2. Chapter 12 Nervous System Cells

3. Introduction The function of the nervous system, along with the endocrine system, is to communicate –Controls and integrates many different functions performed by different organs, tissues, and cells –Communication, Control, Integration, Homeostasis,Survival The nervous system is made up of the –Brain –Spinal cord –Nerves

4. Organization of the Nervous System Organized to… –Detect changes in internal and external environments –Evaluate the information –Initiate an appropriate response Central nervous system (CNS) –Structural and functional center of entire nervous system –Consists of the brain and the spinal cord –Integrates sensory information, evaluates it, and initiates an outgoing response Peripheral nervous system (PNS) –Nerves that lie in “outer regions” of nervous system –Cranial nerves—originate from brain –Spinal nerves—originate from spinal cord

5. Organization of the Nervous System Divisions of the nervous system –Afferent division - incoming sensory pathways –Efferent division - outgoing motor pathways Somatic Nervous System (SNS) – skeletal muscles –Somatic motor division (efferent) –Somatic sensory division (afferent) –Somatic integration centers Autonomic Nervous System (ANS) – smooth/cardiac muscles, glands –Autonomous of voluntary control –Sympathetic division – “fight or flight” (efferent) Middle portions of the spinal cord –Parasympathetic division – “rest and repair” (efferent) Brain and lower portions of spinal cord –Visceral sensory division (afferent) –Autonomic integration centers

7. Cells of the Nervous System Glia –Support the functions of neurons –900 billion in the human nervous system –Ability to do cell division throughout adulthood –Susceptible to abnormalities…cancer –Five types Astrocytes Microglia Ependymal cells Oligodendrocytes Schwann cells

9. Cells of the Nervous System Astrocytes –Star-shaped –Largest –Most numerous type –Cell extensions connect to both neurons and capillaries –Transfer nutrients from the blood to the neurons –Constitute the blood-brain barrier (BBB)

10. Cells of the Nervous System Microglia –Small, usually stationary –In inflamed brain tissue, they enlarge, move about, and carry on phagocytosis Ependymal cells –Resemble epithelial cells and form thin sheets that line fluid-filled cavities in the CNS –Some produce fluid; others aid in circulation of fluid

11. Cells of the Nervous System Oligodendrocytes –Smaller than astrocytes with fewer processes –Hold nerve fibers together and produce the myelin sheath

12. Cells of the Nervous System Schwann cells –Found only in PNS –Support nerve fibers and form myelin sheaths (Figure 12-4) –Gaps in the myelin sheath are called nodes of Ranvier –Neurilemma is formed by cytoplasm of Schwann cell (neurilemmocyte) wrapped around the myelin sheath; essential for nerve regrowth –Satellite cells are Schwann cells that cover and support cell bodies in the PNS

13. Cells of the Nervous System Neurons –Excitable cells that initiate and conduct impulses –Components of neurons –100 billion in the brain (10% of nervous system cells) –Structure Cell body (perikaryon) –Nucleus, cytoplasm, mitochondria, Golgi apparatus, ribosomes, endoplasmic reticulum (Nissl bodies) Dendrites –Receive the stimuli and initiate nerve signals –Conduct nerve signals to the cell body Axon –A single process extending from the axon hillock –Axon collaterals (side branches) –Sometimes covered by a fatty layer called a myelin sheath –Conducts nerve impulses away from the cell body of the neuron –Distal tips of axons are telodendria, each of which terminates in a synaptic knob –Thicker diameter = faster impulse

16. Cells of the Nervous System Classification of neurons –Structural classification—classified according to number of processes extending from cell body Multipolar –One axon and several dendrites Bipolar –Only one axon and one dendrite –Least numerous kind of neuron Unipolar (pseudounipolar) –One process comes off neuron cell body but divides almost immediately into two fibers »Central fiber (toward CNS) »Peripheral fiber (away from CNS)

18. Cells of the Nervous System Classification of neurons –Functional classification Afferent (sensory) neurons –Conduct impulses to spinal cord or brain Efferent (motor) neurons –Conduct impulses away from spinal cord or brain toward muscles or glandular tissue Interneurons –Entirely within CNS

19. Cells of the Nervous System Reflex arc –A signal conduction route to and from the CNS Electrical signal beginning in receptors and ending in effectors –Three-neuron arc Most common Consists of afferent neurons, interneurons, and efferent neurons Afferent neurons—conduct impulses to the CNS from the receptors Efferent neurons—conduct impulses from the CNS to effectors (muscle or glandular tissue) –Two-neuron arc Simplest form Consists of afferent and efferent neurons

20. Cells of the Nervous System Reflex arc (cont.) –Synapse Where nerve signals are transmitted from one neuron to another Located at the junction of the synaptic knob of one neuron and the dendrites or cell body of another neuron Electrical and chemical synapses (more on this later) –Ipsilateral reflex arc Receptors and effectors located on the same side of body –Contralateral reflex arc Receptors and effectors located on opposite sides of the body –Many signals fail to be conducted across a synapse –Not all signals that terminate in effectors start in receptors

22. Nerves and Tracts Nerves –Bundles of peripheral nerve fibers held together by several layers of connective tissue Endoneurium—delicate layer of fibrous connective tissue surrounding each nerve fiber Perineurium—connective tissue holding together fascicles (bundles of fibers) Epineurium—fibrous coat surrounding numerous fascicles and blood vessels to form a complete nerve Tracts –Within the CNS, bundles of nerve fibers are called tracts rather than nerves

23. Nerves and Tracts White matter –PNS—myelinated nerves –CNS—myelinated tracts Gray matter –Made up of cell bodies and unmyelinated fibers –CNS—referred to as nuclei –PNS—referred to as ganglia Mixed nerves –Contain sensory and motor neurons –Sensory nerves—predominantly sensory neurons –Motor nerves—predominantly motor neurons