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4-22-05 Neurotransmitters The Nature Of Nerve Signals 1. Chemical or electrical communication between cells occurs at synapse 2. The same neurotransmitter can produce different effects on different types of cells Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Chemical or electrical communication between cells occurs at synapses • Electrical Synapses. – Action potentials travels directly from the presynaptic to the postsynaptic cells via gap junctions. – Invertebrate giant axons – Present in vertebrate brain in stereotype behavior like a fish flapping its tail to escape a predator. – Are fast connections. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings • Chemical Synapses. – More common than electrical synapses. – Postsynaptic chemically-gated channels exist for ions such as Na+, K+, and Cl-. • Depending on which gates open the postsynaptic neuron can depolarize or hyperpolarize. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Acetylcholine used as neuromuscular junction Action potential causes presynaptic membrane to release a neurotransmitter into synaptic cleft Fig. 48.12 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Neural integration occurs at the cellular level • Excitatory postsynaptic potentials (EPSP) depolarize the postsynaptic neuron. – The binding of neurotransmitter to postsynaptic receptors open gated channels that allow Na+ to diffuse into and K+ to diffuse out of the cell. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings • Inhibitory postsynaptic potential (IPSP) hyperpolarize the postsynaptic neuron. – The binding of neurotransmitter to postsynaptic receptors open gated channels that allow K+ to diffuse out of the cell and/or Cl- to diffuse into the cell making the inside more negative relative to the outside. No sodium movement. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings • Summation: graded potentials (EPSPs and IPSPs) are summed to either depolarize or hyperpolarize a postsynaptic neuron. Fig. 48.14 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings The same neurotransmitter can produce different effects on different types of cells • Acetylcholine (ACh). – Excitatory to skeletal muscle (opens Na channels). – Inhibitory to cardiac muscle (inhibits adenyl cyclase and opens K channels which hyperpolarize the membrane). – Secreted by the CNS, PNS, and at vertebrate neuromuscular junctions. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings • Biogenic Amines. – Epinephrine and norepinephrine. • Can have excitatory or inhibitory effects. • Secreted by the CNS and peripheral nervous system (PNS). • Secreted by the adrenal glands (increase heart rate). • Epinephrine also called adrenalin Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings • Dopamine – Generally excitatory; may be inhibitory at some sites. • Widespread in the brain. • Affects sleep, mood, attention, and learning. – Secreted by the CNS and PNS. – A lack of dopamine in the brain is associated with Parkinson’s disease. – Excessive dopamine is linked to schizophrenia. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings • Serotonin. – Generally inhibitory. • Widespread in the brain. • Affects sleep, mood, attention, and learning – Secreted by the CNS. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings • Amino Acids – Gamma aminobutyric acid (GABA). • Inhibitory –opens chloride channel--IPSPs. • Secreted by the CNS and at invertebrate neuromuscular junctions. • Glycine. . • Secreted by the CNS. • Inhibitory • Glutamate. – Excitatory. – Secreted by the CNS and at invertebrate neuromuscular junctions. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings • Aspartate. – Excitatory. – Secreted by the CNS. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings • Neuropeptides. – Substance P. • Excitatory. • Secreted by the CNS and PNS. • Mediates our perception of pain. • Met-enkephalin (an endorphin). – Generally inhibitory. – Secreted by the CNS. – Decrease our perception of pain. Natural “high”. Long distance runners “feel good” after a long run associated with increased endorphins. Morphine binds to same receptor. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings • Gasses that act as local regulators. – Nitric oxide (NO2). – Carbon monoxide. – Persons suffering angina (chest pain from lack of adequate delivery of oxygen to the heart) take nitroglycrine. An enzyme breaks it down into nitric oxide which relaxes cornary arteries providing almost instant relief. – Viagra slows the degradation of nitric oxide in the erectile tissue veins that causes their relaxation. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings CHAPTER 49 SENSORY AND MOTOR SYSTEMS Sensing, Acting, and Brains 1. The brain’s processing of sensory input and motor output is cyclical rather than linear meaning we are continually analyzing sensory input as we sense the effect of our motor output. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings The brain’s processing of sensory input and motor output is cyclical rather than linear • The way it ISN’T: sensing brain analysis action. • The way it is: sensing, analysis, and action are ongoing and overlapping processes. • Sensations begin as different forms of energy that are detected by sensory receptors. – This energy is converted to action potentials that travel to appropriate regions of the brain. • The limbic region plays a major role in determining the importance of a particular sensory input (whether it gets filtered out). Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Introduction To Sensory Reception 1. Sensory receptors transduce stimulus energy and transmit signals to the nervous system 2. Sensory receptors are categorized by the type of energy they transduce Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings • Definitions: • Sensations are action potentials that reach the brain via sensory neurons. • Perception is the awareness and interpretation of the sensation. • Next slide has example of taste reception. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Sensory receptors transduce stimulus energy and transmit signals to the nervous system. Action potential in most case not generated directly. G-protein >Adenylyl cyclase C-AMP> Protein kinase phosphorylates some protein?? that prevents K+ channel from opening. Action potentials • Sensory Transduction. – The conversion of stimulus energy into a change in membrane potential. – Receptor potential: a sensory receptor’s version of a graded potential (change in voltage across the membrane determined by the strength of stimulus) Change affects the rate of firing of postsynaptic neuron. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings • Amplification. – The strengthening of stimulus energy so that it can be detected by the nervous system. • May be a part of, or occur apart from, sensory transduction. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings • Transmission. – The conduction of sensory impulses to the CNS. – Some sensory receptors must transmit chemical signals to sensory neurons. • The strength of the stimulus and receptor potential affects the amount of neurotransmitter released by the sensory receptor. – Some sensory receptors are sensory neurons. • The intensity of the receptor potential affects the frequency of action potentials. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings • Integration. – The processing of sensory information. • Begins at the sensory receptor. – For example, sensory adaptation is a decrease in responsiveness to continued stimulation. – For example, the sensitivity of a receptor to a stimulus will vary with environmental conditions. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Sensory receptors are categorized by the type of energy they transduce Fig. 49.3 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings • Mechanoreceptors respond to mechanical energy. – For example, muscle spindles is an interoreceptor that responds to the stretching of skeletal muscle. – For example, hair cells detect motion. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings • Pain receptors = nocioceptors. – Different types of pain receptors respond to different types of pain. – Prostaglandins increase pain by decreasing a pain receptors threshold. • Anti-inflammatories work by inhibiting prostaglandin synthesis. • Thermoreceptors respond to heat or cold. – Respond to both surface and body core temperature. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings • Chemoreceptors respond to chemical stimuli. – General chemoreceptors transmit information about total solute concentration. – Specific chemoreceptors respond to specific types of molecules (olfaction). – Internal chemoreceptors respond to glucose, O2, CO2, amino acids, etc. – External chemoreceptors are gustatory receptors (taste) and olfactory receptors (smell). Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings • Electromagnetic receptors respond to electromagnetic energy (migrating animals— birds and whales). – Photoreceptors respond to the radiation we know as visible light. – Electroreceptors: some fish use electric currents to locate objects. If something enters their electric field they sense it. Fish in murky waters in the tropics. Also sharks and duck billed platypus (bill). Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Photoreceptors And Vision 1. A diversity of photoreceptors has evolved among invertebrates 2. Vertebrates have single-lens eyes 3. The light-absorbing pigment rhodopsin triggers a signal-transduction pathway 4. The retina assists the cerebral cortex in processing visual information Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings • Most, if not all, animal photoreceptors may be homologous (common origin)._ Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings A diversity of photoreceptors has evolved among invertebrates • Eye cups are among the simplest photoreceptors – Detect light intensity and direction — no image formation. – The movement of a planarian is integrated with photoreception. Turns away from light Fig. 49.7 • Image-forming eyes. – Compound eyes of insects and crustaceans. • Each eye consists many individual units called of ommatidia, each with its own light-focusing lens. • This type of eye is very good at detecting movement. Neurons directly exit the back of eye Fig. 49.8 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings • Single-lens eyes of invertebrates such as jellies, polychaetes, spiders, and mollusks. – The eye of an octopus works much like a camera and is similar to the vertebrate eye. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Vertebrates have single-lens eyes • Is structurally analogous to the invertebrate single-lens eye. Fig. 49.9 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings • Sclera: a tough white layer of connective tissue that covers all of the eyeball except the cornea. – Conjunctiva: external cover of the sclera — keeps the eye moist. • Cornea: transparent covering of the front of the eye. – Allows for the passage of light into the eye and functions as a fixed lens (refracts light). – Choroid: thin, pigmented layer lining the interior surface of the sclera. Prevents light rays from scattering and distorting the image. – Anteriorly it forms the iris. • The iris regulates the size of the pupil. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings • Retina: lines the interior surface of the choroid. – Contains photoreceptors. • Except at the optic disk (where the optic nerve attaches). Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings • The lens and ciliary body divide the eye into two cavities. – The anterior cavity is filled with aqueous humor produced by the ciliary body. • Glaucoma results when the duct that drain aqueous humor are blocked. – The posterior cavity is filled with vitreous humor. – The lens, the aqueous humor, and the vitreous humor all play a role in focusing light onto the retina. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings • Accommodation is the focusing of light on the retina. – In squid, octopuses, and many fish this is accomplished by moving the lens forward and backward. The cephalopod with a shell, Nautilus has an eye without a lens (pin hole camera). Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings – In mammals accommodation is accomplished by changing the shape of the lens. • The lens is flattened for focusing on distant objects. • The lens is rounded for focusing on near objects. Fig. 49.10 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings