ALS 2304, Week 6: Neurophysiology and the Nervous System
ALS 2304, Week 6: Neurophysiology and the Nervous System ALS 2304
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This 8 page Class Notes was uploaded by Mara DePena on Friday February 26, 2016. The Class Notes belongs to ALS 2304 at Virginia Polytechnic Institute and State University taught by Dr. Cline in Spring 2016. Since its upload, it has received 56 views. For similar materials see Animal Physiology and Anatomy in Agricultural & Resource Econ at Virginia Polytechnic Institute and State University.
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Date Created: 02/26/16
ALS 2304 WEEK SIX NEUROPHYSIOLOGY AND THE NERVOUS SYSTEM NEUROPHYSIOLOGY (CONT.) SYNAPSES Synapse- Where two neurons meet (space in between). Presynaptic neuron- Conducts impulses toward synapse. Postsynaptic neuron- Transmits impulses away from synapse. Synaptic cleft/space- Gap between two neurons. Extracellular fluid is in this space. o Synaptic vesicles- Store neurotransmitters. Action potential opens calcium channel, which floods axon terminal and leads to exocytosis of neurotransmitter. Forms calcium-calmodulin complex, activates kinase which phosphorylates synapsin, which drags the synaptic vesicle down in proximity to membrane (does not cause exocytosis, just shoves vesicles against wall) SNARE proteins- Synaptobrevin attached to vesicle Syntaxin and SNAP 25 attached to membrane. These all entangle, pulling synaptic vesicle very close to plasma membrane. The membranes fuse and exocytosis of neurotransmitter into synaptic space occurs. Diffusion carries these neurotransmitters to the chemically gated channel. This is the rate limiting step in neuronal communication. o When a neurotransmitter is released into a synapse, it can either bind to a receptor, diffuse into a synaptic space and be taken up by a glial cell, be destroyed by degrading enzymes, or be reclaimed by the neuron that released them (reuptake). NEUROTRANSMITTERS Acetylcholine (AcH) o First neurotransmitter identified o Best understood o Released at neuromuscular junction o Synthesized and enclosed in synaptic vesicles o Causes skeletal muscles to contract o Released by all neurons that stimulate skeletal muscle o Some neurons in autonomic nervous system o Cholinergic synapse - Choline acetyltransferase forms acetylcholine, which is then packaged in the synaptic vesicles and waits until calcium arrives at the terminal. Synapsin is then activated, drags acetylcholine to the border and SNARE proteins cause exocytosis. It is then released into the synaptic cleft where it binds with its receptor (chemically gated channel). It may then cause a graded potentional. Acetylcholoinesterase metabolizes acetylcoline and breaks it into acetate and choline. It stops muscle contraction. Choline is pulled back into the terminal and recycled to make a new molecule of acetylcholine. Biogenic amines o Include: Catecholamines Dopamine, norepinephrine, epinephrine o Absence of dopamine results in Parkinson’s and possibly depression Synthesis o All of them are derived from an amino acid, tyrosine o Enzymes present in the cell determine length of biosynthetic pathway. Tyrosine is converted into l-Dopa by tyrosine hydroxylase. It is converted into dopamine by dopa decarboxylase, and into norepinephrine by dopamine b-hydroxlyase, and epinephrine by phenylethanolamine n-methyltransferase, o NE and dopamine are synthesized in axonal terminals o Epi (adrenaline) is released by the adrenal medulla o At the dopaminergic synapse, it is changed from tyrosine into dopamine. Essentially the same process as in any other synapse. o At the bottom of the synapse are COMT and MAO, which produce DOPAC and HVA and break down dopamine. Indolamines Serotonin, histamine (one of the most abundant neurotransmitters, related to consciousness) Serotonin is derived from an amino acid, L- Tryptophan. o Tryptophan comes from the diet o Tryptophan hydroxlase (enzyme) rearranges its structure and changes it into an intermediate called 5-HTP o Another enzyme (l-aromatic amino acid decarboxylase) changes 5-HTP into serotonin (5HT) o This conversion occurs in the synaptic terminal. Tryptophan arrived into synapse through cardiovascular system. After being changed into 5HT, it is stored in a synaptic vesicle. Eventually once synapsin is activated, it drags it down, and SNARE proteins cause exocytosis. o Human depression can come about from an absence of a variety of neurotransmitters, with the absence of serotonin most likely being the main cause. If you inhibit serotonin’s reuptake, the concentration of serotonin in the synapse will increase. This can be used to treat depression. You could also block the degrading enzymes. o Broadly distributed in the brain o Play roles in emotional behaviors and biological clock. AMINO ACIDS Some amino acids themselves function as neurotransmitters. GABA says no. Anytime you have an IPSP, it is because of GABA. PEPTIDES Amino acid derived transmitter. Includes.. o Substance P- Mediator of pain signals. Made by transcription/translation. o Beta endorphins, dynorphin, enkephalins. THE NERVOUS SYSTEM Thalamus- Determines which information runs to the cerebral cortex. It decides if the information is worthy, and the information is dumped if not. Hypothalamus- Below the thalamus. Going down, there is the pons, medulla oblongata, and spinal cord. FUNCTIONAL AREAS OF THE CEREBRAL CORTEX Different regions of the cortex do different things. Two specific regions of the cerebral cortex that are important when dealing with animals are the primary somatosensory cortex and the primary motor cortex. Primary somatosensory cortex- Perception. o Genitals, leg, hip, trunk, neck, head, arm, elbow, forearm, hand, fingers, thumb, eye, nose, face, lips, teeth, gums, jaw, tongue, pharynx, intra-abdominal. o When a cow is shot in the leg, the neurons in this area fire. Primary motor cortex- Movement. o If a neuron is fired in this cortex, it starts a circuit that moves down to the skeletal muscle and causes it to contract. o Toes, nee, hip, trunk, shoulder, arm, elbow, wrist, hand, fingers, thumb, neck, brow, eye, face, lips, jaw, tongue, swallowing. o Area 6- Region is front of primary cortex. Made up of supplemental motor area and premotor cortex. When information comes up from the thalamus, before it goes to the primary motor cortex, it runs through these. SPINAL NERVES AND ROOTS All neurons in the dorsal root have axons that terminate in the spinal cord. Information flows into the dorsal root. Neurons in the dorsal root synapse on interneurons. They are the ones that make the reflex happen. All neurons in the ventral root flow away from the spinal cord. Information flows out of the ventral root. MUSCLE SPINDLE Muscle fibers that all run in parallel. Nothing more than a free dendrite that wraps around these muscle fibers. When muscle cells contract, they get bigger. As they contract, tension is placed on the dendrite. When the muscle is not contracted, or has been stretched apart the neuron fires action potentials spontaneously (that is its default state). When the muscle contracts, it fires action potentials less frequently. o How could this happen? A mechanically gated channel. When it opens, Chlorine goes in and slows down the rate of action potential by overpowering the EPSPs. There are EPSPs occurring on this dendrite by channels that never close, allowing positive ions to flow in during the resting state. Only way to stop this is by increasing the amount of IPSP. Hence why Chlorine comes in. Brain reads firing frequency to tell you where your limb is. If your arm is straight down, it fires a lot more frequently than if you bend your arm upwards, because your arm muscle contracts when it is bent upwards. When you stretch the muscle, the frequency goes up. Indicates amount of contraction the muscle is undergoing. GOLGI TENDON ORGAN Same as muscle spindle, but imbedded in the tendon. Indicates amount of tension the muscle is creating. KNEE JERK (MYOTATIC) REFLEX Evaluates the anatomy associated with the circuits in the spinal column. Checks if all neurocircuits are together. Occurs on one side of the body. Striking tendon with a hammer causes the muscle to stretch. Tendon is like a rope tied to muscle- we yank on it really quick and then let it go. If we didn’t strike it and this reflex happened on its own, we would have to push the leg backward to stretch the muscle. When this happens, the frequency of action potential increases. Shorthand for that on the exam is (up arrow)fAP. This is associated with the release of neurotransmitter in the spinal cord. The amount of release of neurotransmitter is increased with AP. Alpha motor neurons- Neurons that lead the spinal cord and innervate the skeletal muscles. o Two in circuit, running to top and bottom muscle. Muscles always exist in pairs. As one contracts, the other relaxes. Acetylcolene is the transmitter that makes skeletal muscles contract. o On the top muscle, acetylcolene is increased. On the bottom muscle, it is decreased. o On top muscle, frequency of action potential increases (hence more acetylcolene.) The opposite is true for the bottom circuit. If the frequency of action potential went up (top circuit), there have to be EPSPs occurring. If the frequency decreased (bottom circuit), there have to be IPSPs. Anytime you increase frequency of action potential on this neuron, you increase the frequency of action potential on the alpha motor neuron. There is an interneuron in the spinal cord that is stimulated and inhibits the next neuron using GABA (only thing that causes an IPSP.) This decreases the firing frequency. There is a pathway from the spinal cord that goes up through the brain, up to the primary somatosensory cortex. It is stimulated at the same time as the reflex, so you are made aware of what you are doing at the exact same time you are doing it. o How would you suppress the reflex? Go to the decision region and make the decision to suppress the reflex. Goes to the thalamus and then to the primary motor cortex and down. By the time you are trying to suppress the reflex, it has already happened. THE FLEXOR REFLEX Occurs on both sides of the body. If you step on a nail, it triggers a pain receptor, which triggers a reflex. The stimulus goes up and increases the action potential on the circuit. It goes up to the spinal cord, where there are a plethora of interneurons. Whatever happens to the top muscle on one leg, the opposite happens on the opposite leg. Remember, as you work these reflexes, start at the muscle and work backwards. If you step on a nail, you pull the leg back and plant the opposite leg. If you are pulling up your leg, the top muscle stretches. (GABA inhibits acetyl colene.) The back muscle has an EPSP (increases acetyl colene) and contracts. A mirror happens on the opposite leg. Most important part to show on these reflexes are the interneurons. Make sure to include the pathway going to the brain. This is the first thing that is going to happen when the animal is shot in the leg. Next, it runs away. RUNNING/WALKING Circuitry associated with escape. Pathway starts in the primary motor cortex. Descends from the brain, to the spinal cord, to the top and bottom leg muscles. As you walk, the only thing the primary motor cortex does is tell the top muscle to contract. Only one of the circuits is activated at one given time. The spinal cord contributes to the gait by synapsing on an excitatory interneuron and activating (EPSP) the flexor motor neuron on the top muscle. There is another branch on the interneuron that synapses on in inhibitory interneuron. Dumps GABA on excitatory interneuron to decrease the firing frequency on the circuit. Inhibits lower pathway. On your diagram, draw the circuitry associated with the reflex, which happens at one moment in time. The next moment in time, the animal has decided to run away, so you should show this circuitry. On a four- legged animal, this happens four times. Walk vs. running is determined by the duration these two circuits remain on. On comes from the motor cortex, off comes from the spinal cord. Circuit is wired to both muscles but only one pathway is activated at a time. AUTONOMIC NERVOUS SYSTEM Parasympathetic nervous system- Predominates when the body is not in an emergency. o Presynaptic or preganglionic neuron- Has a really long axon. Always releases acetylcolene. Always causes postganglionix neuron (nonmylenated) to release acetyl colene. Needs to be directly wired to something. o Output originates from the vagus nerve. Dorsal motor nucleus- In medulla. When it fires, the vagus nerve is activated. Sympathetic nervous system- Predominates when there is an emergency, like in our diagram situation. o Neuron always releases acetylcolene into ganglion. Second neuron is not mylenated, released norepinephrine onto organs. o Neuron runs through adrenal gland and releases acetyl colene onto it. Adrenal gland responds by release epinephrine and norepinephrine into the cardiovascular system. But this mechanism, the sympathetic nervous system can change the physiology of every cell in the body. o Originates from thoracic and lumbar regions of spinal cord. Salturic conduction-Accelerates rate. Ganglion- Synapse out in the periphery. Decision region of the brain hypothalamus dorsal motor nucleus of vagus nerve to activate parasympathetic nervous system Decision region of the brain hypothalamus thoracic/lumbar region of spinal cord In the diagram, you have to indicate which is more active. NEUROTRANSMITTERS AND RECEPTOR Cholinergic fibers- Ach-releasing fibers (acetylcholine) Adrenergic fibers (neuronergic)- NE releasing fiber (norepinephrine) Cholinergic receptors- Two types of receptors that bind Ach are nicotinic and muscarinic. o Nicotinic receptors- Always cause EPSPs. Sodium always goes through. o Muscarinic receptors- Can either cause EPSPs or IPSPs. Adrenergic receptors- Alpha and beta type. Alpha usually cause stimulation while beta usually cause inhibition. Norepinephrine binds to these. ACTIVATION OF SMOOTH MUSCLE The sympathetic nervous system causes a rise in blood pressure by synapsing on the wall of an artery, causing contraction. Norepinephrine comes over from axon terminal and binds to an alpha receptor. G-protein coupled receptor, activates Gaq. Increases release of calcium, muscles contract. ATP functions as a neurotransmitter as well and binds to a P 2xreceptor. When it binds, the receptor opens a calcium and sodium channel. The calcium coming in contributes to contraction. The sodium causes depolarization (an EPSP). A voltage gated calcium channel opens, driving up the concentration of calcium. Neuropeptide Y is originated from amino acids. It is released and binds onto a receptor and drives up intercellular calcium by an unknown mechanism. DEACTIVATION OF SMOOTH MUSCLE Parasympathetic nervous system. Decreases blood pressure. Epithelial cells surround muscle of artery wall. Acetylcholine binds muscarinic receptor, G-protein coupled. Drives up intercellular calcium and activates an enzyme known as nitric oxide synthase. Nitric oxide diffuses into artery cell, causes activation of guanile cyclase (GC). This causes relaxation of the fiber. Nitric oxide released from terminal itself. Does the same thing as the other nitric oxide. Viagra actually activates this. Vasoactive intestinal peptide (VIP)- Binds g-couple receptor, somehow decreases intercellular calcium concentration, which leads to relaxation.
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