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Neuroanatomy Neuroanatomy refers to the study of the parts and function of neurons. Neurons are individual nerve cells. The entirety of the human body’s neurons make up the nervous system, from the brain to the tips of the toes. Neuron The basic building block of the nervous system -- a nerve cell Neurons perform three basic tasks –Receive information –Carry the information –Pass the information on to the next neuron The Basic Parts of a Neuron A. Dendrites – Thin, branching fibers lined with receptors at which the dendrite receives information from other neurons. The greater the surface area, the greater the amount of information. Some dendrites are covered with spines which greatly increase its surface area. B. Cell Body/Soma –Contains the (C) nucleus and other parts of the cell needed to sustain life D. Axon –Wire-like structure ending in the terminal buttons that extends from the cell body E. Myelin Sheath –An insulating, fatty covering around the axon that speeds neural transmissions. Axons that are myelinated appear white. Known as “white matter.” F. Schwann Cells Provide sheath. for the growth of the myelin G. Nodes of Ranvier – Regularly spaced gaps in the myelin sheath around an axon or nerve fiber. This is where depolarization takes place. H. Terminal Buttons –The branched end of the axon that contains neurotransmitters Neural Transmission Synapse –The space between the terminal buttons on one neuron and dendrites of the next neuron Neurotransmitters Chemicals contained in the terminal buttons that enable neurons to communicate. Neurotransmitters fit into receptor sites on the dendrites of neurons like a key fits into a lock. Neurotransmitters At the terminal buttons, neurotransmitters are released into the synapse and passed along to the dendrites of the next neuron. If enough neurotransmitters have been sent, the next neuron will fire. If not, the message ends. This is called the all-or-nothing principle. After a neuron fires its message, there is a brief period of time before it can fire again. This is called a neuron’s refractory period. During the refractory period, excess neurotransmitters are reabsorbed by the sending neuron, called re-uptake, as well as the cell becoming polarized once again. Resting Potential While in resting potential, a neuron is said to be “Polarized” negative ions are within the cell. Surrounding the cell are positively charged ions. The ions cannot mix while in this stage A neuron has a pre-set level of stimulation that needs to be met or exceeded in order for it to pass the received impulses on to the next neuron. This is called a neuron’s threshold. If the threshold has been met or exceeded, a chain reaction begins. With threshold being met, the cell becomes depolarized and allows positively charged ions into the axon at the nodes of ranvier. This mix of positive and negative ions causes an electrical charge to form (an action potential). At 120 meters per second, the action potential travels to the terminal buttons. Axon – inside and out Resting Potential The state of a neuron when it is at rest and capable of generating an action potential The neuron is set and ready to fire Action Potential A brief electrical charge that travels down the axon of the neuron. A neural impulse Considered an “on” condition of the neuron Refractory Period The “recharging phase” when a neuron, after firing, cannot generate another action potential Once the refractory period is complete the neuron can fire again Neuron firing like a Toilet 1. Like a Neuron, a toilet has an action potential. When you flush, an “impulse” is sent down the sewer pipe Neuron firing like a Toilet 2. Like a neuron, a toilet has a refractory period. There is a short delay after flushing when the toilet cannot be flushed again because the tank is being refilled Neuron firing like a Toilet 3. 4. Like a Neuron, a toilet has a resting potential. The toilet is “charged” when there is water in the tank and it is capable of being flushed again Like a Neuron, a toilet operates on the all-or-none principle – it always flushes with the same intensity, no matter how much force you apply to the handle All-or-None Principle The principle that if a neuron fires it will always fire at the same intensity All action potentials are of the same strength. A neuron does NOT fire at 30%, 45% or 90% but at 100% each time it fires. Click here to see a neuron in action! Click here to see a quick summary! Depending on what type of neurotransmitter has been released, the next neuron will react differently. ….so, since the entire body is a connection of nerves, … Inhibitory vs Excitatory Inhibitory neurotransmitters decrease the likelihood of the firing action potential of a cell while Excitatory neurotransmitters increase the likelihood of action potential Acetylcholine (ACh) Excitatory Involved in muscle action, learning, and memory Undersupply - Alzheimer’s disease Inhibitory: Dopamine Pleasure, Reward and Motivation, Motor Control over Voluntary Movements Excessive dopamine linked to schizophrenia; Serotonin Inhibitory Affects mood, hunger, sleep, and arousal Undersupply is linked to depression; Prozac and other anti-depressants raise serotonin levels Epinephrine and Norepinephrine Excitatory: Used for arousal in the flight/fight response, plays a role in learning and memory retrieval Adrenaline Burst of Energy (small amounts in brain) Undersupply can depress mood GABA Inhibitory: offsets excitatory messages (see Glutamate); helps regulate daily sleep-wake cycles Undersupply linked to anxiety, seizures, tremors, and insomnia Glutamate Excitatory Involved in memory, learning and movement Oversupply can overstimulate the brain, producing migraines or seizures (epilepsy) Endorphins Inhibitory: Natural opiates that are involved in pain perception and positive emotions released in response to pain and vigorous exercise Drugs and Chemicals Interact with Neural Transmission Some drugs that people put into their bodies are classified as agonists. Agonists may either speed up the neural process, cause an over-release or absorption of a neurotransmitter, or block the re-uptake process. Prozac blocking the re-uptake of Serotonin Some agonists mimic the effects of a naturally occurring neurotransmitter Agonist (like morphine – replacing natural endorphines) Dendrite of receiving Neuron After a neuron fires, if reuptake is blocked, the lingering neurotransmitters in the synapse will continue to be absorbed by the receiving neuron until it is gone. Therefore, a lingering feeling will occur Examples of Agonists Cocaine – blocks the reuptake of dopamine MDMA (Ecstasy) – blocks the reuptake of serotonin –Repeated use destroys serotonin producing cells Some drugs that people put into their bodies are classified as antagonists. Antagonists may slow or stop the transmission of a neurotransmitter, or they may bind themselves to receptors on a neuron’s dendrite, thus not allowing a message to be passed on. Examples of Antagonists Curare – a poison that stops the flow of Ach – causes paralysis Antagonist (like curare) Neurotransmitter (such as Ach) Dendrite of receiving Neuron Types of Neurons There are three types of neurons: –Afferent Neurons (Sensory Neurons) –Interneurons –Efferent Neurons (Motor Neurons) Types of Neurons Afferents, or “sensory neurons”, carry information from the body to the brain Types of Neurons Interneurons, found in the spinal cord and the brain interpret incoming information and determine the next course of action Types of Neurons or “motor neurons”, carry information from the spinal cord or the brain to the rest of the body in order to initiate behavior Efferents, The exceptions to the “general pathway” of neural activity are reflexes. •Reflexes are controlled by the spinal cord without any conscious effort on behalf of the brain. Reflexes are primitive responses protect our bodies from danger Coughing Blinking Yawning (too much carbon dioxide in the blood) ….etc……. Reflex Reflex Spinal Cord Sensory to spinal cord to motor… no brain processing involved….