is the input of the cord, it is called the Dorsal Root, which comes from the dorsal root ganglion, contains the cell bodies from the neurons that are gathering information from the periphery. Dorsal white matter sends sensory information from the periphery to the brain Ventral white matter sends motor information from the brain to the periphery Nerve tracts are arranged around the periphery, surrounding the cell bodies, dendrites, and synapses. Motor information leaves the cord through the ventral root Brain: Consists of four primary structures Cerebrum Optic tectum- also referred to as the mid-brain Cerebellum Medulla Develops from a neural tube, with the forebrain being smaller than midbrain and hindbrain at first, then the cerebrum develops to partly envelop the cerebellum? (like the mouse pictures me and Jennifer looked at to figure out if we could see the MCA at 8 days) Forebrain: Telencephalon: contains cortex Dienchephalon Thalamus (sensory processing center) Hypothalamus Pituitary Limbic system MidBrain Tectum Hindbrain Cerebellum (aids in motor corridination, learning) Pons (control of respiration) Medulla oblongata Don?t worry about memorizing the subnuclei of the basal ganglia or the names and functions of the cranial nerves in chapter 3- Autonomic Nervous System (part of the Peripheral Nervous system) Sympathetic Parasympathetic (?para? means ?near?)- the ganglia of the parasympathetic are near the target organs Autonomic efferents ?autonomic outflow?- flight or fight Ventral part of the spinal cord The sympathetic nervous system, there is a short pre-ganglionic fiber. There is a short preganglionic fiber, then the axon from the neuron is within the sympathetic ganglian itself (not the dorsal root ganglion). The fiber that goes into the organ is long (postganglionic fiber). Sympathetic nervous system Mediates ?flight of fight? Preganglionic fibers are SHORT Post are LONG Parasympathetic nervous system Mediates your ?rest and digest? responses Ex. contraction of gut; maintains blood pressure Preganglionic fibers LONG Post ganglionic fibers SHORT because they are near the target organ Enteric Nervous System Resides in the gut and can function semiautonomously (regulates GI tract) Network of connected ganglia Throughout the gut, the lumen that contains the specialized enzymes, then the submucous layer, than two layers of muscle (circular and longitudinal), and there are a small collection of cell bodies (two plexus: myenteric plexus, and the submucosal plexus which communicate with each other through connections). If the longitudinal muscle was stripped away, one would see a type of ?nerve net? that connects the ganglia of the myenteric plexus and signals to the submucosal plexus. Our gut is a primitive brain. What is a signal? A signal is a change in state. There is a basal state, than a change in that state (signal) and then a return to the basal state In the nervous system, the basal state is the resting membrane potential and the different action potentials placed upon it is the signal The basal state has to be very well regulated Terminology Current (denoted as ?I): the movement of charge. Measured in amperes In electrical systems, current is the result of the movement of electrons In biological systems, current is the result of the movements of ions. The typical unit of measurement is the nA. Potential (denoted as ?E? or ?V? aka voltage): the result of the separation of charge. It is measured in Volts. Resting membrane potential (denoted as ?Em?): result of the separation of charge across the membrane. The neuron?s membrane is like a battery. In biological systems, potential is measured in millivolts. Several Cell types possess a resting electrical potential across their membranes: Muscle: skeletal (-30), cardiac(-90) and smooth(-90) Glia (-75) Neurons (-65) These potentials have a negative sign because the inside of the cell is more negative than the outside Resting Membrane Potential Resting membrane potential is measured with the cell is at rest with no perturbing influences such as action potentials or synaptic potentials. Perturbation of the rmp is what happens during the propagation of information down the cell. How is membrane potential recorded? Through to use of microelectrodes (made from micropipette pullers) the micropipette chuck is placed into the cell, there is a difference in the charge between the inside and the outside, and the potential can be recorded. A cell whose potential across its membrane is not equal to zero is said to be polarized As the membrane potential becomes more positive it is said to become depolarized As the membrane potential becomes more negative it is said to become hyperpolarized The Resting Membrane Potential is due to: Specific properties of the cell membrane Differences in the ionic composition between the intracellular and extracellular fluids The cell membrane is a semipermeable membrane Permeability and resistance are measures of how easily and ion can cross the membrane High permeability (low resistance) means that ions can easily cross Low permeabily (high resistance) means the ions cross with great difficulty Semipermeable means that some ions or substances can cross the membrane more easily than others. The actual lipid bilayer is intrinsically impermeable to ions. Therefore: ions must pass through pores or channels. the channels are water-filled so ions may pass through the membrane without having to dissolve in its oily interior. the channels are selective for the ions that can pass through. Remember, ion channels can be gated or non-gated The rmp is established almost exclusively by non-gated ion channels Action potentials are produced primarily by voltage-gated channels Therefore, the permeability of the membrane in the resting state to a particular ion depends on the number of non-gated channels selective for that ion The concentrations of various ions are significantly different inside and outside the cells Major ions: K+(higher concentration inside than outside), Na+, Cl-(lower concentration inside than outside, same as Na+) Minor ions: Ca2+, Mg2+ Intracellular anions: organic anions These concentrations gradients, in combo with the different permeabiliies of the membrane to the various inos lead to the equilibrium (Nerst potentials) When the concentration gradient and electrical gradient counteract each other, you get EQUILIBRIUM At equilibrium: net movement of charge across the membrane ceases Doesn?t mean there is no ion movement across the membrane No energy is required to maintain equilibrium It will last forever with no further changes in either concentrations or membrane potential The equilibrium potential of an ion is the potential required to exactly oppose the concentration gradient of that ion. The equilibrium potential for an ion is also called the ion?s ?Nernst potential?. The Nerst equation essentially converts a concentration gradient into an electrical potentioal When a membrane is only permeable to a single ion, the equ potential for that ion is equal to membrane potential . Eion= (RT/zF)ln[ion0]/ioni =58mV/z log ion/ion at 25degrees C, at 37 degrees, the first term changes It contains several constants: R = the gas constant T = absolute temperature (in °K) F = Faraday?s constant z = valence of the ion Glial cells only have selective, non-gated potassium channels. They are a simple situation because only K is permeanably across the membrane, therefore the Ek=Em. Neurons are more complicated because their membrane is permeable to several ions
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