Func?ons of the Nervous System 11 Fundamentals of the Nervous System and Nervous Tissue: Part A 1. Sensory input Informa?on gathered by sensory receptors about internal and external changes 2. Integra?on Interpreta?on of sensory input 3. Motor output Ac?va?on of effector organs (muscles and glands) produces a response Sensory input Integration Divisions of the Nervous System Central nervous system (CNS) Brain and spinal cord Integra?on and command center Peripheral nervous system (PNS) Paired spinal and cranial nerves carry messages to and from the CNS Motor output Figure 11.1 1
Peripheral Nervous System (PNS) Two func?onal divisions 1. Sensory (afferent) division Soma?c afferent fibers convey impulses from skin, skeletal muscles, and joints Visceral afferent fibers convey impulses from visceral organs 2. Motor (efferent) division Transmits impulses from the CNS to effector organs Motor Division of PNS 1. Soma?c (voluntary) nervous system Conscious control of skeletal muscles Motor Division of PNS Central nervous system (CNS) Brain and spinal cord Integrative and control centers Peripheral nervous system (PNS) Cranial nerves and spinal nerves Communication lines between the CNS and the rest of the body 2. Autonomic (involuntary) nervous system (ANS) Visceral motor nerve fibers Regulates smooth muscle, cardiac muscle, and glands Two func?onal subdivisions Sympathe?c Parasympathe?c Sensory (afferent) division Motor (efferent) division Somatic and visceral sensory Motor nerve fibers nerve fibers Conducts impulses from the CNS Conducts impulses from to effectors (muscles and glands) receptors to the CNS Somatic sensory Somatic nervous Autonomic nervous fiber Skin system system (ANS) Somatic motor Visceral motor (voluntary) (involuntary) Conducts impulses Conducts impulses from the CNS to from the CNS to skeletal muscles cardiac muscles, Visceral sensory fiber smooth muscles, Stomach and glands Skeletal muscle Motor fiber of somatic nervous system Sympathetic division Parasympathetic Mobilizes body division systems during activity Conserves energy Promotes housekeeping functions during rest Sympathetic motor fiber of ANS Heart Structure Function Sensory (afferent) division of PNS Parasympathetic motor fiber of ANS Bladder Motor (efferent) division of PNS Figure 11.2 2
Histology of Nervous Tissue Two principal cell types 1. excitable cells that transmit electrical signals Histology of Nervous Tissue 2. (glial cells) suppor?ng cells: Astrocytes (CNS) Microglia (CNS) Ependymal cells (CNS) Oligodendrocytes (CNS) Satellite cells (PNS) Schwann cells (PNS) Most abundant, versa?le, and highly branched glial cells Cling to neurons, synap?c endings, and capillaries and brace neurons Astrocytes Help determine capillary permeability Guide migra?on of young neurons Control the chemical environment Par?cipate in informa?on processing in the brain 3
Capillary Neuron Astrocyte Small, ovoid cells with thorny processes Migrate toward injured neurons Phagocy?ze microorganisms and neuronal debris (a) Astrocytes are the most abundant CNS neuroglia. Figure 11.3a Cells Neuron Microglial cell Range in shape from squamous to columnar May be Line the central cavi?es of the brain and spinal column Separate the CNS inters??al fluid from the cerebrospinal fluid in the cavi?es (b) Microglial cells are defensive cells in the CNS. Figure 11.3b 4
Fluid-filled cavity (c) Ependymal cells line cerebrospinal fluid-filled cavities. Ependymal cells Brain or spinal cord tissue Branched cells Processes wrap CNS nerve fibers, forming insula?ng myelin sheaths Figure 11.3c Myelin sheath Process of oligodendrocyte Nerve fibers (d) Oligodendrocytes have processes that form myelin sheaths around CNS nerve fibers. Cells and Cells Satellite cells Surround neuron cell bodies in the PNS Schwann cells (neurolemmocytes) Surround peripheral nerve fibers and form myelin sheaths Vital to regenera?on of damaged peripheral nerve fibers Figure 11.3d 5
Satellite cells Cell body of neuron Schwann cells (forming myelin sheath) Nerve fiber (e) Satellite cells and Schwann cells (which form myelin) surround neurons in the PNS. (Nerve Cells) Special characteris?cs: Long- lived ( 100 years or more) Amito?c with few excep?ons High metabolic rate depends on con?nuous supply of oxygen and glucose Plasma membrane func?ons in: Electrical signaling Cell- to- cell interac?ons during development Figure 11.3e Cell Body (Perikaryon or Soma) Biosynthe?c of a neuron Spherical nucleus with nucleolus Well- developed Rough ER called Nissl bodies (chromatophilic substance) Cell Body (Perikaryon or Soma) Network of neurofibrils (neurofilaments) cone- shaped area from which axon arises Clusters of cell bodies are called nuclei in the CNS, ganglia in the PNS 6
Dendrites (receptive regions) Cell body (biosynthetic center and receptive region) Processes Nucleus Nissl bodies (b) Axon hillock Nucleolus Neurilemma Axon (impulse generating and conducting region) Impulse direction Node of Ranvier Schwann cell (one internode) Terminal branches Axon terminals (secretory region) and axons Bundles of processes are called Tracts in the CNS Nerves in the PNS Figure 11.4b Dendrites Short, tapering, and diffusely branched (input) region of a neuron Convey electrical signals toward the cell body as graded poten?als The Axon One axon per cell arising from the axon hillock Long axons (nerve fibers) 7
The Axon Numerous branches (telodendria) Knoblike axon terminals (synap?c knobs or boutons) region of neuron Release neurotransmifers to excite or inhibit other cells Axons: Func?on region of a neuron Generates and transmits nerve impulses (ac?on poten?als) away from the cell body Axons: Func?on Dendrites (receptive regions) Cell body (biosynthetic center and receptive region) Molecules and organelles are moved along axons by motor molecules in two direc?ons: toward axonal terminal Examples: mitochondria, membrane components, enzymes toward the cell body Examples: organelles to be degraded, signal molecules, viruses, and bacterial toxins Nucleus Nissl bodies (b) Axon hillock Nucleolus Neurilemma Axon (impulse generating and conducting region) Impulse direction Node of Ranvier Schwann cell (one internode) Terminal branches Axon terminals (secretory region) Figure 11.4b 8
Myelin Sheath Segmented protein- lipoid sheath around most long or large- diameter axons It func?ons to: Protect and electrically the axon Increase of nerve impulse transmission Myelin Sheaths in the PNS cells wraps many?mes around the axon Myelin sheath concentric layers of Schwann cell membrane Neurilemma peripheral bulge of Schwann cell cytoplasm Myelin Sheaths in the PNS of Ranvier Myelin sheath gaps between adjacent Schwann cells Sites where axon collaterals can emerge Schwann cell plasma membrane Schwann cell cytoplasm Axon 1 A Schwann cell envelopes an axon. Schwann cell nucleus 2 The Schwann cell then rotates around the axon, wrapping its plasma membrane loosely around it in successive layers. Neurilemma Myelin sheath (a) Myelination of a nerve fiber (axon) 3 The Schwann cell cytoplasm is forced from between the membranes. The tight membrane wrappings surrounding the axon form the myelin sheath. Figure 11.5a 9
Unmyelinated Axons Thin nerve fibers are One Schwann cell may incompletely enclose 15 or more unmyelinated axons Myelin Sheaths in the CNS Formed by processes of, not the whole cells Nodes of Ranvier are present No neurilemma Thinnest fibers are White Mafer and Gray Mafer Myelin sheath Process of oligodendrocyte Nerve fibers mafer Dense collec?ons of myelinated fibers mafer Mostly neuron cell bodies and unmyelinated fibers (d) Oligodendrocytes have processes that form myelin sheaths around CNS nerve fibers. Figure 11.3d 10
Structural Classifica?on of Neurons Three types: 1. 1 axon and several dendrites Most abundant Motor neurons and interneurons 2. 1 axon and 1 dendrite Rare, e.g., re?nal neurons Structural Classifica?on of Neurons 3. (pseudounipolar) single, short process that has two branches: Peripheral process more distal branch, oien associated with a sensory receptor Central process branch entering the CNS Table 11.1 (1 of 3) Table 11.1 (2 of 3) 11
Func?onal Classifica?on of Neurons Three types: 1. (afferent) Transmit impulses from sensory receptors toward the CNS 2. (efferent) Carry impulses from the CNS to effectors Func?onal Classifica?on of Neurons 3. (associa?on neurons) Shufle signals through CNS pathways; most are en?rely within the CNS Table 11.1 (3 of 3) 12