Nerve Cells Lecture PDF

Nerve cells (Chapter 12 Tortora & Derrickson online) Cell Biology lecture Dr Hussain [email protected] Learning Objectives: Organisation of the Nervous System Central Nervous System Peripheral Nervous System Cell types Structure of neurons Types of neurog...

Nerve cells (Chapter 12 Tortora & Derrickson online) Cell Biology lecture Dr Hussain [email protected] Learning Objectives: Organisation of the Nervous System Central Nervous System Peripheral Nervous System Cell types Structure of neurons Types of neuroglia  useful text book references Tortora, GJ & Derrickson B (2017). Tortora’s Principles of Anatomy & Physiology. 15 Edn. Chapter 12; th Nervous Tissue: p354 – 390. McKinley M., O’Loughlin V. & Bidle T. (2015). Anatomy & Physiology: An Integrative Approach” McGraw-Hill. Bear MF, Connors BW & Paradiso MA (2020) “Neuroscience: Exploring the brain” (4th edn), Lippincott, Williams & Wilkinson. ALSO a useful Histology textbook by Mitchell BS and Peel S available online via the UoB library: https://www-vlebooks-com.brad.idm.oclc.org/Vleweb/Product/Index/196813?page = The role of the nervous system in physiological function  Basic concepts Sensory (afferent) detection of stimuli and signals Chemoreceptors (GI tract), Photoreceptors light (eyes), Mechanoreceptors (touch skin) (blood pressure; baroreceptors in blood vessels) Integrative Information processing decoding, sorting Motor (efferent) Response -delivering actions Contraction/relaxation (muscle) Secretion (endocrine glands).  Functions of the nervous system The nervous system consists of the: Brain (80 billion neurons) Spinal cord (100 million neurons) Sensory function. Sensory receptors detect internal stimuli, such as an increase in blood pressure, or external stimuli (for example, a raindrop landing on your arm). This sensory information is then carried into the brain and spinal cord through cranial and spinal nerves. Integrative function. The nervous system processes sensory information by analysing it and making decisions for appropriate responses—an activity known as integration. Motor function. Once sensory information is integrated, the nervous system may elicit an appropriate motor response by activating effectors (muscles and glands) through cranial and spinal nerves. Stimulation of the effectors causes The Brain  The Brain and Spinal cord In a freshly dissected section of the brain or spinal cord, some regions look white and glistening, and others appear gray. White matter is composed primarily of myelinated axons- the whitish colour of myelin gives white matter its name. Gray matter contains neuronal cell bodies, dendrites, unmyelinated axons, axon terminals, and neuroglia. It appears grayish, rather than white, because the rough endolasmic reticulum (also called Nissl bodies) impart a gray color and there is little or no myelin in these areas. Blood vessels are present in both white and gray matter. In the spinal cord, the white matter is a thin shell of gray matter is on the outer layer and on the outer layer and surrounds an covers the surface of the largest portions of the brain, the inner core of gray matter Main structures of the Brain The Brain  Spinal cord Spinal cord is a tubular structure made up of nerves that extends from bottom of the brain (medulla oblongata in the brain stem) to the lumbar vertebrae. The vertebrae enclose and protect the spinal cord from damage. Functionally, the S.C. connects the sensory receptors to higher centres in the brain and also transmits motor signals back from the brain to the body, therefore acting as a system of transmission of neural signals.  Nervous System Central nervous system Peripheral nervous system CNS: PNS: Brain Cranial nerves Spinal cord Spinal nerves CNS: consists of the Ganglia brain and the spinal cord. The brain contains ~85 million neurons and the spinal cord contains ~100 Enteric million neurons. plexuses in small intestine PNS: all the components: nerves, ganglia enteric Sensory plexus, sensory receptors receptors. in skin Peripheral Nervous System  Peripheral Nervous System  Somatic Sensory neurons: hearing, taste vision, smell, sensory receptors in the brain and head, limbs and body. Motor neurons: nerves from the skeletal muscle to the CNS; is under voluntary control.  Autonomic Made up of: Sensory neurons in the visceral tissues (e.g. stomach, lungs). Motor neurons from the CNS to cardiac and smooth muscles and the glands (e.g. pancreas). Involuntary not under conscious control Two branches: Sympathetic Parasympathetic  Enteric ~100 million neurons in the enteric plexus in the G.I. tract, can work independently and in conjunction with CNS and ANS. Autonomic Nervous System has two branches: (Sympathetic and parasympathetic)  Enteric Nervous System Found in the gastrointestinal intestinal wall Regulates intestinal functions (e.g. motility and secretions) Submucosal plexus Mesentery (plexus of Meissner) Enteric nerves (also Duct of gland outside Gland in called intrinsic n.s.) is tract (such as mucosa Vein embedded within the pancreas) Glands in gastrointestinal tract. submucosa Mucosa-associated Can work lymphatic tissue (MALT) independently of the autonomic and central Lumen nervous systems but Artery usually has inter- Mucosa: Nerve neuronal connections Epithelium Myenteric plexus with both. Lamina propria (plexus of Auerbach) Muscularis mucosae Neurons of the ENS release many different Submucosa types of neurons such as dopamine, Muscularis: Areolar connective tissue Serosa: acetylcholine, Circular muscle serotonin in the GI Longitudinal muscle Epithelium  Synapses Two types: Chemical and electrical (see later) Neurons  What is a Neuron? Main points: - Basic functional unit of the nervous system - Specialised for transmitting information in the form of electrical and chemical signals - The electrical nature of the signal is the reason why neurones are described as “excitable” cells. As are cardiac and skeletal muscle cells; which also have large action potentials. Main components of a Neuron Dendrites Tree shaped branches that receive signals, contain numerous receptors Cell body Contains the nucleus and other components (soma) typically found inside cells e.g. mitochondria, lysosomes, golgi complex, ribosomes, etc. Axon Takes the signal away from the cell body and to another neuron, muscle fibre or a gland. Axon contains cytoplasm (axoplasm) plus other cellular components but has no ribosome and can not therefore Functional Classification of Neurons  Sensory afferent neurons (take signals to the CNS)  Motor efferent neurons (take signals away from the CNS)  Inter/association neurons Located in the CNS and between sensory and motor neurons  Functional classification of neurons Sensory (afferent) detection of stimuli and signals Chemoreceptors (GI tract), Photoreceptors light (eyes), Mechanoreceptors (touch skin) (blood pressure; baroreceptors in blood vessels) Integrative Information processing decoding, sorting Motor (efferent) Response -delivering actions Contraction/relaxation (muscle) Secretion (endocrine glands). Functional Classification of Neurons Sensory fibres: Touch – myelinated (++) “Fast” pain, temperature – myelinated (+) “Slow” pain, temperature, itch – unmyelinated Motor fibres – myelinated (+++) Based on fibre diameter and conduction velocity (how fast the impulses travel down the axon) Thicker, myelinated fibres tend to have higher conduction velocities than thin, unmyelinated fibres Structural Classification of Neurons Many dendrites ONE main One axon but fused from the cell dendrite plus together with dendrites body the axon from for sensing different the cell body signals e.g. touch, pressure, pain or heat.  Structural Classification of Neurons depends on the number of processes extending from the cell body Multipolar; one axon but many dendrites from the cell body. Motor Unipolar; have only neurones to muscles one axon emerging and glands. Most from the cell body but (99%) of the CNS many dendrites from neurons. the cell body for sensing different signals e.g. chemical, sound, mechanical, temperature, light Bipolar; two processes extending from the cell Pseudounipolar; body; (eyes, ear, nose) unipolar but axon divides in two  Diversity in neurons Neurons are divers in structure and function: Dendrites Cell body Cell bodies can vary from 5 μm to 135 μm in size. Pattern of dendritic branching is varied and distinctive in different areas of the nervous Axon system (see shape of the Purkinje fibres in the cerebellum and the pyramidal cells in Axon cerebral cortex). terminal Axons can either be absent, very short or very long (e.g. motor neurons in the leg) (a) Purkinje cell (b) Pyramidal cell  Non neuronal cells (neuroglia) Also called glial cells, or simply glia are non-neuronal cells that maintain homeostasis, form myelin, and provide support and protection for neurons in the central and peripheral nervous systems. – Support cells – Not electrically excitable – Make up about half the volume of the nervous system – Can multiply and divide – 6 kinds total (4 in CNS, 2 in PNS)  Neuroglia of the Central Nervous System Four types: Astrocytes, Oligodendrocytes, Microglia and Ependymal cells Microglia Function as phagocytes removing debris from microbes and developing or damaged nervous tissue. Astrocytes Two types: Protoplasmic (grey matter) with short branches Fibrous (white matter) with long branches Ependymal cells Cuboidal or columnar cells in a single layer Oligodendrocytes Possess microvilli and line the ventricles of the spinal cord Produce myelin and the brain. (proteolipid) sheath around axons for electrical Function to assist circulation of cerebrospinal fluid and insulation and fast conduction.  Myelination of nerves The myelin (proteolipid) sheath is produced by Schwann cells in the PNS and oligodendrocytes in the CNS. In the peripheral nervous system) In the Central Nervous System) Myelination of Neurons The myelin sheath is made up of multiple layers (up to ~100) wrapped around ~1mm segments of the axon so that the cytoplasmic material of the Schwann cell in the PNS forms the outer most layer called the neurolemma (role in neural regeneration). In unmyelinated neurons multiple neurons are surrounded by one Schwan cell, without the repeated spiralling of the proteolipid  Neuroglia of the Peripheral Nervous System Two cell types: Schwan cells completely surround the axon and cell body. Schwan cells produce myelin Satellite cells (small and flat cells) regulate the exchange of materials between cell bodies and interstitial fluid.  Nodes of Ranvier Gaps between the Schwan cells along the axon create nodes of Ranvier. Each Schwan cell wraps around one neuron between two nodes. Neurofibril nodes Regions between myelinating cells Gaps (approx 1μm) Axon exposed to extracellular space Very important in neurotransmission for the propagation of action potentials (speed up saltatory conduction) Also important in neurodegenerative diseases such as multiple sclerosis  Multiple sclerosis (MS) Inflammatory neurodegenerative disease believed to be of autoimmune aetiology. More common in women than in men. Age of onset is between 20 and 40 years. Inflammatory process of MS results in the gradual destruction of myelin sheaths around myelinated axons of the brain and spinal cord (CNS). Symptoms may include: Muscle weakness and paralysis Impaired coordination and poor balance Depression; impaired speech Memory problems Visual problems; Altered sensory perception; Pain Fatigue Other dysfunctions; e.g. bowel, bladder, and sexual dysfunctions, Resulting from the deficits in neural conduction caused by damage to myelin sheaths and underlying axons in the central nervous system. Some symptoms of MS (depend on the location of the plaques) Gray and white matter in the brain Gray matter ~ 40% of the brain. contains cell bodies of the neurons and unmyelinated axons White matter ~ 60% of the brain. contains myelinated axons of neurons. Axons in the white matter become demyelinated during multiple sclerosis Multiple sclerosis is associated with lesions in the white matter of the brain MS is associated with myelin degradation in the white matter. MS is an autoimmune disease- damage by T (and B) cells that cross the BBB. Brain tries to repairs itself by generation of more myelin by oligodendrocytes, which is why there are periods of remission. Treatments attempt to slow the damage caused by T cells- immunosuppressant  Pathological effects of demyelination Multiple sclerosis – demyelination of axon sections - affects transmission of nerve impulses Inflammation also present – gradually disappears but leaves plaques – glial scarring Remission (partial repair) & relapse (affected axons are now very sensitive to changes, i.e. in temperature) Leprosy – bacillus (gram positive bacteria) enters through skin / mucosa Have sensory loss due to demyelination – subsequent deformity occurs due to inability to sense pain, heat, etc  Nerves- summary of key points: Cell types - neurons and glial Structure – axons & dendrites – information transfer Specialised glial cells – astrocytes, microglia, oligodendrocytes & neurolemmocytes. Myelination neurofibril nodes (Nodes of Ranvier) – important for neural impulse conduction. Different nerve types have different levels of myelination.