Introduction to Nervous System

Nervous Tissue Controls and integrates all body activities within limits that maintain life Three basic functions sensing changes with sensory receptors interpreting and remembering those changes reacting to those changes with effectors (motor function) Controls and integrates all body activities within limits that maintain life Three basic functions sensing changes with sensory receptors fullness of stomach or sun on your face transport sensory information from skin, muscles, joints, sense organs & viscera to CNS interpreting and remembering those changes connect sensory to motor neurons 90% of neurons in the body reacting to those changes with effectors muscular contractions glandular secretions Lecutre 8: Nervous System

The PNS is divided into : 1- a somatic nervous system (SNS) 2- an autonomic nervous system (ANS)

CNS CNS Sensory (Afferent) vs. Motor (Efferent) sensory (afferent) nerve CNS e.g., skin (pseudo-) unipolar neurons conducting impulses from sensory organs to the CNS motor (efferent) nerve CNS e.g., muscle multipolar neurons conducting impulses from the CNS to effector organs (muscles & glands) Gray’s Anatomy 38 1999

Organization Sensory Integration Motor SNS (Sensory) Brain Spinal cord ANS (Motor) ANS (Sensory)

Neuron has three parts: (1) a cell body: perikaryon or soma (2) dendrites (3) an axon

Neurons • Dendrites: carry nerve impulses toward cell body • Axon: carries impulses away from cell body • Synapses: site of communication between neurons using chemical neurotransmitters • Myelin & myelin sheath: lipoprotein covering produced by glial cells (e.g., Schwann cells in PNS) that increases axonal conduction velocity dendrites cell body axon with myelin sheath Schwann cell synapses Moore’s COA5 2006

Notice that action potential propagation is unidirectional

Neurons Cell body Nissl bodies Golgi apparatus Neurofilaments (IFs) Microtubules Lipofuscin pigment clumps Cell processes Dendrites Axons Functional unit of nervous system Have capacity to produce action potentials electrical excitability Cell body single nucleus with prominent nucleolus Nissl bodies (chromatophilic substance) rough ER & free ribosomes for protein synthesis neurofilaments give cell shape and support microtubules move material inside cell lipofuscin pigment clumps (harmless aging) Cell processes = dendrites & axons Lecutre 8: Nervous System

Structure of neurons Axoplasm: cytoplasm of axon Axolemma: cell membrane of axon Axon hillock: where axon originates from soma Synaptic boutons: swelling of axon terminal Synapse: junction axon makes with cell acting upon Synaptic vesicles

Axon Dendrite Becomes much thinner (tapering) short Branches profusely The cytoplasm of its base is similar to cell body Dendtric spines (points of synapse with other neurons) Always unmyelinated Nearly constant diameter Much Longer Branches less profusely Distal end forms terminal arborization and terminal boutons Mostly myelinated, could be unmyelinated Axoplasm contains mitochondria, microtubules, neurofilaments and SER but not RER and ribosomes Bidirectional transport along the axon

presynaptic neuron Synaptic vesicles contain the neurotransmitter synaptic cleft postsynaptic neuron

Axonal transport Anterograde: movement away from soma Retrograde: movement up toward soma

Neurons: Sensing, thinking, remembering, controlling muscle activity, and regulating glandular secretions Do not divide (no centriols!!!) Long lived High metabolic activity Electrically excitable Neuroglia : Support, nourish, and protect neurons Divide Smaller cells but they greatly outnumber neurons

Parts of a Neuron Neuroglial cells Nucleus with Nucleolus Axon or Dendrites Cell body

Multipolar neurons usually have several dendrites and one axon motor neurons 2. Bipolar neurons have one main dendrite and one axon the retina of the eye 3. Unipolar neurons (pseudounipolar neurons) sensory neurons Structural classification of neurons

Don’t produce action potential Regulatory function Figure 9–4 Structural classes of neurons. Shown are the three main types of neurons, with short descriptions. (a) Most neurons, including all motor neurons and CNS interneurons, are multipolar. (b) Bipolar neurons include sensory neurons of the retina, olfactory mucosa, and inner ear. (c) All other sensory neurons are unipolar or pseudounipolar. (d) Anaxonic neurons of the CNS lack true axons and do not produce action potentials, but regulate local electrical changes of adjacent neurons.   4- Anaxonic neuron: CNS Lack true axon Don’t produce action potential Regulatory function Figure 9-4

1. Tissues: neurons vs. glia 2. Position: CNS vs. PNS 3. Function 1: sensory vs. motor 4. Function 2: somatic vs. visceral glial cell Gray’s Anatomy 38 1999

Continuous versus Saltatory Conduction Continuous conduction (unmyelinated fibers) Saltatory conduction (myelinated fibers) A.P. Na Na Na Na Na Na Na Continuous conduction (unmyelinated fibers) step-by-step depolarization of each portion of the length of the axolemma Saltatory conduction depolarization only at nodes of Ranvier where there is a high density of voltage-gated ion channels current carried by ions flows through extracellular fluid from node to node Na Na Na Lecutre 8: Nervous System

Saltatory Conduction Nerve impulse conduction in which the impulse jumps (Salta) from node to node Na A.P. Na Na Na Na Local anesthetics!!!!!!

Types of synapses

Clusters of Neuronal Cell Bodies a ganglion (plural is ganglia) a cluster of neuronal cell bodies located in the PNS. a nucleus: a cluster of neuronal cell bodies located in the CNS.

Bundles of Axons A nerve: is a bundle of axons that is located in the PNS. Cranial nerves connect the brain to the periphery spinal nerves connect the spinal cord to the periphery A tract: is a bundle of axons located in the CNS. Tracts interconnect neurons in the spinal cord and brain.

Figure 9–26 Peripheral nerve connective tissue: Epi-, peri-, and endoneurium. (a) The diagram shows the relationship among these three connective tissue layers in large peripheral nerves. The epineurium (E) consists of a dense superficial region and a looser deep region that contains the larger blood vessels. (b) The micrograph shows a small vein (V) and artery (A) in the deep epineurium (E). Nerve fibers (N) are bundled in fascicles. Each fascicle is surrounded by the perineurium (P), consisting of a few layers of unusual squamous fibroblastic cells that are all joined at the peripheries by tight junctions. The resulting blood-nerve barrier helps regulate the microenvironment inside the fascicle. Axons and Schwann cells are in turn surrounded by a thin layer of endoneurium. X140. H&E. (c) As shown here and in the diagram, septa (S) of connective tissue often extend from the perineurium into larger fascicles. The endoneurium (En) and lamellar nature of the perineurium (P) are also shown at this magnification, along with some adjacent epineurium (E). X200. PT. (d) SEM of transverse sections of a large peripheral nerve showing several fascicles, each surrounded by perineurium and packed with endoneurium around the individual myelin sheaths. Each fascicle contains at least one capillary. Endothelial cells of these capillaries are tightly joined as part of the blood-nerve barrier and regulate the kinds of plasma substances released to the endoneurium. Larger blood vessels course through the deep epineurium that fills the space around the perineurium and fascicles. X450.   Figure 9-26