PSYC 3510 Behavioral Neuroscience Exam 2 Materials
PSYC 3510 Behavioral Neuroscience Exam 2 Materials PSYC 3510-001
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This 21 page Bundle was uploaded by Erica Britton on Thursday October 13, 2016. The Bundle belongs to PSYC 3510-001 at Auburn University taught by Dr. Barker in Fall 2015. Since its upload, it has received 4 views. For similar materials see Behavior Neuroscience in Psychology (PSYC) at Auburn University.
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What are four similarities between human and vertebrate brains? Spinal cord, bilateral, front and back end, identifiable divisions How old is vertebrate brain? Human brain? 500 million years old / 200,000 years old With what family was there an explosion in brain size? When? 2 million years ago in Hominids, esp. homo erectus What gene is the genetic determinant for brain size? When is it expressed? Human acceleration region 1/1f/5. It’s expressed in the gestational period around 712 weeks. Are bigger brains smarter? Not necessarily, the difference in brain size is insignificant without taking in genetic and environmental factors. What are some environmental factors that are related to brain size (3)? Poverty, parental education, household income According to our wide pelvis, our birth canal and brain coevolved. What makes us question the future of our constraints? 1/3 births are Csections Convolutions increase surface area without increasing size. 6 layers of neurons make up a 1/8 inch thick neocortex. What kind of problems are faced in disorders with no brain folds (gyri) or grooves (sulci)? Lots!!!! Our brain is bilateral and cephalocaudal. What does the encephalon encompass? Everything under the skull and the spinal cord Label the pictures with anatomical directions. When you cut the brain horizontally, you cut it to where you can look down at the brain. When you cut the brain from a frontal/coronal view, you can look at the brain from the front. When you cut the brain from a sagittal view, you can see left and right hemispheres. Threepart division of the brain: forebrain, midbrain, hindbrain Fivepart division of the brain: telencephalon, diencephalon, mesencephalon, metencephalon, myelencephalon What, of the fivepart division, make up the threepart division? Forebrain is tel and die, midbrain is mes, and hindbrain is met and mye Talk about the two theories/ideas that support that vertebrates, mammals, and humans have the same basic brain design. 1. Herricks Vertebrate Plan firs to show that both human and vertebrate brains have similar design, used a tiger salamander 2. McLean’s Reptilian brain old reptilian brain evolved to be our brain stem, that brain stem and the thalamus created the old mammalian brain, and the old mammalian brain and the neocortex created our new mammalian brain. What is corticalization? When does it occur in fetal development? Increase in growth of neurons; last period of fetal development What makes up the brain and is the smallest structural unit? neurons Neurons in cerebral hemisphere, cerebellum, and spinal cord? 1215 billion, 70 billion, 1 billion How much of the body’s power does the brain use? 20% What is the process called that naturally makes us lose neurons? What is the only place in the brain that can create new neurons, and what is the process called? Programmed cell death; hippocampus What two nerve systems make up the autonomic system? Where are they located and what do they do? Parasympthatic, top and bottom of spinal cord makes you have to move around. Sympathetic, middle of spinal cord, relaxes What are meninges? What are three for the brain? Protective coverings of the brain. Dura mater, subarachnoid cortex, and pia mater What makes up the subarachnoid space? CSF fluid and What are central ventricles? Fluid filled cavities, four I the brain What produces CSF? Where is the excess fluid absorbed? Choroid plexus, subarachnoid space What does the telencephalon include? Lobes of cerebral cortex Diencephalon? Describe each part. Thalamus and hypothalamus. Thalamus involved in motor. Hypothalamus is biological homeostasis with the pituitary gland Mesencephalon? Tectum (inferior and superior colliculus) and tegmentum (substantia niagra, red nucleus, and periquadectial grey) Metencephalon? Cerebellum (fine motor movement and timing) and pons Myelencephalon? Medulla, spinal cord (tracts to communication brain and body), reticular formation (complex network of fibers) What are the general functions for each lobe of the brain? Frontal is voluntary motor movements, EF, speech production, theory of mind. Occipital is vision and visual consciousness. Temporal is auditory. Parietal is integrative, sense of touch. What lobes are involved in paying attention? All but temportal Damage to the frontal lobe creates a loss of consciousness and predictability. Dorsal lateral prefrontal cortex EF Ventromedial prefrontal cortex theory of mind Arculate fasciculus / when it’s developed good readers, K3 grade Broca’s area speech production Premotor cortex? Motor homunculus? Central fissure? Pre is planning, motor is executing, and central fissure separates motor and parietal lobe Difference in cats/rats and humans? Sensory and motor are different areas in humans What else is the motor cortex involved with that helps control muscle movement? Cerebellum, basal ganglia, and thalamus Where in the body are fibers crossed? Uncrossed? Uncrossed from neck up, crossed from neck down What area does the parietal lobe use for integration? Association cortex Where does the sense of touch derive from in the brain? How are the neurons numerically represented? Somatosensory cortex in the parietal lobe, neurons are related to sensitivity or amount of muscle control Upper parietal involved in? Right parietal? Lower portion? Spatial orientation, awareness/sensory neglect, visual and auditory integration Posterior parietal lobe Damage to the parietal lobe can cause alexia, agraphia, Artists have more grey matter in what three parts of the brain? Motor cortex, thalamus, What are the two main cortex in the temporal lobe? Primary auditory cortex and secondary auditory cortex What is the secondary cortex referred to as? Damage to this area causes what? Wernicke’s area, production of nonmeaningful words, inability to read and understand language What is the hippocampus involved in? creating new neurons Three disruptions that stem from KluverBucy syndrome? Socially innapropriate talk, hypo or hyper sexuality, and dominance hierarchy changes Two cortex of the occipital lobe? Damage does what? Striate cortex is and extrastriate cortex The limbic system regulates the four F’s of motivation feeding, fighting, fleeing, fucking Limbic system is also called the emotional regulation cortex Cingulate cortex pain and visceral responses Cerebellum fine motor movement and timing Uniqueness of human consciousness results from neocortex and fibers interacting with thalamus. CHAPTER 3 ANATOMY OF THE NERVOUS SYSTEM 3.1 DIVISIONS OF THE NERVOUS SYSTEM The vertebrate nervous system is composed of two divisions: the CNS and the PNS 1. The central nervous system (CNS) is located within the skull and spine. It is composed of two divisions: the brain and the spinal cord. 2. The peripheral nervous system (PNS) is the division located outside the skull and spine. It is composed of two divisions: the somatic and autonomic nervous system. a. The somatic nervous system (SNS) is the part that interacts with external environment. It uses afferent nerves that carry sensory signal to the CNS and efferent nerves that carry motor signals from the CNS to the muscles. b. The autonomic nervous system (ANS) is the part that regulates the body’s internal environment. It uses afferent and efferent nerves in the same manner, but with internal organs. The ANS has two types of efferent nerves: i. Sympathetic nerves project from the CNS in the lumbar and thoracic regions of the spinal cord. The sympathetic nerves are a twostage neural path, going only part of the way to the organ before synapsing on other neurons at a far distance from the target organ. ii. Parasympathetic nerves project from the brain and sacral region of the spinal cord. Parasympathetic nerves also use a twostage neural path, but they synapse much close to their target organ. **Appendix I Three important principles in the functions of sympathetic and parasympathetic systems. 1. Sympathetic nerves stimulate, organize, and mobilize energy in threatening situations. Parasympathetic nerves conserve energy. 2. Each autonomic target receives opposing sympathetic and parasympathetic input and its activity is controlled by the levels of each activity. 3. Sympathetic changes indicate psychological arousal and parasympathetic indicate relaxation. ** Appendix II Most nerves of the peripheral system project from the spinal cord, but there are 12 exceptions. These 12 pairs of cranial nerves project from the brain and include purely sensory nerves. The functions are very specific so we can use them to locate tumors and other brain pathology. **Appendix IV The brain and spinal cord are the most protected organs in the body. The are covered by three protective members, or three meninges. The out meninx is the dura mater. Immediately inside is the fine, arachnoid membrane. Beneath the arachnoid membrane is the subarachnoid space which contains many large blood vessels and cerebrospinal fluid and then the delicate pia mater. The cerebrospinal fluid (CSF) fills the subarachnoid space. The central canal is a small central channel that runs the length of the spinal cord. The cerebral ventricles are the four large internal chambers of the brain. The sub. space, central canal, and ventricles are interconnected and form a single reservoir. Cerebrospinal fluid cushions the brain. It is continuously produce by the choroid plexuses (networks of capillaries) that protrude into the ventricles from the pia mater. Excess fluid is absorbed from subarachnoid into dural sinuses then into large jugular veins in the neck. The bloodbrain barrier impedes the passage of many toxic substances from the blood into the brain. How? The cells of the blood vessel walls, unlike everywhere else in the body, cells are tightly packed, forming a barrier. 3.2 CELLS OF THE NERVOUS SYSTEM There are two types of cells in the nervous system: neurons and glial cells. Neurons are cells specialized for the reception, conduction, and transmission of electrochemical signals. They vary in shape and size. Pg. 55 / Figure 3.5 know the anatomy of a neuron. The neuron cell membrane is composed of a lipid bilayer, or two layers of fat molecules. Neurons are classified by the number of processes (projections) protruding from their cell bodies. Multipolar neurons have more than two, unipolar neurons have one, and bipolar neurons have two. Neurons with short axons or none at all are called interneurons. There are two kinds of gross neural structures in the nervous system those made up of cell bodies and those made up of axons. In the CNS, a cluster of cell bodies are called nuclei; in the peripheral nervous system they are called ganglia. In the CNS, bundles of axons are called tracts; in the peripheral nervous system, they are called nerves. There are also glial cells that make up the nervous system. Several types of glial cells are below: 1. Oligodendrocytes have extensions that wrap around the axons of some neurons in the CNS. The extensions are rich in myelin, a fatting insulating substance, and the myelin sheaths that they form increase the speed and efficacy of axonal conduction. 2. Schwann cells have a similar function in the PNS. Schwann cells only have one myelin segment though, while oli. has several. Another difference is that Schwann cells can guide axonal regeneration after damage, and oli. can’t. 3. Microglia are smaller than other glia and respond to injury/disease. 4. Astrocytes are the largest glial cells and they play a role in allowing the passage of some chemicals from the blood into CNS neurons and in blocking others. 3.3 NEUROANATOMICAL TECHNIQUES AND DIRECTIONS Neurons are tightly packed and intricately intertwined, making them hard to visualize. They key to studying them is to prepare neural tissue in a variety of ways that permits a clear view of a different aspect of neuronal structure. Golgi stains made it possible to see individual neurons (shadows) and are used when the overall shape of neurons is of interest. Nissl stains are used to estimate the number of cell bodies in an area by using the dye cresyl violet. Electron microscopy provides information about the details of neuronal structure. The nature of light makes it impossible to reveal fine anatomic details of neurons, so to see greater detail we obtain an electron micrograph. A scanning electron microscope shows electron micrographs in three dimensions but doesn’t have a strong magnification. There are two types of neuroanatomical tracing techniques: anterograde (forward) tracing methods and retrograde (backward) tracing methods. Anterograde methods are used when we want to trace the paths of axons projecting away from cells bodies. Retrograde methods are used to trace the paths of axons projecting into an area. Directions in the vertebrate nervous system are described in relation to the orientation of the spinal cord. It has three axes: anteriorposterior, dorsalventral, and mediallateral. Anterior means toward the nose end / posterior means toward the tail end Dorsal means toward the surface or the top of the head / ventral means toward the surface of the chest or the bottom of the head Medial means toward the midline of the body / lateral means away from the midline towards the surface Some more directional terms Superior and inferior refer to the top and bottom of the primate head, respectively Proximal means closer to the CNS and distal means farther from the CNS. We can also slice the brain in three different planes: horizontal sections, frontal sections, and sagittal sections. A section cut down the center of the brain, between two hemispheres, is called a midsagittal section. A section cut at a right angle to any long, narrow structure like the spinal cord or a nerve, is a cross section. 3.4 SPINAL CORD The spinal cord is comprised of two different areas: an inner Hshaped core of gray matter and a surrounding area of white matter. Gray matter is composed of cell bodies and unmyelinated interneurons, whereas white matter is composed largely of myelinated axons. The two dorsal arms of the gray matter are called dorsal horns and the two ventral arms are called the ventral horns. All dorsal root axons are sensory (afferent) unipolar neurons, grouping together cell bodies to form the dorsal root ganglia. In contrast, the neurons of the ventral root are motor (efferent) multipolar neurons with their cell bodies in the ventral horns. 3.5 FIVE MAJOR DIVISIONS OF THE BRAIN In the vertebrate embryo, the tissue that develops into the CNS is recognizable as a fluidfilled tube. The indications of a brain are three swellings at the end of this tube. The three swellings develop into the forebrain, midbrain, and hindbrain. Before birth, these three become five. The forebrain swelling grows into the telencephalon (cerebral hemispheres) and diencephalon. The midbrain is referred to as the mesencephalon. The hindbrain swelling grows into the metencephalon and the myelencephalon. Memorize the order by remembering that telencephalon is on the top and the other four are in alphabetical order. Figure 3.19 / Pg. 64 The telencephalon undergoes the greatest growth during development. The other four are referred to collectively as the brain stem the stem on which the cerebral hemispheres sit. The myelencephalon is called the medulla. 3.6 MAJOR STRUCTURES OF THE BRAIN The myelencephalon/medulla is the most posterior division of the brain and is composed of tracts carrying signals between the rest of the brain and the body. It has a reticular formation, a complex network of nuclei. The metencephalon has many tracts and is a part of the reticular formation. The pons is on the brain stem’s ventral surface; the other division of the metencephalon is the cerebellum, a large convoluted structure of the brain stem’s dorsal surface. It is an important sensorimotor structure. The mesencephalon has two divisions: the tectum and the tegmentum. In mammals, the tectum is composed of two pairs of bumps called colliculi. The posterior pair, called the inferior colliculi are auditory. The anterior paid called the superior colliculi are visual. The tegmentum is ventral to the tectum. It is composed of the periaqueductal gray, the substantia nigra, and the red nucleus. The periaqueductal gray is the gray matter around the cerebral aqueduct, which connects the third and fourth ventricles. The substantia nigra (black substance) and the red nucleus are important in the sensorimotor system. The diencephalon is composed of the thalamus and the hypothalamus. The thalamus is a large, twolobed structure that is the top of the brain stem. The lobes are joined by the massa intermedia. The most well understood thalamic nuclei are the sensory relay nuclei that receive signals from sensory receptors, process them, and then transmit them to sensory cortex areas. The hypothalamus is located below the anterior thalamus. It is important in regulating motivated behaviors like eating and sleeping by releasing hormones from the pituitary gland. The optic chiasm and the mammillary bodies appear on the inferior surface of the thalamus as well. The optic chiasm is where the optic nerves from each eye cross. The mammillary bodies are a pair of spherical nuclei behind the pituitary gland. Figure 1 thalamus, figure 2 hypothalamus. The telencephalon is the largest division of the brain and mediates the most complex functions. The cerebral hemispheres are covered by a layer of tissue called the cerebral cortex. The convolutions in the cerebral cortex can increase the amount of matter without increasing the overall volume of the brain. The large furrows in a convoluted cortex are fissures and the small ones are sulci. The ridges are called gyri. The hemispheres are completed by the largest of fissures: the longitudinal fissure. The hemispheres are connected by a few tracts (cerebral commissures) spanning the longitudinal fissure, these hemisphereconnecting tracts are called cerebral commissures. The largest cerebral commissure is the corpus callosum. Two important landmarks on the lateral surface of the brain are the central fissure and the lateral fissure. These fissures divide each hemisphere into four lobes: the frontal, parietal, temporal, and occipital lobe. Occipital lobe visual input Parietal lobe sensations from the body / perceiving the location of both objects and our own bodies and in direction our attention Temporal lobe hearing and language / complex visual patterns / certain kinds of memory Frontal lobe motor functions / complex cognitive functions About 90% of human cerebral cortex is neocortex, a sixlayered cortex of recent evolution. There are three important characteristics of neocortical anatomy: 1. Cortical neurons are either pyramidal cells (large multipolar neurons with pyramid shaped cell bodies, large dendrites, and long axon) or stellate cells (small starshaped interneurons with a short or no axon). 2. The six layers differ from one another in terms of size and density of cell bodies and proportion of pyramidal to stellate cell bodies they have. 3. Long axons move vertically, creating a columnar organization, neurons in a vertical column that often form a minicircuit that performs a single function. The hippocampus is an important area that isn’t part of the neocortex. It looks like a sea horse, hence the name hippocampus. It is involved in memory, particularly memory for spatial location. Aside from the neurons in the telencephalon that are projecting to and from the neocortex, there are large groups of neurons considered to be a part of either the limbic system or the basal ganglia system. 1. The limbic system is a circuit of midline structures that circle the thalamus. It is involved in the regulation of motivated behaviors (the four F’s) fleeting, feeding, fighting, and sexual behavior. a. Major structures mammillary bodies, hippocampus, amygdala (involved with emotion, esp. fear), the fornix, the cingulate cortex, and the septum. 2. The basal ganglia play a role in the performance of voluntary motor responses. Parkinson’s disease deals with this group of structures a. Major structures amygdala, caudate, putamen, striatum, globus pallidus CHAPTER 4 – NEURAL CONDUCTION AND SYNAPTIC TRANSMISSION 4.1 RESTING MEMBRANE POTENTIAL The membrane potential is the difference in electrical charge between the inside and the outside of a cell. To record the membrane potential, you position the tip of an electrode inside the neuron and the tip of another outside the neuron in the extracellular fluid. The very thin intracellular electrodes are called microelectrodes. When both electrodes are outside the cell, the membrane potential is 0. The steady membrane potential of about 70mV is called the resting potential. In it’s resting state, the neuron is said to be polarized. Why are resting neurons polarized? The salts in the neural tissue separate into positively and negatively charged particles called ions. The resting potential results from the uneven ratio of negative to positive charges between the inside and outside of the neuron. The unequal distribution of charges can be explained by four interactions (two homogenizing factors and two features that counteract these effects): Random motion ions are in constant random motion and tend to become evenly distributed because they move down concentration gradients (going from high areas of concentration to low) Electrostatic pressure an accumulation of charges in one area tends to be dispersed by the repulsion among like charges and the attraction of opposite charges concentrated elsewhere Differential permeability (passive) K+ and CL ions pass readily through the membrane, NA+ ions have difficulty, and negatively charged proteins do not pass at all. o Ions pass through the neural membrane at specialized pores called ion channels. Active mechanisms K+ ions are always being driven out and Na+ are being driven in. The active mechanisms in the cell membrane counteract the influx of Na+ ions by pumping them out and they counteract the efflux of K+ ions by pumping K+ in as rapidly as they pass out. o Sodiumpotassium pumps exchange three Na+ ions inside the neuron for two K+ ions outside. Other transporters (mechanisms in the membrane that actively transport ions across the membrane) also occur. 4.2 GENERATION AND CONDUCTION OF POSTSYNAPTIC POTENTIALS When neurotransmitter molecules bind to postsynaptic receptors, they typically have one of two effects. Both are graded responses, meaning the amplitudes of them are proportional to the intensity of the signal that elicits them (weak signals elicit small potentials and strong signals elicit large ones). Excitatory postsynaptic potentials (EPSPs) will depolarize the receptive membrane, decreasing the resting potential and increase the likelihood that the neuron will fire. Inhibitory postsynaptic potentials (IPSPs) will hyperpolarize the membrane, increasing the membrane potential and decreasing the likelihood that the neuron will fire. EPSPs and IPSPs have two characteristics. First, they are rapid (almost instantaneous). Second, they are decremental, meaning they decrease in amplitude as they travel through the neuron, like a sound wave grows fainter, thus they never travel far along the axon. 4.3 INTEGRATION OF POSTSYNAPTIC POTENTIALS AND GENERATION OF ACTION POTENTIALS If the sum of the depolarizations and hyperpolarizations adjacent to the axon hillock (structure at the junction between the cell body and the axon) is sufficient to depolarize the membrane referred to as it threshold of excitation, an action potential is generated near the axon hillock. The action potential (AP) is the reversal of the membrane potential from about 70 to +50mV. Unlike graded responses, their magnitude is not related to the intensity. Instead, they are allor none responses; meaning they either occur to their full extent or not at all. Multipolar neurons add together all graded (excitatory and inhibitory) potentials and decides to fire or not on the basis of their sum. Adding a number of individual signs into one overall sign is called integration, and neurons integrate signals in two ways (over space and over time). Space/spatial summation Spatial summation occurs when local EPSPs/IPSPs are summed to form greater EPSPs/IPSPs, or when local EPSPs and IPSPs are combined to cancel each other out. Time/temporal summation when postsynaptic potentials produced in rapid succession at the same synapse are added together to form a great signal, that is temporal summation. 4.4 CONDUCTION OF ACTION POTENTIALS Ionic basis of action potentials Action potentials are produced and conducted along the axon through the action of voltage activated ion channels, ion channels that open or close in response to changes in the level of membrane potential. Things suddenly change when the membrane potential of the axon is reduced to the threshold of excitation. 1. The voltageactivated sodium channels open and Na+ ions rush in. 2. The change in potential is associated with the influx of Na+ ions and then triggers the opening of voltageactivated potassium channels. 3. K+ ions are driven out of the cell through these channels, and then the sodium channels close. 4. This marks the end of the rising phase and begins the repolarization by the continued efflux of K+ ions. 5. Once repolarization is achieved the potassium channels gradually close. Because they close gradually, too many K+ ions leave and it is left hyperpolarized for a brief period of time. Refractory periods There is a brief 12 milliseconds after initiation of an action potential during which it is impossible to elicit a second one. This period is called the absolute refractory period. It is followed by the relative refractory period, during which it is possible to fire the neuron again but with higherthannormal levels of stimulation. The end of the relative period is when the amount of stimulation necessary to fire a neuron returns to its baseline. The refractory period is responsible for two important characteristics of neural activity. The fact that action potentials travel along axons in only one direction. An action potential cannot reverse direction. The fact that the rate of neural firing is related to the intensity of the stimulation. Axonal conduction of action potentials Action potentials differ from the conduction of EPSPs and IPSPs in two ways. Action potentials are nondecremental (they don’t grow weaker as they travel along the axonal membrane) They are conducted more slowly than postsynaptic potentials. The reason for these differences is that unlike EPSPs and IPSPs, the conduction is largely active. Once an action potential is generated, it travels along the membrane and opens the voltage activated sodium channels, allowing another action potential to be generated. This is repeated until a fullblown action potential is triggered in all the terminal buttons. (Remember the mouse trap set up example, pg. 83) An action potential can travel in both directions. Either from the terminal end to the axon (antidromic conduction), or in the natural direction from cell body to terminal buttons (orthodromic conduction). Conduction in myelinated axons Ions can pass through the axonal membrane only at the nodes of Ranvier, the gaps between adjacent myelin segments. When an action potential is generated in a myelinated axon, the signal is conducted passively (instantly and decrementally). The signal diminishes when it reaches the node, but it is strong enough to open the VA sodium channels to generate another action potential. This is repeated. Myelination increases the speed of axonal conduction. Because the conduction is passive, it occurs instantly and the signal thus “jumps” along the axon from node to node. This transmission of action potentials in myelinated axons is called saltatory conduction (saltare means to skip or jump). The velocity of axonal conduction The speed of action potentials depends on two properties of an axon: Conduction is faster in largediameter axons It is faster in axons that are myelinated. Mammalian motor neurons are large and myelinated and conduct at speeds of 100 meters per second. Conduction in neurons without axons Action potentials are the means by which axons conduct all or none signals nondecrementally over long distances. Many neurons in mammalian brains, however, do not have axons are have very short ones, and many of these neurons do not display action potentials. These interneurons are typically passive and decremental. The HodgkinHuxley model in perspective The HodgkinHuxley model is presented as a factual account of neural conduction and its mechanisms, rather than as a theory. It provides a simple introduction to the general ways in which neurons conduct signals. The problem is that it does not represent the variety, complexity, and plasticity of the neurons in the mammalian brain. The model was based on the study of squid motor neurons, making it difficult to apply the model directly to the mammalian brain, especially cerebral neurons. Here are some properties of cerebral neurons that are not shared by motor neurons: Cerebral neurons fire continually even when they receive no input Axons can actively conduct graded signals and action potentials Action potentials of all motor neurons are the same, but they differ for cerebral neurons Cerebral neurons have no axons and do not display action potentials The dendrites of some cerebral neurons can actively conduct action potentials 4.5 SYNAPTIC TRANSMISSION: CHEMICAL TRANSMISSION OF SIGNALS AMONG NEURONS Now we will look at how action potentials arriving at terminal buttons trigger the release of neurotransmitters into synapses and how neurotransmitters carry signals to other cells. Structure of synapses There are many types of synapses, like axodendritic synapses (synapses of axon terminal buttons on dendrites), or axosomatic synapses (synapses of axon terminal buttons on somas). These synapses terminate on dendritic spines (nodules of various shapes that are located on the surfaces of many dendrites). Synapses can either be directed or nondirected. Directed synapses are synapses at which the site of neurotransmitter release and the site of neurotransmitter reception are in close proximity. Nondirected synapses have distance between the release and reception. Synthesis, packaging, and transport of neurotransmitter molecules There are two basic categories of neurotransmitter molecules: small and large. There are several types of small neurotransmitters. Small neurotransmitters are typically synthesized in the cytoplasm of the terminal button and packaged in synaptic vesicles by the golgi complex. Once filled with neurotransmitter, the vesicles are stored in clusters next to the presynaptic membrane. All large neurotransmitters are neuropeptides, short proteins. Large neurotransmitters, neuropeptides, are assembled in the cytoplasm of the cell body on ribosomes, then packaged in vesicles by the cell body’s golgi complex and transported by microtubules to the terminal buttons. The vesicles that hold neuropeptides are usually larger and more distributed from the presynaptic membrane than smallmolecule neurotransmitters. Many neurons contain two neurotransmitters (coexistence); most cases of coexistence involve one smallmolecule neurotransmitter and one neuropeptide. Release of neurotransmitter molecules Exocytosis is the process of neurotransmitter release. Vesicles that contain smallmolecule neurotransmitters tend to congregate around the presynaptic membrane, areas particularly rich in voltageactivated calcium channels. When stimulated by action potentials, the channels open and Ca2+ ions enter the button, causing the vesicles to fuse with the presynaptic membrane and empty their contents into the synaptic cleft. In most cases, one action potential causes the release from one vesicle. Small neurotransmitters are released in a pulse each time an action potential triggers a momentary influx of Ca2+ ions, but neuropeptides are released gradually in response to general increases in the level of intracellular Ca2+ ions. Activation of receptors by neurotransmitter molecules Once released, neurotransmitter molecules produce signals in postsynaptic neurons by binding to receptors in the postsynaptic membrane. Each receptor is a protein that contains binding sites for only particular neurotransmitters. Any molecule that binds to another is referred to as its ligand, thus a neurotransmitter is said to be a ligand of its receptor. Most neurotransmitters bind to several different types of receptors. These different types are called receptor subtypes for that neurotransmitter. One advantage of these subtypes is that it allows a neurotransmitter to transmit different kinds of messages to different parts of the brain. The binding of a neurotransmitter to one of its receptor subtypes can influence a postsynaptic neuron in two ways, depending on whether the receptor is ionotropic or metabotropic. Ionotropic receptors are associated with ligandactivated ion channels. When neurotransmitter molecules bind to an ionotropic receptor, the associated ion channels opens or closes immediately, inducing an immediate postsynaptic potential. Metabotropic receptors are associated with signal proteins and G proteins (guanosine triphosphatesensitive proteins). These are more prevalent and their effects are slower to develop, longerlasting, more diffuse, and more varied. There are many kinds but each is attached to a serpentine signal protein that winds back and forth through the cell. The receptor is attached to a portion of the signal protein outside the neuron; the G protein is attached to a portion of the signal protein inside the neuron. o When a neurotransmitter binds to a metabotropic receptor, a subunit of the associated G protein breaks away. Then, one of two things can happen. The subunit may move along the inside surface of the membrane and bind to a nearby ion channel, inducing an EPSP or IPSP It may trigger the synthesis of a chemical called a second messenger (neurotransmitters are considered to be the first). Once created, it will diffuse through the cytoplasm and may influence the activities of a neuron in a variety of ways. o One type of metabotropic receptors warrants special mention autoreceptors. They have two unconventional characteristics: they bind to their neuron’s own neurotransmitter molecules and they are located on the presynaptic, rather than postsynaptic, membrane. Their usual function is to monitor the number of neurotransmitter molecules in the synapse and either reducing or increase the releases depending on their levels. Differences between small and large (neuropeptide) neurotransmitters suggest they serve different functions. Smallmolecule transmitters tend to be released into directed synapses and active ionotropic/metabotropic receptors that act directly on channels. Their function seems to be the transmission of rapid, brief excitatory/inhibitory signals to adjacent cells. Neuropeptides, or largemolecule transmitters, virtually all bind to metabotropic receptors that act through second messengers. Their function seems to be the transmission of slow, diffuse, longlasting signals. Reuptake, enzymatic degradation, and recycling To keep a neurotransmitter molecular from remaining active in the synapse/clogging the channel of communication, two mechanisms exist to terminate synaptic messages. Reuptake involves the neurotransmitters almost immediately being drawn back into the presynaptic buttons. This is the more common of the two. Enzymatic degradation involves breaking apart the neurotransmitters in the synapse by the use of enzymes (proteins that stimulate/inhibit biochemical reactions without being affected by them). Terminal buttons are very efficient. When neurotransmitter molecules or their breakdown products are released, they are drawn back into the button and recycled. Even the vesicles are drawn back into the neuron and are used to create new vesicles. Glial function and synaptic transmission Gap junctions are narrow spaces between adjacent neurons that are bridged by fine tubular channels, called connexins, that contain cytoplasm. The cytoplasm is continuous, allowing electrical signals and small molecules to pass from one neuron to the next. Although they are less selective than synapses, gap junctions have to advantages. Communication is very fast because it doesn’t involve active mechanisms. They permit communication in either direction. 4.6 NEUROTRANSMITTERS There are three classes of conventional smallmolecule neurotransmitters: amino acids, monoamines, and acetylcholine. The fourth group of various transmitters is referred to as unconventional neurotransmitters for their unusual mechanisms of actions. In contrast, there is only one class of largemolecule transmitters: the neuropeptides. Neurotransmitters produce either excitation or inhabitation, but not both. The neurotransmitters in the majority of fastacting, directed synapses in the CNS are amino acid neurotransmitters building blocks of proteins. The four most widely studied are glutamate (most prevalent excitatory neurotransmitter), aspartate, glycine, and gamma aminobutyric acid (GABA). The first three are common in proteins we consume, but GABA is synthesized by a simple modification of the structure of glutamate. GABA is the most prevalent inhibitory neurotransmitter. Monoamines are synthesized from a single amino acid, hence the name monoamine. Monoamine neurotransmitters are slightly larger than amino acid neurotransmitters, and their effects tend to be more diffuse. They are released into the extracellular fluid. There are four monoamine neurotransmitters: dopamine, epinephrine, norepinephrine, and serotonin. Dopamine, norepinephrine, and epinephrine are catecholamines are are synthesized from the amino acid tyrosine. Serotonin is synthesized from the amino acid tryptophan and is classified as an indolamine. Neurons that release norepinephrine are called noradrenergic and those that release epinephrine are called adrenergic. Acetylcholine is created by adding an acetyl group to a choline molecule. It is the neurotransmitter at neuromuscular junctions, at many synapses in the ANS, and at synapses in several parts of the CNS. Acetylcholine is broken down in the synapse by the enzyme acetylcholinesterase. Neurons that release acetylcholine are called cholinergic. Unconventional neurotransmitters act in ways that are different from others. One class is the solublegas neurotransmitters, which includes nitric oxide and carbon monoxide. They are produced in the neural cytoplasm and immediately diffused through the cell membrane. Soluble gas neurotransmitters have been shown to be involved in retrograde transmission, which seems to regulate the activity of presynaptic neurons by transmitting signals from the postsynaptic neuron back to the presynaptic neuron. Another class are the endocannabinoids, neurotransmitters that are similar to the main psychoactive constituent of marijuana. The most widely studied of the two endocannabinoids is anandamide (translates to eternal bliss). Like the solublegas neurotransmitters they are produced immediately before released. Neuropeptide transmitters and their actions depend on its amino acid sequence. It is usual to loosely group neuropeptide transmitters into five categories: The pituitary peptides contain neuropeptides that were identified as hormones released by the pituitary gland The hypothalamic peptides contain neuropeptides that were identified as hormones released by the hypothalamus Braingut peptides contain neuropeptides that were discovered in the gut. The opioid peptides contain neuropeptides similar in structure to the active ingredients in opium. Miscellaneous peptides is the catchall category that contains all neuropeptide transmitters that don’t fit into one of the other four categories. Drugs can either facilitate synaptic transmission/neurotransmitters (agonists) or inhibit the effects of neurotransmitters (antagonists).
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