Chapter 11: Fundamentals of the Nervous System and Nervous Tissue PDF

Chapter 11: Fundamentals of the Nervous System and Nervous Tissue  With the endocrine system, the nervous system shares the responsibility of maintaining Homeostasis Keep controlled conditions within limits that maintain life. The nervous system is responsible for responding to chan...

Chapter 11: Fundamentals of the Nervous System and Nervous Tissue  With the endocrine system, the nervous system shares the responsibility of maintaining Homeostasis Keep controlled conditions within limits that maintain life. The nervous system is responsible for responding to changes via nerve impulses (or action potentials).  An Introduction to the Nervous System The Nervous System Includes all neural tissue in the body Neural tissue contains two kinds of cells 1. Neurons Cells that send and receive signals 2. Neuroglia (glial cells) Cells that support and protect neurons Organs of the Nervous System Brain and spinal cord Sensory receptors of sense organs (eyes, ears, etc.) Nerves connect nervous system with other systems  Divisions of the Nervous System Anatomical Divisions of the Nervous System Central nervous system (CNS) Consists of the spinal cord and brain Functions of the CNS are to process and coordinate: Sensory data from inside and outside body Motor commands control activities of peripheral organs (e.g., skeletal muscles) Higher functions of brain intelligence, memory, learning, emotion Peripheral nervous system (PNS) Includes all neural tissue outside the CNS that carries sensory information and motor commands in PNS Cranial nerves — connect to brain Spinal nerves — attach to spinal cord Functions of the PNS Deliver sensory information to the CNS Carry motor commands to peripheral tissues and systems  Divisions of the Nervous System Functional Divisions of the PNS – Afferent or Sensory division Carries sensory information From PNS sensory receptors to CNS – Efferent or Motor division Carries motor commands From CNS to PNS muscles and glands Functional Division of the PNS  Neurons Neurons The basic functional units of the nervous system The structure of neurons Cell body (soma) Short, branched dendrites Long, single axon Dendrites Perikaryon Cell body Nucleus Telodendria Axon This color-coded figure shows the four general regions of a neuron.  Neurons Dendrites Highly branched Dendritic spines Many fine processes Receive information from other neurons 80–90% of neuron surface area The axon (“nerve fiber”) Is long Carries electrical signal (action potential) to target Axon structure is critical to function  Neurons Structures of the Axon Telodendria Axon hillock Fine extensions of distal axon Thick section of cell body Synaptic terminals (synaptic end Connects cell body to axon knobs, synaptic end bulbs) Collaterals Tips of telodendria. Contains vesicles Branches of a single axon filled with neurotransmitters Axoplasm- cytoplasm of the axon Axolemma- “PM” of the axon; specialized; can be exposed or covered.  Neurons The synapse Area where a neuron communicates with another cell (E.g., another neuron, muscle cell, gland cell, etc.)  Neurons The Structure of Neurons – The synapse Presynaptic cell – Neuron that sends message Postsynaptic cell – Cell that receives message The synaptic cleft – The small gap that separates the presynaptic membrane and the postsynaptic membrane  Neurons The Synapse – The synaptic Telodendrion terminal Synaptic terminal Is expanded area of Endoplasmic Mitochondrion reticulum axon of presynaptic Synaptic neuron vesicles Presynaptic Contains synaptic membrane Postsynaptic Synaptic membrane cleft vesicles of neurotransmitters  Neurons Three Functional Classifications of Neurons 1. Sensory neurons Afferent neurons of PNS 2. Motor neurons Efferent neurons of PNS 3. Interneurons Association neurons  Neurons 1. Sensory Neurons Functions of Sensory Neurons Monitor internal environment (visceral sensory neurons) Monitor effects of external environment (somatic sensory neurons) Structures of Sensory Neurons Processes (afferent fibers or axons) extend from sensory receptors to CNS  Neurons Three Types of Sensory Receptors 1. Interoceptors Monitor internal systems (digestive, respiratory, cardiovascular, urinary, reproductive) Internal senses (taste, deep pressure, pain) 2. Exteroceptors External senses (touch, temperature, pressure) Distance senses (sight, smell, hearing) 3. Proprioceptors Monitor position and movement (skeletal muscles and joints)  Neurons 2. Motor Neurons Carry instructions from CNS to peripheral effectors Via efferent fibers (axons) that enter the CNS Two major efferent systems 1. Somatic nervous system (SNS) Includes all somatic motor neurons that innervate skeletal muscles 2. Autonomic (visceral) nervous system (ANS) Visceral motor neurons innervate all other peripheral effectors Smooth muscle, cardiac muscle, glands, adipose tissue  Neurons 3. Interneurons (a.k.a., association neurons or processing neurons) – Most are located in brain, spinal cord, and autonomic ganglia Between sensory and motor neurons – Are responsible for: Distribution of sensory information Coordination of motor activity – Are involved in higher functions Memory, planning, learning Most common neurons in the nervous system…the multipolar neuron! Structures of the Nervous System The nervous system is a complex, highly organized network that consists of... billions of… Neurons (nerve cells) AND even more… Neuroglia (glial)- support cells, Nervous System Histology Less than 20% of the nervous system is extracellular space. This affords for very closely packed cells. Even though the nervous systems is highly complex, it consists of only TWO (2) major types of cells. 1. Neurons- which have the ability to produce an action potential or nerve impulse; have property of electrical excitability; conductive or transmissive  In addition to this property neurons are also know for.... – Their extreme longevity- with good nutrition, neurons can perform optimally for more than 100 years. – Being amitotic-they lose the ability to undergo mitosis. – Their high metabolic rate- requires continuous supply of oxygen and glucose; without oxygen the neuron will only survive a few minutes. 2. Neuroglia or Glial Cells supportive cells; surround the more delicate neurons; non-conductive or non-transmissive.  The Myelin Sheath (1 of 6) Myelin Sheath – layers of plasma membrane of Schwann cell or oligodendrocyte in PNS and CNS respectively (Figures 11.8, 11.9): Myelination – neuroglial cells wrap multiple layers of membrane (myelin) around axon – Lipid content of myelin sheath insulates axon (prevents ion movements) like rubber around copper wire; increases speed of action potential conduction – Myelinated axons conduct action potentials about 15–20 times faster than unmyelinated axons  The Myelin Sheath (2 of 6) Differences between myelination in PNS and CNS: – Neurolemma – on outer surface of myelinated axons in PNS; Schwann cell nucleus, organelles, and cytoplasm; not present in CNS (Figure 11.8a, b) – Number of axons myelinated – oligodendrocytes have multiple processes that myelinate multiple axons in CNS; Schwann cell only myelinates one axon in PNS – Timing of myelination – myelination begins early in fetal development in PNS and much later in CNS; very little myelin in brain of newborn  The Myelin Sheath (3 of 6) The myelin sheath in the PNS and CNS.  The Myelin Sheath (4 of 6) The myelin sheath in the PNS and CNS.  The Myelin Sheath (5 of 6) Axons in both CNS and PNS are generally longer than neuroglial cells so multiple cells must provide complete myelin sheath – Internodes – segments of axon covered by neuroglia – Node of Ranvier – gap between adjacent neuroglia; where myelin sheath is absent  The Myelin Sheath (6 of 6) Small axons in CNS and PNS are usually unmyelinated White matter – composed of myelinated axons; appear white Gray matter – composed of neuron cell bodies, unmyelinated dendrites and axons; appear gray Unmyelinated peripheral axons and Schwann cells.  Regeneration of Nervous Tissue Regeneration or replacement of damaged tissue is nearly nonexistent in CNS; limited in PNS; neural tissue can regenerate only if cell body remains intact Repair of axon damage in the PNS. Regeneration of Nervous Tissue Regeneration steps (Figure 11.10): – Axon and myelin sheath degenerate distal to injury (Wallerian degeneration); facilitated by phagocytes – Growth processes form from proximal end of axon – Schwann cells and basal lamina form regeneration tube – Single growth process grows into regeneration tube; directs new axon toward its target cell – New axon reconnects to its target cell Regeneration of Nervous Tissue Repair of axon damage in the PNS. Impulse Transmission (Pathway) Dendrites: receptive or input region. –> Cell Body–> Axon: singular process–> Axon Terminal–> synaptic knobs/end bulbs: secretory or output regions; release: neurotransmitters.  Ion Channels in Neurons Reminder: Channels allow for the flow on ions across the membrane through a process of diffusion through a channel (passive transport) Neural Anatomy 1. Functional Regions of Neuron a. Receptive region- receives stimulus; PM exhibits chemically- gated ion channels. b. Conducting region- generates or transmits action potentials; PM exhibits voltage-gated Na+/K+ channels. c. Secretory region- releases neurotransmitters ; PM exhibits voltage-gated Ca+ channels.