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PSYB65H3 (479)
Ted Petit (185)
Chapter 2


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University of Toronto Scarborough
Ted Petit

University of Toronto 1 PSYB65 :Human Brain and Behavior Sayanutha Niranjan Chapter 2: Neuroanatomy Humans have aggregates of specialized cells that perform specialized functions Neurons: Cells in the nervous system that are responsible for communication of neural impulses and therefore behavior. They learn and store information about their external environment. The neuron shape allows it to receive, conduct, and transmit signals. The neuron consists of three main components Dendrites: Receives incoming information from other neurons Increase the surface area available for the reception of signals from the axons to other neurons Covered in tiny spines which grow and retract in response to experience Extension of branching give an indication of the # of connections or synapses it makes with incoming axons Soma: cell body, that contains the genetic machinery & most of the metabolic machinery needed for common cellular functions Axon Info sender: sends neural information to other neurons Covered with insulation, called myelin Helps speed the rate of info transfer The ends of the axon is called the terminal buttonreleases neurochemical message (neurotransmitters) *Synapse Gap in which information is passed from the axon to the dendrite (20- 50nanometers wide) Presynaptic events that occur in the axon Postsynaptic events that happen in the dendrite Information is sent to the rest of the neuron in the form of an electrical charge or AP Neuron has only one axon and can divide in its far end into many branches Look at Figure 2.1 Internal Anatomy of the Neuron Covered by a membrane plasma membrane bilayer of continuous sheets of phospholipids that separate two fluid (h20) environments Within membrane, are proteins and channels that allow passage of materials in/out Organelles nucleus (packages and controls genetic info & genetic info to code proteins), ribosome, endoplasmic reticulum and mitochondria Structure and Functions of Neurons There are 3 types: Unipolar: have only one process coming from the cell body. Bipolar: have only 2 processes coming from the cell body. Multipolar: have numerous processes coming from the cell body. 1 University of Toronto 2 PSYB65 :Human Brain and Behavior Sayanutha Niranjan Neurons with short or no axons are known as interneurons and relay information within structures instead of between structures (ex. Connect the sensory and motor neurons of the reflex arc to connect the responses). Afferent neurons send information to the brain Efferent neurons send information away from the brain. * Neurons do vary in size, shape and function and that a neuron can change shape as a result of experience GLIA: cells in the nervous system that are responsible for support and maintenance of neurons. There are 3 types: Astrocytes: Largest & Star shaped glia Fill the space between neurons, involved in the blood-brain barrier supply nutrients to neurons, metabolic functions, regulate the chemicals in extracellular space, regulate and store neurotransmitters. Oligodendrocytes: Glia that make myelin (fatty substance that insulate axons). They wrap around axons of the CNS. Axons outside of CNS, myelin provided by Schwann cells Microglia: Smallest glia Made outside the CNS by microphages, which are phagocytes that remove debris from the nervous system. Excessive microglia can cause neurodegenerative disease (eg multiple sclerosis and Alzheimers disease) Satellite Cells: these are support cells outside of the brain and spinal cord. Communication within the Neurons: The Action Potential Neurons at rest have an electrical change of -70mv resting potential Extracellular fluid contains HIGH [Na+] Intracellular fluid contains HIGH [K+] Neurons have two properties that promote the uneven distribution of ions: 1. Ions cross the membrane through proteins embedded in the membrane ion channels At rest, K+ can easily pass through membrane and Na+ cannot easily 2. Sodium-Potassium Pump Actively pumping 2K+ in and 3Na+ out Depolarization when membrane potential gets 50+mv Action potential occurs releases neurotransmitters [Na+ channels open] 2 University of Toronto 3 PSYB65 :Human Brain and Behavior Sayanutha Niranjan As neuron becomes depolarized, K+ channels OPEN and K+ ions rapidly leave neuron hyperpolarization (-70mv) Absolute Refractory Period: the inability to open Na channels after depolarization, causing a period without action potential. AP are all or none all action potentials are the same size Nodes of Ranvier: the gaps along the axon where there is no myelin. Action potentials occur at these nodes; therefore ion movement only occurs here. When an action potential enters an axon it moves to the first node where depolarization occurs producing a new action potential uniform in size. Saltatory Conduction: the jumping of action potential from one node of Ranvier to another, and occurs down the length of an axon. AP is actively propagated, neural transmission in myelinated neurons is faster than transmission in neurons without myelination Communication b/w Neurons: The Synapse Synapse: the gap between the axon of one neuron and the dendrites of another where information is passed. Presynaptic events are those that occur before the synapse and postsynaptic are those that occur after the synapse. There are 3 types of synapses: Axodendritic: consist of axons that form synapses with other dendrites. Axosomatic: occur between the axon and soma of the neurons. Dendrodentritic: occur between dendrites. Active Zones area of protein accumulation on the membrane that allow the vesicle to deposit its contents into the synapse (neurotransmitters) Action potential cause calcium channels to open and causes the neurotransmitter to be released into the synapse by exocytosis Membrane of vesicle fuses with the axonal membrane which results in an opening in the vesicle allowing the neurotransmitter to flow into the synapse Neurotransmitter binds to a protein embedded in the post-synaptic membrane known as a receptor (usually very specific) Types of Receptors on the Postsynaptic Membrane 1. Transmitter-Gated Ion Channels (Ionotropic Receptors) Proteins on the dendrite that control an ion channel. When a neurotransmitter binds to this, the channel changes conformation (open or closed) and are required for fast responses. EPSP (Excitatory Postsynaptic potential) moved towards producing an AP 3 University of Toronto 4 PSYB65 :Human Brain and Behavior Sayanutha Niranjan IPSP (Inhibitory Postsynaptic potential) moved away from producing an AP 2. G-Protein Coupled Receptors (Metabotropic receptors) Produce more slower, more diverse, more sustained responses Occur more frequently in the nervous system Multistep process Neurotransmitter bound sub-unit of G-protein breaks away and can either move along the side of the membrane can bind to an ion channel OR trigger synthesis of other chemicals Can result in IPSPs or EPSPs result in changes in gene expression 3. Autoreceptors (found on pre-synaptic membrane) Metabotropic receptors Regulate and monitor the amount of neurotransmitter in the synapse Two mechanisms are responsible for terminating the activity of Neurotransmitters Reuptake Presynaptic neuron reabsorbing the NT from synapse and repackaging Enzyme degradation NT is broken down into inactive form by an enzyme present in the synapse NEUROTRANSMITTERS Divided into small and large molecules NT Smaller NTS: Released in a directed fashion, activating either Ionotropic or metabotropic receptors that act directly on ion channels Associated with fast responses 1. Acetylcholine (Ach) Cholinergic neurons release this NT Used by all motor neurons in brain and spinal cord Synthesized by enzymatic conversion from choline Ach is deactivated into choline and acetic acid by acetylcholinesterase (AChE) Degradation speed is very fast
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