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The Nervous System AP Biology Why do animals need a nervous • Because the world system? is always coming at you! Remember… Poor thinkbunny! about the bunny… The Nervous System • Function: environment is constantly changing – nervous system detects those changes and helps the organism respond/adapt • Irritability: ability to respond to a stimulus The Nervous System • Nervous System detects (sensory input), processes (integration), and responds (motor output) • Peripheral Nervous System detects and responds • Central Nervous System processes information Nervous System cerebrum cerebellum spinal cord • Central nervous system – brain & spinal cord • Peripheral nervous system – nerves from senses – nerves to muscles cervical nerves thoracic nerves lumbar nerves femoral nerve sciatic nerve tibial nerve The Neuron (Nerve Cell) • Three types of neurons: –Sensory – carry impulses from the sense organs (receptors) to the CNS –Motor – carry impulses from the CNS to the muscles or glands (effectors) –Interneurons – connect and carry impulses between sensory and motor neurons Neurons 1. Cell body – largest part; most metabolic activities take place here; contains nucleus 2. Dendrites – carry impulses from the environment or other neurons toward the cell body Neurons 3. Axon – long fiber that carries impulses away from the cell body • Terminal branches – branching of axon • Synaptic knobs – ends of axon; contain vesicles with neurotransmitters Fun facts about neurons • Most specialized cell in animals • Longest cell – blue whale neuron • 10-30 meters – giraffe axon • 5 meters – human neuron • 1-2 meters Nervous system allows for 1 millisecond response time The Nerve Impulse • Resting potential – a nerve cell has an electric potential because OPPOSITELY charged ions are on each side of the membrane • Anions are mainly on the inside of the the cell; cations on the outside ++++++++++++++++++++++++ ----------------------------------------- The Nerve Impulse • Ungated ion channels allow ions to diffuse across the plasma membrane –These channels are always open • This diffusion does not achieve an equilibrium since sodium-potassium pumps transport these ions against their gradients Hyperpolarization • Gated K+ channels open K+ diffuses out of the cell the membrane potential becomes more negative Depolarization • Gated Na+ channels open Na+ diffuses into the cell the membrane potential becomes less negative The Nerve Impulse • Action Potential – a rush of Na+ flow into the membrane causing an imbalance in the charge on each side of the membrane • This causes the POLARITY to shift and a wave (impulse) moves down the length of the neuron +++++++++++++++++--------------------------------------+++++++ + Step 1: Resting State Step 2: Threshold Step 3: Depolarization Step 4: Repolarizing Step 5: Undershoot Na+ gates close & K+ gates open Na+ gates open Stimulus K+ gates close • During hyperpolarization or undershoot, Na+ channels are closed –Neuron cannot depolarize in response to another stimulus: refractory period –The refractory period assures impulse conduction is unidirectional Myelin • Is composed of 80% lipid and 20% protein • Used for insulation and to help speed up the nerve impulse • Wraps around the axon of some neurons Myelin • Gaps in myelin sheath cells called Nodes of Ranvier – allow impulses to move more quickly down neurons Myelin • In Saltatory Conduction, only the Nodes of Ranvier depolarize and therefore conduct an impulse faster The Synapse • SYNAPSE: the space between the axon of one neuron and the dendrite of another • Axon terminals have vesicles containing chemicals: NEUROTRANSMITTERS • These chemicals are secreted from the axon of one neuron stimulates receptor sites on the effector or the dendrite of the next neuron Neurotransmitter Action at Synapse 1. Action potential arrives at axon terminal of presynaptic neuron 2. Synaptic vesicles rupture, releasing neurotransmitter into synapse 3. Neurotransmitter diffuses across synapse & binds to receptor protein on postsynaptic cell 4. Postsynaptic cell is excited or inhibited 5. Neurotransmitter in synapse is deactivated Synapse Junction between nerve cells – 1st cell releases chemical to trigger next cell – where drugs affect nervous system synapse • Neurotransmitters are the chemicals which allow the transmission of signals from one neuron to the next across synapses. • They are also found at the axon endings of motor neurons, where they stimulate the muscle fibers. • They and their close relatives are produced by some glands such as the pituitary and the adrenal glands. • They are chemicals that communicate information throughout our brain and body. • The brain uses neurotransmitters to tell your heart to beat, your lungs to breathe, and your stomach to digest. • They can also affect mood, sleep, concentration, weight, and can cause adverse symptoms when they are out of balance. • Neurotransmitter levels can be depleted many ways. • Stress, poor diet, neurotoxins, genetic predisposition, drug (prescription and recreational), alcohol and caffeine usage can cause these levels to be out of optimal range. Types of Neurotransmitters • Two kinds of neurotransmitters – INHIBITORY and EXCITATORY. • Excitatory neurotransmitters are not necessarily exciting – stimulate the brain. • Inhibitory - calm the brain and help create balance are called . – balance mood and are easily depleted when the excitatory neurotransmitters are overactive. Small molecule neurotransmitters Type Neurotransmitter Acetylcholine Amino acids Biogenic amines Postsynaptic effect Excitatory Gamma aminobutyric Inhibitory acidGABA Glycine Inhibitory Glutamate Excitatory Aspartate Excitatory Dopamine Inhibitory Nor adrenaline Excitatory Serotonin Inhibitory Histamine Excitatory ACETYLCHOLINE • Acetylcholine was the first neurotransmitter to be discovered. • It is responsible for much of the stimulation of muscles, including the muscles of the gastro-intestinal system. • It is also found in sensory neurons and in the autonomic nervous system, and has a part in scheduling REM (dream) sleep. • There is a link between acetylcholine and Alzheimer's disease: There is something on the order of a 90% loss of acetylcholine in the brains of people suffering from Alzheimer's, which is a major cause of senility. • Outside the brain, acetylcholine is the main neurotransmitter in the parasympathetic nervous system – the system that controls functions such as heart rate, digestion, secretion of saliva and bladder function. • The plant poisons curare cause paralysis by blocking the acetylcholine receptor sites of muscle cells. • The well-known poison botulin works by preventing the vesicles in the axon ending from releasing acetylcholine, causing paralysis. SEROTONIN • SEROTONIN is an inhibitory neurotransmitter – which means that it does not stimulate the brain. • Adequate amounts of serotonin are necessary for a stable mood and to balance any excessive excitatory (stimulating) neurotransmitter firing in the brain. • If you use stimulant medications or caffeine in your daily regimen – it can cause a depletion of serotonin over • Low serotonin levels leads to an increased appetite for carbohydrates (starchy foods) and trouble sleeping, which are also associated with depression and other emotional disorders. It has also been tied to migraines, irritable bowel syndrome, and fibromyalgia. • Low serotonin levels are also associated with decreased immune system function. • In addition to mood control, serotonin has been linked with a wide variety of functions, including the regulation of sleep, pain perception, body temperature, blood pressure and hormonal activity • Largest amount of serotonin is found in the intestinal mucosa. • Although the CNS contains less than 2% of the total serotonin in the body, serotonin plays a very important role in a range of brain functions. It is synthesized from the amino acid tryptophan. • Gamma amino butyric acid(GABA) is the major inhibitory neurotransmitter that is often referred to as “nature’s VALIUMlike substance”. When GABA is out of range (high or low excretion values), it is likely that an excitatory neurotransmitter is firing too often in the brain. GABA will be sent out to attempt to balance this stimulating over-firing. • People with too little GABA tend to suffer from anxiety disorders, and drugs like Valium work by enhancing the effects of GABA. Lots of other drugs influence GABA receptors, including alcohol and barbiturates. If GABA is lacking in certain parts of the brain, epilepsy results. HISTAMINE • Amino acid Histidine is the precursor of an important neurotransmitter histamine. • Histamine is present in venom and other stinging secretions. • Histamine is a biogenic amine involved in local immune responses • Regulate physiological function in the gut • Act as a neurotransmitter. • Triggers the inflammatory response. Nervous System Organization • Cnidaria - nerve net –loose organization of bi-directional neurons –no centralization • Flatworms - ladder –2 anterior ganglia (rudimentary brain) with paired, longitudinal nerve cords –paired sensory organs (eyespots) Nervous System Organization • Segmented worms - advanced ladder –prominent brain –solid, fused, ventral nerve cord –segmentally arranged ganglia Nervous System Organization • Arthropods –prominent brain –solid, fused, ventral nerve cord –extensive fusion of ganglia –well-developed sensory organs –exhibit complex behaviors Organization of the NS • The human nervous system is divided into 2 major divisions: –Central Nervous System (CNS) • Control center of body, brain and spinal cord –Peripheral Nervous System (PNS) • Nerves (bundles of axons) CNS: Parts of the Brain • Forebrain • Midbrain • Hindbrain Hindbrain • Cerebellum –coordinates muscular movements • Medulla oblongata –regulates heart rate, blood pressure and breathing –contains reflex centers for vomiting, swallowing, sneezing, hiccupping, and coughing • Pons –helps regulate respiration Forebrain • Thalamus – switching station for sensory input for all senses but smell; relays sensory info to cerebrum and motor info from the cerebrum • Hypothalamus – control hunger, thirst, fatigue, anger, and body temp; regulates pituitary gland Forebrain • Cerebrum – divided into left and right hemispheres • Corpus callosum – major connection between hemispheres –Left hemisphere primarily responsible for right side of body; right hemisphere primarily responsible for left side Forebrain • Cerebral cortex – outer covering of gray matter –The more convoluted the surface, the more surface area, the more neurons Forebrain • Cerebrum – divided into frontal, temporal, parietal, and occipital lobes • Frontal lobe –Contains the primary motor cortex (controls actions of skeletal muscles) and olfactory cortex (smell) Forebrain • Parietal lobe –Contains the primary somatosensory cortex and gustatory cortex (taste) • Temporal lobe –Contains auditory cortex (sound) • Occipital lobe –Contains visual cortex (sight) PNS • Sensory – transmits impulses from the sense organs (such as the ears and taste buds) to the CNS • Motor – transmits impulses from the CNS to the muscles or glands (somatic or autonomic) • Somatic – conscious movement of the body • Autonomic – regulates activities that are automatic or involuntary –Sympathetic (stress, high energy) and Parasympathic (leisure, rest) are antagonistic systems that turn an autonomic response on or off Sympathetic effects: • dilates pupil • accelerates heartbeat & respiration • inhibits stomach & intestine activity • relaxes urinary bladder Parasympathetic effects: • constricts pupil • slows heartbeat & respiration • stimulates stomach & intestine activity • contracts urinary bladder Reflex Arc • Some actions don’t/can’t wait for your brain to interpret the signal • Reflexes are involuntary actions; they travel from ____ to ____: –Receptors (nerve “endings”) –Sensory neurons –Interneurons –Motor neurons –Effectors (muscles or glands) Types of Sensory Receptors • Thermoreceptors – detect heat and cold • Pain receptors (nocioceptors) – detect chemicals released from injured cells • Mechanoreceptors – detect mechanical energy (touch, pressure, vibration) Types of Sensory Receptors • Chemoreceptors – detect chemicals • Photoreceptors – detect light energy • Electroreceptors – detect electrical fields How are sounds sensed? • The ear captures, transmits, and converts sound into electrical signals • Ear has three basic parts: 1. Outer ear 2. Middle ear 3. Inner ear How are sounds sensed? • Outer ear: external ear (pinna) and auditory canal –Funnels sound –Sound waves vibrate the tympanic membrane How are sounds sensed? • Middle ear –Tympanic membrane (ear drum) –Three tiny bones: malleus (hammer), incus (anvil), stapes (stirrup); transfer vibrations to the oval window on the cochlea –Eustachian tube – equalize pressure; connects middle ear to pharynx How are sounds sensed? • Inner ear: cochlea –converts vibrations into electrical signals –As the oval window vibrates, it sets the cochlear fluid in motion –Moving fluid brushes over hairs –Bending of hairs is sensed by mechanorecptors and sends the signal to the brain (auditory nerve) Equilibrium • Equilibrium is maintained by the semicircular canals Equilibrium • The semicircular canals are arranged in the X, Y, and Z planes –Therefore, any movement in any direction will be perceived • Fluid in the canals brushes over hairs • Movement of hairs is sensed and the signal is sent to the brain Equilibrium • Dizziness can be due to the momentum of the fluid in the canals –You’ve stopped moving, but the movement of the fluid in the semicircular canals makes you think you’re still moving Fish “Hearing” – Lateral Lines • Contains mechanoreceptors that function similarly to mammalian inner ear • Gives info about direction and velocity of water flowing over fish’s body How is light sensed? • Sclera – tough, white layer • Conjunctiva – external cover of sclera; keeps eye moist; conjuctivitis (pink eye) How is light sensed? • Cornea – transparent covering in front of eye • Choroid – thin, pigmented layer lining interior surface of the sclera; prevents light rays from scattering and distorting the image • Iris regulates size of pupil/amount of light into eye How is light sensed? • Lens focuses light on retina • Retina – Contains photoreceptors (Except at the optic disk where the optic nerve attaches) –Rods: Black and White –Cones: Color • Optic nerve takes electric signals from eye to brain Rods and Cones • ~ 125 million rod cells –Rod cells are light sensitive but do not distinguish colors • ~ 6 million cone cells –Not as light sensitive as rods but provide color vision –Most highly concentrated on the fovea – area of retina lacking rods How are scents sensed? • Insects smell through their legs and antennae Male silkworm moth Bombyx mori Sensory hairs on antennae detect pheromones released by female How are scents sensed? • Olfactory nerves are stimulated when chemicals touch them • Different chemicals create different responses in the olfactory nerves; hence we detect different smells How are tastes sensed? • Taste buds on tongue act just like the olfactory nerves –Different chemicals stimulate the nerves in the taste buds differently; hence we detect different tastes • Four “primary” tastes are bitter, sour, salty, and sweet