14_Nervous System and Nervous Tissue.pptx

Nervous Tissue and Organization of the Nervous System Chapter 11, Human Anatomy (LibreTexts) "Cerebellum Cross Section" by Berkshire Community College Bioscience Image Library is in the Public Domain, CC0 Organization of the Nervous System • Nervous system can be divided structurally (by organs)...

Nervous Tissue and Organization of the Nervous System Chapter 11, Human Anatomy (LibreTexts) "Cerebellum Cross Section" by Berkshire Community College Bioscience Image Library is in the Public Domain, CC0 Organization of the Nervous System • Nervous system can be divided structurally (by organs) and functionally (by function): • Structural division: central and peripheral nervous system • Functional division: somatic, autonomic, and enteric nervous system Structural Division of the Nervous System • Central Nervous System (CNS): composed of brain and spinal cord; processes and integrates the signals coming from the peripheral nervous system and initiates the response • Peripheral Nervous System (PNS): composed of ganglia and nerves (cranial and spinal); senses the external and internal environment, sends the information to the central nervous system and carries out the response from it Divisions of the Nervous System "Structural Divisions of the nervous system" by Chiara Mazzasette is licensed under CC BY 4.0 / A derivative from the original work Ganglion, Nuclei, Nerves and Tracts • Depending on the location within a structural division of the nervous system, certain features can be called differently. • Neurons are located in ganglia (plural of ganglion) in the peripheral nervous system and in nuclei (plural of nucleus) in the central nervous system. • The projections of neurons are called tracts (or columns) within the central nervous system and nerves when outside of the central nervous system. Cranial nerves emerge from the brain and spinal nerves exit from the spinal cord. Basic Functions of the Nervous System • The basic functions of the nervous system are sensation, integration and response. • Sensation: the nervous system receives information on changes of the external and internal environment. These changes are called stimuli (plural for stimulus) and are perceived by peripheral sensory structures. • Integration: stimuli are communicated to the brain and spinal cord where that information is processed. • Response: the nervous system produces a motor response to the stimulus. The response involves movement of skeletal muscles (voluntary), cardiac or smooth muscles or glands (involuntary). Functional Division of the Nervous System • Based on the responses, the nervous system can be divided into: • Somatic nervous system: responsible for conscious perception and voluntary motor responses of skeletal muscles. Includes sensory ganglia, structures of special senses, brain, spinal cord and nerves. • Autonomic nervous system: responsible for involuntary control of cardiac muscle, smooth muscle and glands everywhere in the body. Includes autonomic neurons in the brain and spinal cord, some cranial and spinal nerves and autonomic ganglia. • Enteric nervous system: responsible for involuntary control of the smooth muscle and glands in the digestive system. Includes nervous structures within the digestive system. Somatic, Autonomic and Enteric Divisions of the Nervous System Somatic Sensation Integration Conscious perception of environmental changes (light, sounds, molecules in food, movement, temperature, touch, etc) through somatic sensory neurons of PNS Information processed Voluntary and reflex in brain and spinal cord response via skeletal (CNS) muscles Unconscious perception of external or internal changes (light, Autonomic molecules, organ stretch, etc) through visceral sensory neurons in both CNS and PNS Enteric Unconscious perception of internal changes (molecules, movement, stretch) within the gastrointestinal tract through visceral sensory neurons in gastrointestinal tract Motor Response Information processed in brain (particularly hypothalamus and brainstem) and spinal cord (CNS) Involuntary movement of cardiac muscle, smooth muscle, glands Information processed within the gastrointestinal tract (PNS) Involuntary movement of smooth muscle and glands of digestive system Table Credit: Chiara Mazzasette Nervous Tissue • Located in the central and peripheral nervous system. • Cells: • Neurons: responsible for the electrical signals that communicate information about sensations, and that produce movements in response to those stimuli, along with inducing thought processes within the brain. Large, non-mitotic cells. • Glia (or neuroglia, or glial cells): considered to be supporting cells, and many functions are directed at helping neurons complete their function for communication. Nervous Tissue Smear "Labeled Nervous Tissue Smear" by Chiara Mazzasette is licensed under CC BY 4.0 / A derivative from the original work Neurons • Conduct nerve impulses from one place to another • Special characteristics: • • • • • High metabolic rate: rely on glucose and oxygen Extreme longevity: sometimes a lifetime Nonmitotic: cannot divide and form new cells Excitable: generate electrical signals Conductive: propagate electrical signals Neuron Structure • Soma: cell body of a neuron, containing nucleus and organelles • Dendrites: processes (extensions) that receive information • Axon: process that carry the information from the soma. Neurons have only one axon. The axon emerges from the axon hillock and is covered by glial cells that form the myelin. The axon ends in branches at the axon terminal and the branches enlarge at their ends to form a synaptic end bulb. Axons that exit the CNS are bundled together in nerves. • Nodes of Ranvier: gaps between the myelin that Parts of a Neuron - Image "Labeled parts of a neuron" by Chiara Mazzasette is licensed under CC BY 4.0 / A derivative from the original work Structural Classification of Neurons • This classification depends on the number of processes attached to the cell body: one, two or multiple. • Unipolar neurons: have only one process (axon and dendrite combined) emerging from the soma. Found in ganglia. • Bipolar neurons: have two processes (one axon and one dendrite) emerging from the soma. Found in olfactory epithelium and retina. • Multipolar neurons: have multiple processes emerging from the soma. Found everywhere else in the nervous system. Found in CNS and ganglia. Neuron Classification by Shape "Neuron Shape Classification" by OpenStax is licensed under CC BY 4.0 Other Classifications of Neurons • Neurons can also be classified on the basis of where they are found, who found them, what they do, or even what chemicals they use to communicate with each other. Other Neuron Classifications "Other Types of Neurons" by OpenStax is licensed under CC BY 4.0 Functional Classification of Neurons • This classification depends on which function the neuron is carrying out: sensation, integration or motor. • Sensory neurons: their dendrites are receiving sensory information, sometimes directly from the stimulus itself. Found in PNS. They can be unipolar, bipolar or multipolar. They can be somatic or visceral. • Interneurons: carry out integrative functions (such as retrieve, process and store information) and facilitate communication between sensory and motor neurons. Only found in CNS. They are multipolar neurons and the majority of neurons. • Motor neurons: carry commands from the brain and spinal cord to muscles and glands. Found in CNS (somatic) or autonomic ganglia. They are multipolar neurons. Glial Cells • Neuroglia (glial cells) protect and nourish neurons; help forming synapses and maintaining them; clear out dying neurons; adjust electrical signals of neurons. • Found in both CNS and PNS • Smaller and more numerous than neurons • Capable of mitosis • Brain tumors are more likely to be derived from glial cells than neurons Glial Cells of the CNS • Astrocytes: have many processes extending from their main cell body that extend to interact with neurons, blood vessels, or the connective tissue covering the CNS. Contribute to the blood-brain barrier (BBB). • Ependymal cells: filter blood to make cerebrospinal fluid (CSF), the fluid that circulates through the CNS. • Oligodendrocytes: insulate axons in the CNS. One oligodendrocyte will provide the myelin for multiple axon segments. • Microglia: smaller than most of the other glial cells. They are the CNS-resident macrophages that ingest and digest those cells or the pathogens that cause disease. Glial Cells of the CNS - Image "Glial Cells of the CNS" by OpenStax is licensed under CC BY 4.0 Glial Cells of the PNS • Satellite cells: surround cell bodies of neurons and provide support to them. Found in sensory and autonomic ganglia. • Schwann cells: insulate axons with myelin in the PNS. Wrap around a portion of only one axon segment and no others. Glial Cells of the PNS - Image "Glial Cells of the PNS" by OpenStax is licensed under CC BY 4.0 Myelinated and Unmyelinated Axons • Myelinated nerves: the glial cell is wrapped around each axon several times with little to no cytoplasm between the glial cell layers. The segment of axons is entirely covered by myelin. • Unmyelinated nerves: axons are not covered completely and one side of the axon remains unmyelinated. The Process of Myelination "Myelinated Neuron" by OpenStax is licensed under CC BY 4.0/ Micrograph provided by the Regents of University of Michigan Medical School © 2012 Unmyelinated Axons "Unmyelinated Nerve" by Chiara Mazzasette is a derivative from the original work of Daniel Donnelly and is licensed by CC BY 4.0 Axon Regeneration • In the PNS: when nerves are damaged, the axons distal to the injury degenerate. The Schwann cells that were wrapping the injured axons proliferate to form a new tube of myelin that guides the growing axons through physical and chemical support. Most of the times, axons can cross the injury site and can successfully grow back to the original targets. • In the CNS: when nerves are damaged, the oligodendrocytes respond to injury by undergoing apoptosis or becoming inactive. This contributes to the formation of scar tissue. Injury-activated astrocytes release molecules that inhibit axonal growth. Injured axons in the CNS are not able to grow back to their original target and nerve injuries in the CNS are permanent (e.g. spinal cord injuries) Axon Regeneration in the PNS ”Axon Regeneration in the PNS" by Chiara Mazzasette is a derivative from the original work of Daniel Donnelly and is licensed by CC BY 4.0 Synaptic Communication • Neurons generate electrical impulses called action potentials that are carried down the axon to the axon terminal. • The axon terminal enlarges to form a synaptic end bulb which delivers the electrical impulse to the target cell. • The synapse is the connection between a neuron and its target cell, which can be another neuron or skeletal, cardiac or smooth muscle cells, or glands. • Presynaptic cell: neuron that generates the action potential • Postsynaptic cell: cell that receives the action potential Presynaptic and postsynaptic neurons "Neuron Part 1" by BruceBlaus is licensed under CC BY-SA 4.0 Myelin Sheath • The insulation for axons in the nervous system is provided by oligodendrocytes in the CNS and Schwann cells in the PNS. • Myelin is a lipid-rich sheath that surrounds the axon and by doing so creates a myelin sheath that facilitates the transmission of electrical signals along the axon. • Saltatory conduction: For myelinated axons, there are gaps of myelin called nodes of Ranvier that allow the electrical impulse to jump from myelin to myelin, thus conduction is faster. • Continuous conduction: For unmyelinated axons, one side of the axon does not contain myelin, so electrical impulse needs to travel the entire length of the axon, thus conduction is slow. Electrical Synapses • A neuron and another cell form a physical connection through gap junctions. • The gap junctions allow an action potential to pass through its channels and change the electrical properties of the target cell. • Found throughout the nervous system and between excitable cells other than neurons (e.g. smooth muscle cells in the intestines and cardiac muscle cells in the heart). Electrical Synapse ”Electrical Synapse" by Chiara Mazzasette is a derivative from the original work of Daniel Donnelly and is licensed by CC BY 4.0 Chemical Synapses • A neuron and another cell are spaced by a synaptic cleft (no direct contact). • A chemical signal, called a neurotransmitter, is the messenger between the two cells. • The neurotransmitter is packed into synaptic vesicles inside the synaptic end bulb. • When the action potential reaches the synaptic end bulb, the vesicles fuse with the membrane and the neurotransmitters are released in synaptic cleft. There it binds to a receptor on the postsynaptic membrane of the target cell and generates a response. Chemical Synapse "Chemical Synapse" by Young, KA., Wise, JA., DeSaix, P., Kruse, DH., Poe, B., Johnson, E., Johnson, JE., Korol, O., Betts, JG., & Womble, M. is licensed under CC BY 4.0 Endocrine vs Nervous Systems • Endocrine System: Endocrine glands transmit chemical signals in the form of hormones through blood stream to cells far away, so the distance traveled is far and response is slow. • Nervous System: Neuron scan affect only cells they contact (make a synapse with) and release their electrical impulses or chemical neurotransmitters to, so the distance traveled is short and response is fast.