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The Nervous System Ch. 37 Who makes it, has no need of it. Who buys it, has no use for it. Who uses it can neither see nor feel it. What is it? Coffin • What gets wetter and wetter the more it dries? A Towel • What are the next two numbers in this series: 2, 9, 16, 23, .. 30 and 37 • What does this say? TOIMWN I’m in town Vertebrate Nervous System • The Central Nervous System- consists of brain and spinal cord • The Peripheral Nervous System- consists of nerves and ganglia outside the Central nervous system Nervous Tissue • 2 Cells of the Nervous System • Neurons • Neuroglia Neuroglia • Also called glial cells • Microglia- phagocytic cells that remove bacteria and debris • Astrocytes- provide metabolic and structural support to neurons • Oligodendrocytes- form myelin sheath around axon Neurons • Consist of 3 major parts • Cell Body- contains a nucleus and other organelles • Dendrites- short, highly branched processes that receive signals and transmit them to cell body. • Axon- convey information to other neurons or cells • • Sometimes covered by myelin sheath Often called nerve fibers Types of Neurons • Motor Neurons • • Take nerve impulses from the CNS to muscles or glands. Said to have multipolar shape because they have many dendrites and a single axon Types of Neurons • Sensory (afferent) Neurons • • Take nerve impulses from sensory receptors to CNS Structure is unipolar because process from cell body branches to the periphery and CNS Types of Neurons • Interneurons • • Occur only within CNS Have multipolar shape and convey impulses between various parts of the CNS Transmitting Nerve Impulses • First hypothesized in early 1900s, but not able to be tested until 1960s. • Electrodes were inserted into a giant axon of a squid. Voltage was then measured between the inside and the outside of the axon. • Voltage is the electrical potential difference between two points • The electrical difference across a membrane is called the membrane potential. Transmitting Nerve Impulses • Resting Potential • When the axon is not conducting an impulse. • There is a difference in ion distribution on either side of the axonal membrane caused in part by the sodium-potassium pump. • Voltage is about – 70 mV. • Action Potential • A rapid change in polarity across a portion of an axonal membrane • A threshold is the minimum change in polarity that is required to generate an action potential. • Voltage swings from -70 mV to 40 mV. Transmitting Nerve Impulses • In nonmyelinated axons the action potential moves down an axon one section at a time at a speed of about 1 m/s. • In myelinated axons, the action potential “jumps” from node to note. • When the action potential moves on, the section goes through a refractory period, in which Na+ gates are unable to open. This causes the action potential to only move in one direction. Transmission across a Synapse • The ends of the axon are branched into axon terminals. • The terminals are close to the dendrites of another neuron. The space between the 2 neurons is called a synapse. Transmission across a Synapse • Impulses cannot travel across a synapse, so molecules called neurotransmitters carry it. • When an impulse reaches the axon terminals an gated channel for Ca2+ opens and stimulates a vesicle to release neurotransmitters. • The neurotransmitters then attach to the dendrites of the next neuron to carry on the impulse. • After the impulse is passed on the neurotransmitters are removed from the synapse. Antidepressants • Antidepressant drugs (ie: Prozac and Wellbutrin) work by preventing the reuptake of neurotransmitters serotonin and norepinephrine. • This helps prolong the effect of these neurotransmitters and level out the emotional state. The Central Nervous System • 3 Specific Functions • Receives sensory input • Performs integration • Generates motor outputs The Central Nervous System • The brain and spinal cord are wrapped in 3 protective membranes called meninges and the spaces between the meninges are filled with cerebrospinal fluid. The Spinal Cord • The spinal cord is a bundle of nervous tissue enclosed in the vertebral column. • 2 functions of the spinal cord • • Reflex actions Provides communication between the brain and spinal nerves The Spinal Cord • Gray matter • Consists of cell bodies and unmyelinated fibers • Contains portions of sensory and motor neurons • White matter • Made up of bundles of myelinated long fibers of interneurons called tracts • Tracts are like super highways, constantly sending information between the brain and the rest of the body The Spinal Cord • Damage to sections of the spinal cord can result in paralysis. • Amyotrophic Lateral Sclerosis (ALS), also known as Lou Gehrig’s disease, is caused by motor neurons in the brain and spinal cord dying. This eventually leads to paralysis and patients cannot breath properly. • The Cerebrum The Brain • The largest, outermost portion of the brain • Communicates with other parts of the brain and coordinates their activities • Split into 2 hemispheres, left and right, by the longitudinal fissure. Each hemisphere receives information and controls the opposite side of the brain. • Sulci, shallow grooves, divide the hemispheres into lobes. • • • • Frontal lobes control motor functions, memory, reasoning, and judgment Parietal lobes control sensory reception and integration, as well as taste. Temporal lobes receives auditory information Occipital lobes receive information from the eyes The Brain • The Cerebral Cortex • A thin layer of thin, convoluted grey matter covering the cerebral hemispheres. • Accounts for sensation, voluntary movement and all the thought processes required for learning, memory, language and speech. • A stroke that affects the cerebral cortex could lead to paralysis of one side of the body. The Brain • Basal Nuclei • Masses of gray matter within white matter that integrate motor commands, ensuring the correct muscle groups are activated. • Parkinson Disease results from loss of cells in the basal nuclei that normally control release of dopamine. The Brain • Hypothalamus • Helps maintain home0stasis by regulating hunger, sleep, thirst, body temp., and water balance. • Thalamas • Receives all sensory input except smell • Integrates the information then sends the appropriate portion to the cerebrum • Pineal Gland • Secretes melatonin, the hormone that regulates sleep-wake cycle The Brain • Cerebellum • • Largest part of the hind brain Receives sensory input from the eyes, ears, joints, and muscles about current body position • Brain Stem • • Contains midbrain, the pons, and the medulla oblongata This is where the tracts cross so right brain controls left side of body • Medulla Oblongata • Regulates heart beat, breathing, swallowing and blood pressure • Pons • Contains bundles of axons that form “bridges” between the cerebellum and the rest of the CNS. The Brain • The Reticular Activating System • Contains reticular formation, a complex network of nuclei and nerve fibers that run the length of the brain stem • Receives sensory signals and sends them up to higher centers • Receives motor signals and sends them to the spinal cord The Brain • The Limbic System • Blends higher mental function and primitive emotions into one • 2 significant structures • Hippocampus- makes prefrontal area aware of past experiences • Amygdala- causes experiences to have emotional overtones Alzheimer Disease • Patients have abnormal neurons throughout the brain, but especially in the hippocampus and amygdala. • The abnormalities are plaques containing beta amyloid accumulated around axons and bundles of fibrous proteins around the nucleus. • Symptoms can be treated with cholinesterase inhibitors, which increase levels of acetylcholine in the brain. (Neurotransmitter that either excites or inhibits smooth muscles or glands depending on their location) The Peripheral Nervous System • Contains Nerves • 2 types of nerves • Cranial Nerves • • • Humans have 12 pairs A mixture of motor and sensory nerves Concerned mostly with the head, neck, and facial regions of the body • Spinal Nerves • • Humans have 31 pairs Extend from spinal cord by 2 short roots • The dorsal roots contain axons of sensory neurons • The ventral roots contain axons of motor neurons The Peripheral Nervous System • Contains 2 divisions • The somatic system • The autonomic system The Peripheral Nervous System • The Somatic System • Take sensory information from external sensory receptors in the skin and joints to the CNS • Carries motor commands from CNS to the skeletal muscles The Peripheral Nervous System • Reflexes • • • Voluntary control of skeletal muscles originates in the brain Reflexes are involuntary response to stimuli Ex: Hand touches a sharp pin The Peripheral Nervous System • The Autonomic System • Regulates activity of the cardiac and smooth muscles and glands • Divided in to two divisions: sympathetic and parasympathetic • Both divisions: • Function automatically and usually involuntarily • Innervate all internal organs • Use two neurons and one ganglion for each impulse The Peripheral Nervous System • Sympathetic Division • Arises from the middle portion of the spinal cord and terminate in ganglia near the cord • Used in emergency situations- “fight or flight” • Increases heartbeat and dilates the bronchi while inhibiting the digestive system The Peripheral Nervous System • Parasympathetic Division • Includes a few cranial nerves and fibers that arise from the bottom of the spinal cord • Called the “rest and digest division” • Promotes internal responses associated with a relaxed state, ie: pupils to contract, digestion of food, and slowed heatbeat