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Biology 30 NERVOUS SYSTEM 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. Nervous System Overview The Neuron Reflex Arc The Action Potential The Synapse / Neurotransmitters Nervous System Diseases Drugs PNS CNS The Brain The nervous system is responsible for maintaining homeostasis by responding quickly and efficiently to internal and external stimuli. The nervous system has 5 general functions: 1. Reception – receiving information from the external and internal environments 2. Conduction – the passage of information to specific parts of the brain / body 3. Interpretation – organizing sensory input into events that incorporate past experience and present sensation 4. Organization – coordinating a thought or action n response to internal or external stimulus 5. Transmission – sending information required to execute a reaction to stimulus Nervous System Organization The Neuron The neuron (nerve cell) is the basic unit of the nervous system. It consists of several basic parts: the cell body, the dendrites, the axon, the mylelin sheath, and the nodes of Ranvier. Some myelinated nerves are covered with a membrane called neurilemma, which promotes regeneration of damaged nerve cells in some organisms. Glial cells such as Schwann cells, nourish and support neurons. There are 3 types of nerve cells: Sensory Neurons – (afferent) have long dendrites and short axons - carry nerve impulses from the sensors to the central nervous system Motor Neurons – (efferent) have short dendrites and long axons - carry nerve impulses from the central nervous system to the effectors (muscles and glands) Interneurons – short dendrites and long or short axons (may be multipolar) - carry nerve impulses within the central nervous system (brain and spinal chord) Reflex Arc Reflexes are automatic, quick, involuntary responses to internal or external stimuli. They follow a special pathway of conduction, called a reflex arc, that does not immediately involve the brain. This allows quicker reaction times to potentially harmful stimulus such as touching a hot stove. http://www.brainviews.com/abFiles/AniPatellar.htm The reflex arc also involves a one-way flow of information. Sensory neurons may stimulate a number of inter-neurons, which take impulses to different parts of the central nervous system. This is why we are usually conscious of stimuli that we reflexively react to. Action Potential The Action Potential is an electrochemical event, whereby a nerve impulse is transmitted down the length of the axon. The action potential occurs in a series of stages, each marked by a specific chemical event. 1. Resting Potential Voltage is (-65 mV) when a cell is resting, it must be in a polarized state where there is an unequal distribution of + and – charges across the membrane. This is achieved because [Na+] is higher on the outside, while [K+] and [Cl-] concentrations are higher on the inside. The different concentrations of Na+ and K+ are maintained by a sodium / potassium pump. Gates for Na+ and K+ are closed in this state. http://www.lifesci.ucsb.edu/~mcdougal/neurobehavior/modules_ho mework/lect2.dcr 2. Stimulation / Depolarization (+ 20 mV) stimulation by a change in pH, pressure, or an electrical stimulus cause the Na+ gates to open, and Na+ ions rush into the cell. This causes the membrane to become depolarized, with the outside of the cell being less positive than the inside. 3. Re-polarization (-70 mV) the change in electrical potential causes the opening of the K+ channels. This results in K+ ions rushing out of the cell, restoring the polarized state (except the ion concentrations are reversed from the resting state). 4. Refractory period (-65 mV) before the neuron can fire again, the original resting potential must be restored. Na+ ions are pumped out of the cell and K+ ions are pumped back into the cell, using ATP energy The Action Potential In order for a nerve impulse to occur, the nerve must be adequately stimulated, that is a minimum number of sodium channels must open. The Action Potential in Action Neuron Action Potential Threshold level – minimum depolarization that must be reached before sufficient Na+ gates open to continue the action potential All or None Response – if the threshold level is not reached, the action potential will not occur at all. If the threshold is reached or exceeded a full action potential will result. Saltatory Action – the speed of the nerve impulse is increased by the impulse jumping from node of Ranvier to node of Ranvier, this is how myelinated neurons are able to conduct impulses faster than non-myelinated neurons Propagation of the Action Potential The intensity of the nervous response is determined by: 1. the number of neurons that fire simultaneously 2. the frequency at which the neurons fire 3. the threshold level of different neurons (lower threshold neurons are more likely to fire, and are found in more “sensitive” areas The Synapse and Neurotransmitters Once the action potential has reached the end of the axon, the electrical impulse cannot continue by the same mechanism. The information must be chemically carried to the next axon, where the action potential will be initiated again. The electrical impulse stimulates the release of a neurotransmitter from the synaptic vesicles in the pre-synaptic membrane. The neurotransmitter will diffuse across the synaptic cleft, and be picked up by receptors on the post-synaptic membrane. The Synapse Neurotransmitter substances – are chemicals that are stored in the synaptic vesicles of the axon, and are released across the synaptic cleft. These substances may continue or inhibit the nerve impulse. They can be either excitatory or inhibitory. 1. excitatory neurotransmitters – cause the movement of Ca2+ ions into the cell causing depolarization or the opening of Na+ channels to cause depolarization 2. inhibitory neurotransmitters – prevent depolarization by blocking Na+ channels, opening K+ channels (causing hyper-polarization), or moving Cl- ions into the neuron Summation – whether or not a neuron fires depends upon the net effect of excitatory and inhibitory neurotransmitters received by the neuron. If there is adequate excitation to reach the threshold, the neuron will fire. Integration – the degree of sensation felt, or the degree of response created by the brain depends on the number of neurons that fire within a nerve bundle, and how the brain organizes all incoming stimuli. Let’s look at nerve conduction overall… http://www.lifesci.ucsb.edu/~mcdougal/neurobehavior/mo dules_homework/lect3.dcr Neurotransmitters There are 9 universally recognized neurotransmitters: aspartate, glycine, GABA, glutamate, dopamine, nor-epinephrine, epinephrine, seratonin, and acetylcholine. Once a neurotransmitter is released, it must act quickly before it is broken down by enzymes or is reabsorbed by the pre-synaptic vesicles. Some of the more common neurotransmitters (and their enzymes) include: Nor-epinephrine – (NE) an excitatory neurotransmitter in the autonomic nervous system, responsible for the fight or flight reflex Dopamine – an excitatory neurotransmitter often associated with behavioral states and muscle contraction - broken down by a class of enzymes called MAO inhibitors GABA – an inhibitory neurotransmitter in the CNS, may be involved with promoting “responsible” and appropriate behavior Seratonin – an excitatory neurotransmitter in the with behavioral states such as mood, sleep, attention, learning and memory - broken down by a class of enzymes called monoamine oxidases MAO’s Acetylcholine – (Ach)an excitatory neurotransmitter in both the CNS and PNS, often responsible for skeletal muscle contraction - broken down by the enzyme acetylcholinesterase (AchE) Once a neurotransmitter has conveyed the message to the next neuron, its action must be blocked. This can occur by the following mechanisms. 1. Degradation by enzymes in the synaptic cleft 2. Re-uptake by the pre-synaptic membrane 3. Diffusion out of the synaptic cleft 4. Inability to bind due to competitive inhibitors Drugs often interfere with the normal neurotransmitter function. Close to Home Animation: Cocaine Chemical Diseases of the Nervous System Parkinson’s Disease: wide-eyed, unblinking expression, involuntary tremor, muscle rigidity, shuffling gait - due to dopamine deficiency, or the malfunction of dopamine receptors Alzheimer’s Disease: characterized by loss of memory, senility, deterioration of cells in the basal nuclei, presence of tangles and plaques - possibly due to a malfunction of acetylcholine - seems to be linked to a gene located on chromosome #21 Schizophrenia: delusions, random thoughts, disjointed thoughts, sensory hallucinations - may be the result of excessive activity of brain neurotransmitters Chemical Diseases Cont. Huntington’s Disease: progressive deterioration of the nervous system that leads to writhing movements, insanity and eventually death - seems to be caused by the malfunction of the inhibitory neurotransmitter GABA Depression: low affect, feeling blue, lack of or excessive sleep and eating patterns - seems to be linked to malfunctions in dopamine and seratonin, perhaps caused by an excess of monoamine oxidase enzymes The Effects of Drugs A drug is – anything that is not food that alters the normal bio-chemistry of the body in some way. Drugs affect the reticular activating system (RAS) in the brain. This system sorts out incoming stimuli to the brain, and affects the limbic system which creates pleasant or unpleasant sensations, feelings or emotions. Some drugs block or promote the activity of certain neurotransmitters. Here are some examples. Some examples… Alcohol: - seems to block GABA’s ability to cause Cl- uptake, leads to lack of coordinated response, and loss of normal social inhibitionsClose to Home Animation: Alcohol Marijuana: may have an impact on the activity of seratonin in the brain, not physically addicting, however this is a gateway drug and may be psychologically addicting Cocaine: blocks the re-uptake of dopamine, causing an adrenaline like effect from the dopamine, as dopamine levels increase in the synapse, the body produces less, thus making cocaine very physically addicting Heroine: binds to the body’s receptors for endorphins which block the “pain neurotransmitters” called substance P, when the drug is removed, there is a flood of pain neurotransmitter causing a physical dependence and addiction to the drug. Close to Home Animation: Heroin Peripheral Nervous System (PNS) The PNS is composed of: Cranial nerves – 12 pairs of sensory, motor and mixed nerves that control the face, neck and shoulders. Spinal Nerves – 31 pairs of nerves that emerge from the spinal cord by two roots (branches) (one pair for each segment) Dorsal root nerves – contain sensory neurons and ganglia Ventral root nerves – contain motor neurons All other nerves not part of the CNS The PNS is subdivided into two major parts: The Somatic Nervous System – contains all the nerves that serve the musculo-skeletal system and the sensory organs. The actions are generally conscious and deliberate. The Autonomic Nervous System – contains all the nerves that serve the internal organs. The actions are unconscious and automatic. The autonomic nervous system is also divided into the: – A. Sympathetic nervous system – responsible for the fight or flight response dilation of the pupils, increased heart rate, increased breathing rate, slowed digestion, enhanced performance, increase in blood sugar – B. Parasympathetic nervous system – responsible for the relaxation response (after fight or flight) – http://itc.gsw.edu/faculty/gfisk/anim/autonomicns.swf Central Nervous System (CNS) Is primarily responsible for the processing and organization of information. The CNS consists of two major structures: – 1. The Brain – 2. The Spinal Cord The Spinal Cord Central Cavity – contains cerebrospinal fluid Dorsal Root Ganglion – entry of sensory neurons to spinal cord and CNS, ganglion is the collection of cell bodies Ventral Root – exit of motor neurons from the spinal cord White Matter – contains myelinated nerve cells Grey Matter – contains un-myelinated nerve cells Meninges – 3 protective membranes surrounding the brain (dura mater, arachnoid, pia mater) Cerebrospinal Fluid – circulates between the inner and middle layers in the central canal and spinal cord. Provides protection, nutrient / waste exchange, etc. Spinal Cord Functions center for reflex action provides a pathway for communication between the brain and peripheral nerves The Brain The Unconscious Brain Midbrain – reflex center for head movements in response to visual stimuli, connects cerebrum to other parts of the brain Hindbrain – important for autonomic functions required for survival cerebellum – responsible for muscle co-ordination, posture, coordinated muscle movement and balance medulla oblongota – controls heartbeat, respiration, blood pressure, reflex center for vomiting, sneezing, hiccupping, coughing and swallowing pons – connects the cerebrum to other parts of the brain, regulates breathing rate The Conscious Brain (Forebrain) Localization of Function Frontal Lobe – controls voluntary muscle movement, concentration, problem solving, judgement of behavioral consequences, conscious thought, higher intellectual processes Temporal Lobe –responsible for hearing and smell, association areas interpret sensory experiences such as visual scenes, music, etc. Parietal Lobe – responsible for perceptions of touch, temperature, pressure, pain, etc from the skin, also includes association areas for understanding speech, thoughts and feelings. Occipital Lobe – responsible for vision, combining images with other sensory experiences The Secret Life of the Brain : 3-D Brain Anatomy Other parts of the brain Thalamus – central relay station for sensory impulses traveling upward from other parts of the spinal cord and brain to the cerebrum Hypothalamus – contains neuro-secretory cells that produce some hormones, controls thirst, hunger, and controls many of the pituitary hormones Limbic System – includes the frontal lobes, temporal lobes, thalamus and hypothalamus, controls pain / pleasure feelings, emotions, memory and learning Corpus Callosum – structure that connects the two halves of the brain (left and right), allowing for integrated thoughts and coordinated responses Left brain – verbal, linguistic dominant Right brain – spatial, artistic, visual dominant Viewing the Brain PET – Positron Emission Tomography – measures emissions from radioactively labeled chemicals that have been injected into the bloodstream and uses the data to produce two- or three-dimensional images of the distribution of the chemicals throughout the brain and body. SPECT-Single Photon Emission Computed Tomography – Similar to PET, this imaging procedure also uses radioactive tracers and a scanner to record data that a computer uses to construct two- or threedimensional images of active brain regions. MRI-Magnetic Resonance Imaging - uses magnetic fields and radio waves to produce high-quality two- or three dimensional images of brain structures without injecting radioactive tracers. EEG-Electroencephalography - uses electrodes placed on the scalp to detect and measure patterns of electrical activity emanating from the brain. CT-Computed Tomography Scan - use a series of X-ray beams passed through the head. The images are then developed on sensitive film. This method creates cross-sectional images of the brain and shows the structure of the brain, but not its function.