Organization and Cells of the Nervous System PDF

Clinical Science I CCPA 5031 Neuroscience Organization and Cells of the Nervous System Edwin E. Nyambi, DMSc, MPAS, PA-C [email protected] Instructional Objectives Outline the major anatomic components of the central nervous system (CNS) and peripheral nervous system (PNS). Diagram the functional areas of the brain and spinal cord. Describe the types of nerves and ganglia in the PNS and their structure. Distinguish the different categories of neurons. Identify the neuronal organelles and their biochemical functions. Diagram the specialized neuronal processes and their functions. Neuroscience Introduction Overview: The Nervous system The nervous system mediates a wide range of functions, from detection of environmental stimuli, to control of muscle contraction, to problem solving, language, and memory. The nervous system is divided into 2 main parts, the central nervous system (CNS) and the peripheral nervous system (PNS). Anatomically, the CNS is divided into the brain and spinal cord, whereas the PNS is composed of ganglia and nerves, including cranial and spinal nerves and their branches. Functionally, the PNS can be divided into the somatic, autonomic (visceral), and enteric nervous systems. In the overall ﬂow of information, the PNS detects and relays sensory information about the external and internal environments to the CNS. The CNS receives, integrates, and stores information and controls the output to the PNS to generate responses and behavior. Neuroscience Overview: The Nervous system At the cellular level, the nervous system is composed of neurons and glial cells. Neurons (also called nerve cells or neuronal cells) are the main signaling cells that communicate with other neurons, muscles, or glands. Neurons can be categorized by their function as sensory neurons, motor neurons, or interneurons or by their morphology or neurotransmitter. Glial cells (also called neuroglia or glia) are the support cells in the nervous system. Astrocytes and satellite cells provide structural and metabolic support, oligodendrocytes and Schwann cells furnish myelination of axons, pericytes regulate capillaries, ependymal cells synthesize cerebrospinal ﬂuid (CSF), microglia are immune cells, and enteric glia are part of the gastrointestinal (GI) tract. Neurons have a cell body where the nucleus and majority of cellular organelles are located and many biochemical activities occur. Neuroscience Overview: The Nervous system Neurons also contain specialized processes and regions that allow them to send and receive signals rapidly and precisely. The axon is a process by which electrical signals are conducted and where signals are sent to other neurons or target cells. The dendrites are branched processes that receive signals from other neurons. The synapse is a structure where signals are transmitted, via synaptic transmission, from the axon to its target. Neurons can receive thousands of synaptic inputs, can form neural circuits, and function in networks that underlie sensations, cognitive functions, and the generation of responses and behavior. Neuroscience CNS Components, Coverings & Vasculature The brain and spinal cord compose the CNS. The brain is enclosed within the cranial cavity and is protected by the cranium (skull) and meninges. The meninges are a 3-membrane system that covers, protects, and nourishes the brain and spinal cord. The outer dura mater is a thick tough membrane that is connected to the cranium and protects the brain. The middle membrane, called the arachnoid mater, cushions the brain. Below the arachnoid is the subarachnoid space, which contains CSF and where specialized regions called arachnoid granulations resorb CSF. The innermost layer, the pia mater, is a thin layer that adheres to the surface of the brain and follows its contours, forming a barrier but with many capillaries that nourish the brain and spinal cord. Neuroscience CNS Components, Coverings & Vasculature The brain has 3 main regions: the forebrain, brainstem, and cerebellum. The inner spaces of these regions form the ventricles, which produce and circulate CSF and are connected to form the ventricular system. Both the meninges and brain are highly vascularized. Two main pairs of arteries supply blood to the brain. The internal carotid arteries, which are branches from the common carotid artery, supply the anterior brain, whereas the vertebral arteries, which are branches from the subclavian artery, supply the posterior brain and brainstem. The main venous blood outﬂow from the brain is via the jugular veins. Neuroscience Basic Brain Anatomy The forebrain is the largest part of the brain and contains the cerebrum and diencephalon. The cerebrum is formed by the large left and right cerebral hemispheres, which are separated by the medial longitudinal ﬁssure, and contains the outer cerebral cortex, inner cerebral white matter, and subcortical nuclei. The cerebrum encloses the lateral ventricles and overlies the diencephalon, a structure that contains the thalamus and hypothalamus and that surrounds the third ventricle. Functionally, the forebrain is involved in receiving sensory information from the PNS and generating outgoing motor information and is where executive and cognitive functions are generated. Neuroscience Basic Brain Anatomy The oldest part of the brain, the brainstem, is composed of the midbrain, pons, and medulla and serves to relay information from the spinal cord and cerebellum to the forebrain and vice versa. In addition, the brainstem regulates vital functions, such as breathing, consciousness, and control of body temperature. The cerebral aqueduct and fourth ventricle lie inside the brainstem. Connected to the pons, the cerebellum forms the posterior-most region of the brain and is involved in control and coordination of movement and some cognitive tasks. Neuroscience Basic Brain Anatomy Examination of postmortem ﬁxed brain tissue reveals that each of these brain regions contains gray and white matter areas. Gray Matter In living tissue, gray matter appears pinkish light brown. Gray matter contains mainly neuronal cell bodies, their dendrites, and associated glial cells. In the brain, 2 types of gray matter are present #. Cortical gray matter forms the outer regions of the cerebrum and cerebellum and is distinguished by its layered organization of neurons. #. The other type of gray matter is called a nucleus, an aggregate of cell bodies with similar morphology and function found below the cortex (subcortical nuclei) and in the brainstem and cerebellum. Neuroscience Basic Brain Anatomy White Matter White matter contains predominantly myelinated axons (which, because of their fatty rich myelin membrane, produce the white appearance) and white matter glial cells. White matter contains bundles of myelinated axons that are referred to as tracts in the CNS. In the brain, the white matter tracts include projection tracts that connect neurons in the forebrain to neurons in the brainstem or spinal cord, association tracts that connect one cortical region to another, and commissural tracts, which connect areas from one side of the brain to the other. Neuroscience Basic Brain Anatomy The exterior surface of the cerebrum is distinguished by many gyri (singular: gyrus) and sulci (singular: sulcus) that produce the characteristic folded appearance of the human and many mammalian brains. A sulcus is a groove or furrow in the cerebral cortex, whereas a gyrus is a crest or ridge. The folding created by gyri and sulci facilitates a larger surface area of cerebral cortex to ﬁt inside the skull. Deep sulci separate the cortex into 4 cortical lobes on each side, called the frontal, parietal, temporal, and occipital lobes, named for the cranial bones that overlie each. Neuroscience Basic Brain Anatomy The central sulcus forms the division between the frontal and parietal lobe. The lateral sulcus (also called the Sylvian ﬁssure) separates the temporal lobe from the frontal and parietal lobes. The parieto-occipital sulcus forms the boundary between the parietal and occipital lobes. Many additional sulci and gyri are present in the cerebral cortex, with all cortical gyri and sulci containing an outer layer of cortical gray matter and a thin layer of underlying white matter. Each of the 4 main lobes contains distinct anatomic and functional areas. Complete Anatomy Neuroscience Basic Spinal Cord Anatomy The spinal cord emerges caudally from the brainstem, within the spinal canal, and is protected by the vertebral column (also called the spine) and meninges. Along their length, the vertebral column and spinal cord inside are separated into 5 regions, called cervical, thoracic, lumbar, sacral, and coccygeal segments. Similar to the brain, the spinal cord is composed of gray and white matter regions but with an opposite organization. Spinal gray matter, composed of neuronal cell bodies, dendrites, and glia, is located medially and is surrounded by spinal white matter, which contains tracts and glia, located in the lateral areas of the spinal cord. The gray matter surrounds the inner central canal, which provides CSF to the spinal cord. Spinal gray matter is separated anatomically and functionally into dorsal (posterior) and ventral (anterior) horns on each side. Sensory information is carried by afferent axons of spinal nerves, which enter the cord via the dorsal roots. Neuroscience Basic Spinal Cord Anatomy These sensory axons branch, and one branch can synapse on interneurons in the dorsal horn, whereas the other branch can ascend to the brain. These axons form the ascending tracts in the spinal cord. Descending tracts in the white matter provide outgoing motor information from the cerebrum or brainstem. The axons in the descending tracts synapse on motor neuron cell bodies in the ventral/anterior horns. The ventral horn motor neurons extend their axons out of the cord via the ventral root, and their axons form the motor components of the spinal nerves. Because the lower motor neurons cell bodies lie in the spinal cord, while their axons form the motor components of the spinal nerves, they are considered part of both the CNS and PNS. Neuroscience Peripheral Nervous System (PNS): Functional Divisions The PNS has 3 functional divisions: the somatic, autonomic (also called visceral), and enteric nervous systems. Somatic Nervous System (SNS) The somatic nervous system mediates conscious/voluntary movement via regulation of skeletal muscle contraction and provides sensory information from the skin, muscles, and joints. Neuroscience Peripheral Nervous System (PNS): Functional Divisions The PNS has 3 functional divisions: the somatic, autonomic (also called visceral), and enteric nervous systems. Autonomic Nervous System (ANS) The autonomic nervous system involves unconscious/involuntary control of cardiac muscle, smooth muscle, and glands. Its sensory component, often called the visceral sensory system, provides information from the viscera, the internal organs, and vasculature. The autonomic motor system is divided into the sympathetic and parasympathetic nervous systems. Neuroscience Peripheral Nervous System (PNS): Functional Divisions The PNS has 3 functional divisions: the somatic, autonomic (also called visceral), and enteric nervous systems. Autonomic Nervous System (ANS) A. Sympathetic Nervous System The sympathetic nervous system, referred to as the “ﬁght or ﬂight” system, is activated under conditions requiring mobilization of energy. B. Parasympathetic Nervous System The parasympathetic nervous system, referred to as the “rest and digest” or “feed and breed” system, is activated when organisms are in a relaxed state. Neuroscience Peripheral Nervous System (PNS): Functional Divisions The PNS has 3 functional divisions: the somatic, autonomic (also called visceral), and enteric nervous systems. Enteric Nervous system (ENS) The enteric nervous system, which is also under involuntary control, governs the gastrointestinal system. Although it receives considerable input from the autonomic nervous system, the enteric nervous system can function independently and is considered a separate system in the PNS. Neuroscience Peripheral Nervous System (PNS): Anatomic Components Anatomically, the PNS is composed of ganglia and nerves. Ganglia are clusters of functionally related neuronal cell bodies and their accompanying glial cells in the PNS. Cell bodies of somatic sensory neurons form the dorsal root ganglia, whereas cell bodies of autonomic neurons form the sympathetic and parasympathetic ganglia. Neuroscience Peripheral Nervous System (PNS): Anatomic Components The sympathetic ganglia lie outside of but close to the spinal cord and communicate to form the sympathetic chain. The parasympathetic ganglia in the body lie close to the organ they innervate. The cranial ganglia contain parasympathetic or sensory cell bodies. Nerves are bundles of axons ensheathed in connective tissue that innervate all parts of the body, sending messages to and receiving messages from the CNS. The neuronal cell bodies that give rise to nerves do not lie within the nerves themselves. Rather, their cell bodies reside within the brain, spinal cord, or ganglia. Neuroscience Peripheral Nervous System (PNS): Anatomic Components Nerves can contain both efferent and afferent axons. Efferent axons transmit motor signals from the CNS to the PNS and can be somatic or autonomic. Afferent axons transmit sensory signals from the PNS to the CNS; afferents can be somatic or visceral. Afferent and efferent axons are protected by several layers of connective tissue, which together with glia and blood vessels form nerves. Neuroscience Functional Categories of Neurons Neurons are signaling cells that transmit electrical and chemical signals. The vast majority of neurons are electrically excitable, meaning they can produce and conduct action potentials. Most neurons function as part of neuronal circuits. Neurons are distinguished from other cells by a variety of features: Morphologically, they extend specialized membrane processes including axons (for sending information), dendrites and small protrusions called dendritic spines (for receiving information), and membrane subdomains called synapses (where information is transformed and transferred from the axon to the receiving cell). Biochemically, neurons synthesize, package, and release neurotransmitters. Physiologically, they produce and conduct action potentials along the axon and graded potentials along the dendrites and cell bodies, and their receptors can detect different forms of energy, such as light and sound waves, and convert those into electrical or chemical signals. Each of these specialized functions ensures that neurons communicate speciﬁcally and rapidly with their targets. Neuroscience Functional Categories of Neurons Hundreds of different types of neurons have been identiﬁed in the human nervous system. One-way neurons can be categorized is by their general function, as sensory neurons, motor neurons, or interneurons. Sensory Neurons There are 3 types of sensory neurons (also called afferent neurons) that detect and convey signals from the external and internal environments. Special sense neurons are located in special sense organs of the CNS (eg, photoreceptor cells) or the PNS (eg, hair cells). Somatosensory and visceral sensory neuron cell bodies are found in the PNS, although their axons enter the CNS and branches form components of ascending sensory tracts in the CNS. Neuroscience Functional Categories of Neurons Hundreds of different types of neurons have been identiﬁed in the human nervous system. One-way neurons can be categorized is by their general function, as sensory neurons, motor neurons, or interneurons. Motor Neurons Four types of motor neurons (also called efferent neurons) are involved in control of the somatic and autonomic motor systems. Upper motor neuron cell bodies are located in either the motor cortex or brainstem, and their axons project to lower motor neurons, and those lie entirely in the CNS. Somatic and autonomic (preganglionic) lower motor neuron cell bodies are found in the spinal cord or brainstem, with their axons emerging from those regions to form the motor efferents of the spinal and cranial nerves. Thus, lower motor neurons are considered part of both the CNS and PNS. Both the cell bodies (in sympathetic or parasympathetic ganglia) and axons of postganglionic autonomic motor neurons lie entirely in the PNS. Neuroscience Functional Categories of Neurons Hundreds of different types of neurons have been identiﬁed in the human nervous system. One-way neurons can be categorized is by their general function, as sensory neurons, motor neurons, or interneurons. Interneurons All of the other neurons in the CNS are called interneurons (or in some cases, just neurons). There are 2 types of interneurons, which are distinguished by whether they function locally or send their axons to other parts of the CNS. Local interneurons (also called local circuit neurons) work within the same brain region, usually have short unmyelinated axons, and form circuits with nearby neurons. Projection neurons (also called principal or relay neurons) extend their axons, which are usually long and myelinated, to another brain or spinal cord region. Interneurons are also named by their morphology, neurotransmitter they release, type of response they produce in their target cells, electrophysiology properties, and/or by the person who ﬁrst discovered them. Neuroscience Biologic membranes and Ions Approximately 90% of brain neurons use either glutamate or γ-aminobutyric acid (GABA) as their neurotransmitter, with the other 10% of neurons using acetylcholine or one of the biogenic amines (ie, norepinephrine, epinephrine, dopamine, or serotonin). Neurons that release glutamate are called glutamatergic neurons. Glutamatergic neurons are found throughout the brain and spinal cord. Although glutamate can act on several types of postsynaptic receptors, the majority (in terms of numbers) of glutamate receptors are ionotropic receptors that produce an excitatory response. Therefore, glutamatergic neurons are also referred to as excitatory neurons. An important class of glutamatergic neurons are the pyramidal neurons in the hippocampus and cerebral cortex, many of which are projection/principal neurons. Neuroscience Biologic membranes and Ions Neurons that release GABA as their neurotransmitter are GABAergic neurons. The most prevalent effect of GABA is an inhibitory response, and accordingly, GABAergic neurons are categorized as inhibitory interneurons. Inhibitory interneurons are found throughout most regions of the brain and are abundant in the cerebral cortex, cerebellum, and striatum. Cerebellar Purkinje neurons are GABAergic and are one of the few types of projection/principal neurons that are inhibitory. The neurotransmitter glycine also produces inhibition, and glycinergic neurons, as well as GABAergic neurons, are inhibitory interneurons in the spinal cord. Neuroscience Biologic membranes and Ions Other brain neurons release acetylcholine (called cholinergic neurons) or one of the monoamines. In the brain, many cholinergic neuron cells bodies are located in the basal forebrain and project to many areas of the cerebrum. Neurons that release biogenic amines, called noradrenergic, adrenergic, dopaminergic, or serotonergic neurons, have cell bodies located in the brainstem. Because these neurons produce a variety of excitatory, inhibitory, and modulatory effects on their targets, they are usually named for their neurotransmitter. In the PNS, somatic lower motor neurons are classiﬁed as excitatory neurons because they release acetylcholine at the neuromuscular junction, which always produces an excitatory response in the muscle cell. Neuroscience Neuronal Organelles Similar to other cells, neurons possess a plasma membrane that functions as a selective permeability barrier to the extracellular ﬂuid and that encloses the cytoplasm containing the nucleus, cytosol, and typical complement of cellular organelles and biochemical activities. The cell body of the neuron, also called the cell soma, can vary in diameter from approximately 100 µm to approximately 10 µm. The organelles include the nucleus, smooth and rough endoplasmic reticulum (ER), Golgi apparatus, lysosomes, proteasomes, and mitochondria, several of which are also localized in the axon and dendrites. Neurons contain a robust cytoskeleton with typical ﬁlaments and cytoskeletal motor proteins, which are important in the development, structure, and function of axons and dendrites. Neuroscience Neuronal membranes The neuronal plasma membrane and other membrane-enclosed organelles are formed from a phospholipid bilayer and transmembrane proteins. Membranes also contain cholesterol and sphingolipids. In neurons, lipids are synthesized in the smooth Endoplasmic Reticulum (ER) or, in the case of cholesterol, obtained from the diet or liver. Transmembrane proteins become incorporated into the ER membrane during translation. Following translation in the rough ER, transmembrane and secreted proteins and peptides are transported via small vesicles to the Golgi, where they are further modiﬁed and then sorted and packaged into vesicles, called secretory vesicles, at the trans-Golgi network to be transported to their ﬁnal destination. Called the biosynthetic or secretory pathway, it is involved in the synthesis and delivery of lipids and transmembrane proteins to the plasma membrane; the secretion of proteins and peptides outside the neuron; formation of organelles in the endosomal pathway, including the early endosome and lysosome; and production of the precursors needed to form synaptic vesicles. Proteins and lipids move between membrane compartments by a process called membrane trafﬁcking. Neuroscience Neuronal membranes Proteins and lipids are also retrieved from the plasma membrane through trafﬁcking in the endosomal pathway. Plasma membrane lipids and transmembrane proteins undergo endocytosis, where a small patch of the membrane folds inward into the cytoplasm and pinches off to form a small endocytic vesicle. This vesicle then trafﬁcs to and fuses with the early endosome, where the proteins are sorted into small vesicles that bud off and are trafﬁcked to different compartments. Transmembrane proteins can travel by recycling vesicles back to the plasma membrane. Neuroscience Neuronal membranes A speciﬁc type of endocytosis, called receptor-mediated endocytosis, is used to bring key nutrients such as cholesterol and iron into the cell. In this process, the receptors bind and deliver their nutrients to endosomal compartments and can recycle back to the plasma membrane. Transmembrane proteins that have been damaged will trafﬁc to the late endosome and then fuse with the lysosome. A lysosome is a membrane-bound compartment with an acidic pH that contains a variety of degradative enzymes inside, which can degrade proteins, nucleic acids, and lipids. Lysosomes can also function to degrade cytoplasmic proteins or damaged organelles, such as mitochondria, by fusing with compartments in the autophagy pathway. Neuroscience Neuronal energetics It has been estimated that the brain consumes approximately 20% of the energy and oxygen in the body, although it only composes approximately 2% of the body’s mass. In order to meet its energy demands, neurons require energy substrates, oxygen, and mitochondria located in the cell soma, axons, and dendrites. The main energy currency in the cell is adenosine triphosphate (ATP). ATP can be generated by glycolysis; a process that takes place in the cytoplasm, converting glucose into pyruvate, reduced nicotinamide adenine dinucleotide (NADH), and ATP. Neuroscience Neuronal energetics Pyruvate can be converted to lactate or shuttled with NADH into mitochondria, which synthesize ATP via the enzymes in the Krebs cycle and oxidative phosphorylation, with a yield of approximately 16 ATP molecules per pyruvate/NADH. Neurons do not contain large stores of glucose in glycogen, and therefore, they depend on nearby astrocytes to take up glucose from capillaries and release glucose and its glycolytic product lactate into the extracellular space. Neurons convert lactate back into pyruvate via lactate dehydrogenase and shuttle the pyruvate into mitochondria. Neurons are highly dependent on mitochondria for the synthesis of ATP, and conditions that lead to anoxia can rapidly lead to neuronal damage and cell death. Neuroscience Axon The axon is a structure that is unique to neurons. It is a process that is specialized for the generation and conduction of electrical signals called action potentials, which are sent along the axon, in some cases over long distances, to the end of the axon called the presynaptic terminus. At the terminus, the action potential electrical signal is converted into a chemical signal in the form of neurotransmitter release. Axons can be small or large in diameter, ranging from less than 1 to 10 µm. Axons can be long or short and unmyelinated or myelinated. Projection neurons and sensory neurons extend the longest axons, which are usually myelinated and can be centimeters to a meter in length. Local circuit neurons are usually unmyelinated and only a few millimeters in length. Axons can be unbranched with 1 main output or be highly branched, with the branches called collaterals, each of which can form a separate output. Neuroscience Axon Axons contain cytoskeletal proteins that provide structure and important functions. The largest ﬁlaments, called microtubules (MTs), give the axon stability and provide tracks to move organelles and large protein complexes up and down the axon. MTs are formed by polymerization of tubulin subunits with microtubule-associated proteins (MAPs) that associate and regulate polymerization and bundling of MTs. In addition, MT-based motors bind MTs and use the energy released by ATP hydrolysis to move cargoes along microtubules (MT) ﬁlaments in fast axonal transport, a type of axoplasmic transport Neuroscience Axon Two other cytoskeletal ﬁlaments, an intermediate ﬁlament called neuroﬁlament and the microﬁlaments composed of actin, are expressed in neurons. Mechanically strong, neuroﬁlaments serve a mainly structural role and ensure the diameter of the axon does not diminish along its length. Actin microﬁlaments are found associated with the plasma membrane, along with dozens of actin-binding proteins that regulate the assembly, disassembly, and bundling of the actin ﬁlaments and binding to the plasma membrane. During development, actin ﬁlaments and the actin-based motor, called myosin, are used to provide the mechanical forces that drive the motility of the growing axon. In mature axons, actin is localized in a mesh that underlies the axonal plasma membrane and at the presynaptic terminus. Because microtubules do not extend into the axon terminus, actin and myosin are involved in transporting cargoes in the axon terminal and providing scaffolds that tether transmembrane proteins at speciﬁc regions of the axonal or presynaptic membrane. Neuroscience Dendrites and Synapses Dendrites are branched processes that are specialized for receiving information. Presynaptic axon terminals form synapses on dendrites, which then produce postsynaptic signals that are passively transmitted to the cell body. Dendrites can be highly branched and referred to as the dendritic tree or arbor. The number of inputs that a neuron receives is proportional to its dendritic area. Dendrites can be distinguished from axons by their appearance. Unlike axons, which have a constant diameter, dendrites taper as they extend from the cell body. Dendrites are usually shorter than axons and may be studded with dendritic spines. Similar to the axon, dendrites contain mitochondria, secretory and endocytic vesicles, early endosomes, and an organized cytoskeleton, including MTs, microﬁlaments, and their regulatory proteins and motors. In addition, axons and dendrites contain ribosomes and mRNA that mediate local protein synthesis within these processes. The dendrites of some neurons contain dendritic spines along their length. Dendritic spines are actin-rich small protrusions that can have a bulbous head and are the regions where the majority of excitatory glutamate synapses occur on the dendrite. Neuroscience Dendrites and Synapses The synapse is the structure where synaptic transmission occurs. The axon terminal is the part of the axon that forms a synapse with another neuron, muscle, or gland. When a neuron makes a synaptic connection with another cell, it is said to innervate that cell. The connections between an axon and a muscle or gland are also referred to as junctions. In neurons, synapses occur between an axon and a dendrite (axodendritic synapses), an axon and the cell body (axosomatic synapses), or an axon and another axon (axoaxonic synapses). If the axons form short branches at their ends and synapse on dendrites or cell bodies, these branches are called the terminal arbor. Some axons form bulbous presynaptic swellings and synapse on a dendrite without terminating there but continue on to form additional synapses. Neuroscience Dendrites and Synapses Two different types of synapses, named electrical and chemical synapses, mediate transmission. Although fairly rare in the human CNS, electrical synapses allow for the direct ﬂow of ions (current) through gap junctions between the pre- and postsynaptic neuron. The vast majority of synapses in the human brain are chemical synapses, which involve the release of neurotransmitter from the presynaptic terminus and production of electrical and other signaling responses in the postsynaptic neuron. In chemical synapses, the presynaptic terminus is distinguished by the presence of synaptic vesicles and specializations called active zones. The synaptic cleft (20 to 40 nm wide) separates the pre- and postsynaptic membrane, although synaptic adhesion molecules from the presynaptic and postsynaptic membrane bind across the synaptic cleft to help produce the stability and speciﬁcity of the synapse. The postsynaptic membrane contains neurotransmitter receptors that are tethered at the synapse by scaffolding and cytoskeletal proteins, forming a region called the postsynaptic density in excitatory/glutamatergic synapses. Neuroscience Action Potential Watch the video Neuroscience Question Which of the following neurotransmitters has excitatory effects when released in the central nervous system? A. Glycine B. GABA C. Chloride D. Glutamate Neuroscience Questions?

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