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Nervous System 3 stages in processing information by NS: 1. sensory input 2. integration 3. motor output Neuron: basic cell of nervous system cell body contains nucleus and organelles axon: long process conducting impulses away from cell body - usually single and very long relative to cell dendrites: receive impulses from stimulus from other neurons - usually many, short relative to cell 1 synapse: junction between axon of one neuron and dendrite of next - presynaptic cell = transmitting neuron - postsynaptic cell = receiving cell chemical messengers = neurotransmitters = communication between the pre- and post- 2 sensory neurons: - receive information from body and senses - transmit to spinal cord and brain motor neurons: - transmit information from brain and spinal cord - control muscles and glands interneuron: between sensory and motor neurons; in brain. glial: support cells for neurons Schwann cells (PNS): wrap peripheral axons of one and/or many neurons in myelin sheath = nerve - Lipid; faster, more efficient conduction (insulation) o Nodes of Ranvier: gaps between the myelin sheaths 3 - Action potential only occur at these nodes and travels to the next node = Saltatory conduction Transmission of Impulses in Neurons membrane potential: inside of neuron negative relative to outside (outside = + ; inside = -) “+” ions (cations) on inside and outside cell, but “-“ inside due to “-“ proteins and other anions (“-“) inside cell that cannot cross cell membrane. resting potential: not transmitting signal (about —70 mV = refers to the inside of the membrane) - we’ll get back to this after … 4 ACTION POTENTIAL (AP): signals that carry info. along axon It’s all about Na+ AND K+ reversal in membrane potential (outside = - ; inside = +) = depolarization How? - controlled by gated protein channels in the membrane - stimulus small depolarization triggers opening of Na+ ion channels - if stimulus causes enough Na+ to move in = threshold: a stimulus causes minimum depolarization required to trigger an AP - increases permeability to Na+ - rushes into cell o Domino affect = one Na+ opening triggers the next and so on… causes membrane potential increases to +35 mV (outside = - ; inside = +) • all or none principle: all the way to +35 mV or not o So long as they can reach the threshold of the cell, strong stimuli produce no stronger action potentials than weak ones, just more of them. 5 o So, the strength of the stimulus is encoded in the frequency of the action potentials that it generates. Na+ gated channel closes - inactivated: will not reopen until resting potential restored How? repolarization: - K+ ion gated channels open = K+ migrates out It keeps on moving down the axon…propagation of action potential: movement of impulse down neuron • influx of Na+ depolarizes adjacent membrane • impulse travels with AP 6 Now what is the problem here? Hummmm We have resting membrane potential again after repolarization (outside = + ; inside = -), but… Na+ and K+ are on the wrong sides of the membrane! How to fix it… Na+/K+ pump: 3 Na+ out for every 2 K+ pumped in High energy cost! (1/3 of daily energy!) 7 Now, let’s go back to how the stimulus is received… Synapses Synapses located at branches of axons (terminal end) of presynaptic cell and cell body or dendrite of postsynaptic cell electrical synapses: transmit action potential directly between neurons - formed by gap junctions between cells **chemical synapses: use chemicals to transfer impulses when action potentials are not transmitted from one neuron to the next, directly…needs a “communicator” = neurotransmitter (NT) - Located in vesicles of neuron - Binds to receptor portion of gated channels of postsynaptic cell most common = **Acetylcholine (Ach) others: Norepinephrine & Epinephrine (also act as hormones – adrenal glands) = increases heart rate; therefore blood flow Dopamine & Serotonin = affect sleep, mood, attention, and learning And more… 8 Where are the NT’s released?… - synaptic cleft (synapse): space between neurons HOW? incoming potential: AP ends at presynaptic terminal of presynaptic cell - This depolarization of presynaptic terminal opens gated channels for Ca2+ (Ca2+ in synapse) = = enters presynaptic terminal Ca2+ in - triggers fusion of synaptic vesicles with presynaptic membrane NT released into synaptic cleft via exocytosis NT bind to receptors on gated channels in postsynaptic membrane - specific ion channels opened by binding • EPSP: Excitatory PostSynaptic Potential - Na+ ion channels open on postsynaptic membrane - Bring membrane potential of postsynaptic neuron to threshold = membrane depolarization AP If an inhibitory NT is released… • IPSP: Inhibitory PostSynaptic Potential 9 - K+ ion channels open and leaves cell - Causes the interior to be more negative - increases polarization (hyperpolarization) = inhibits impulse propagation = no AP - due to membrane moving away from threshold o Gives, AP “a break” - Need twice the amount of Na+ rushing into get AP to go again. 10 Types of Nervous Systems (Evolutionary Trends) o nerve net: no central control (Cnidarians) o ganglia: clusters of nerve cells (other many inverts.) o cephalization: nerve centers and sense organs at anterior end - flatworms: simple brain with two nerve trunks - annelids & arthropods: obvious brain plus ventral nerve cord The Vertebrate Nervous System Central Nervous System (CNS) = includes brain and spinal cord BRAIN Gray matter on outside – mostly unmyelinated axons o modulates the distribution of action potentials White matter on inside – myelinated axons o Function: tissue through which messages pass between different areas of gray matter; o affects how the brain learns and dysfunctions 4 ventricles in deep interior • Contains cerebrospinal fluid – formed from filtrate of blood; drains into veins 11 • Contains nutrients, oxygen, hormones, NT’s, and cellular waste. • Cushions the brain (& spinal cord) Breaking it down into parts: 1. Cerebrum (forebrain): processing and integration of sensory information, speech, and emotion. • includes: cerebral cortex – divided into 2 halves; R. & L. sides = cerebral hemispheres o corpus callosum: connects halves of cerebral cortex; communication between right and left cerebral hemispheres - R. side of brain controls L. side of body - L. side of brain controls R. side of body 12 o thick bands of axons (mammals: integration of vision) 2. hypothalamus (forebrain): regulation of homeostasis, endocrine sys. 3. Brainstem (midbrain and hindbrain): axons extend into the cerebral cortex - Midbrain = sends sensory info to forebrain o Auditory center (hearing) – localizes sound o optic lobe: visual centers (mammals: coordinate visual reflexes) - hindbrain o cerebellum: coordination of movement o medulla oblongata: control of autonomic functions = breathing, heartbeat, vessel activity, digestion, swallowing, vomiting o pons: helps regulate the medulla oblongata 13 SPINAL CORD o carries information to and from brain o simple reflexes (knee jerk, etc.) reflex arc: • sensory neuron brings afferent signal to spinal cord • impulse synapses to motor neuron • motor neuron delivers efferent impulse to effector 14 Peripheral Nervous System (PNS) o Divided into 2 regions: 1. somatic (voluntary) 2. autonomic (involuntary) sensory: inputs signals to brain (afferent) - somatic: senses external stimuli, [from] skeletal muscles ex. Step on sharp rock - autonomic: senses organs, [from] involuntary muscles ex. Bolus in esophagus 15 motor: relays commands from brain (efferent) - somatic: voluntary muscles ex. Move foot off sharp rock - autonomic: involuntary muscles and organs ex. Peristaltic contractions moving bolus to stomach. More on the 2 regions: 1. Somatic NS - carries signals to and from skeletal muscles 2. Autonomic NS - Regulates internal environment: peristalsis, heart, other organs - Divided into 3 regions (we’re looking at 2 of them): 1. sympathetic: generally prepares body for action (“fight or flight”) - increases heart rate, raises metabolism **[What part of the endocrine system would this control?]** 2. parasympathetic: generally relaxes and rebuilds - stimulating digestion, slowing heart 3. (enteric: controls digestive tract, pancreas and gallbladder) [don’t worry about this] 16 Receptors and Effectors receptors: receive information from external or internal environment from reception to perception: Types of Receptors mechanical: pressure, touch, stretch, balance, sound - bending/stretching membrane increases permeability to Na+ & K+ depolarization generates AP 17 - hair cells: detect motion (sound and balance, pressure in fish) chemical: taste and smell, osmolarity, pheromones in insects - chemicals bind to membrane receptor molecules - changes membrane permeability thermal: separate responses to heat and cold - hypothalamus receptors regulate body temp pain: response to excess heat, pressure or chemicals - comprised of naked dendrites electromagnetic: visible light, infrared, electricity, magnetism - earth’s magnetic fields aid in migration Sensory Receptors of Human Skin: 18