Fundamentals of the Nervous System and Nervous Tissue Chapter 11

Nervous System The master controlling and communicating system of the body Functions Sensory input – monitoring stimuli occurring inside and outside the body Integration – interpretation of sensory input Motor output – response to stimuli by activating effector organs

Levels of Organization Central Nervous System (CNS) Brain and Spinal Cord Integration and Control Centers Associative neurons (interneurons) Peripheral Nervous System (PNS)- Cranial and Spinal Nerves Sensory - afferent -input to CNS Somatic fibers – impulses from skin, skeletal muscles, & joints Visceral fibers – transmit impulses from visceral organs Motor -efferent-output from CNS Somatic -voluntary control (skeletal muscles) Autonomic –involuntary (glands, cardiac and visceral muscles) Sympathetic –stress Parasympathetic - normal conditions Levels of Organization

Histology of Nerve Tissue The two principal cell types of the nervous system are: Neurons – excitable cells that transmit electrical signals Supporting cells – cells that surround and wrap neurons The supporting cells (neuroglia or glial cells): Provide a supportive scaffolding for neurons Segregate and insulate neurons Guide neurons to the proper connections Promote health and growth Astrocytes Most abundant, versatile, and highly branched glial cells They cling to neurons and their synaptic endings, and cover capillaries Functionally, they: Support and brace neurons Anchor neurons to their nutrient supplies Guide migration of young neurons Control the chemical environment

Neuroglia (cont.) Central Nervous System Glial Cells Microglia – small, ovoid cells with spiny processes Phagocytes that monitor the health of neurons Ependymal cells – range in shape from squamous to columnar They line the central cavities of the brain and spinal column Oligodendrocytes – branched cells that wrap CNS nerve fibers Peripheral Nervous System Glial Cells Schwann cells (neurolemmocytes) – surround fibers of the PNS Satellite cells surround neuron cell bodies with ganglia

Neuron Excitable cells that generate and conduct impulses Amitotic - extreme longevity High metabolic rate – require glucose and O2 Structure Cell body - site of protein synthesis nucleus with prominent nucleolus,no centrioles Nissl bodies (chromatophilic substance) Dendrites - carry impulses toward cell body many per neuron, short and branched respond to neurotransmitters postsynaptic terminal Axon next page

Myelinated Axons Axons - carry impulses away from cell body long thin process, one per neuron branched at end (axon terminals -presynaptic knobs) secrete neurotransmitters from the axonal terminals Schwann cells form Myelin Sheath: Envelopes an axon in a trough Encloses the axon with its plasma membrane Has concentric layers of membrane that make up the myelin sheath Nodes of Ranvier – gaps between Schwann cells. Neurilemma – remaining nucleus and cytoplasm of a Schwann cell

Nerve Fibers (Axons) and Myelin Sheath Axon conduction speed depends on:: Diameter of axon Degree of myelination Myelin Sheath Whitish, fatty (protein-lipoid cell membrane) Protect the axon Electrically insulate fibers from one another Increase the speed of nerve impulse transmission Unmyelinated axons Schwann cell surrounds nerve fibers but coiling does not take place Schwann cells partially enclose 15 or more axons Axons of the CNS Both myelinated and unmyelinated fibers are present Myelin sheaths are formed by oligodendrocytes White matter – dense collections of myelinated fibers Gray matter – mostly soma and unmyelinated fibers

Classification of Neurons Multipolar - motor, associative many extensions most numerous Bipolar - special sensory two extensions olfactory, rods and cones not common Unipolar - most sensory one process from cell body peripheral extension responds to a stimuli cell bodies form ganglia Reflex Arc Neural Circuits

Electrical Current and the Body - Neurophysiology Reflects the flow of ions rather than electrons There is a polarized potential on either side of membranes when: The number of ions is different across the membrane (ion gradient) The membrane provides a resistance to ion flow until a stimulus occurs. Resting Membrane Potential - polarized Na+ high outside cell K+ high inside cell Negative ions do not move across the cell membrane Na+K+ pump maintains the ion concentration K+ leak out faster than Na+ leak in  negative inner membrane For a nerve impulse to be propagated in a nerve cell, this resting potential (polarization) must be disturbed or depolarized. Action potentials, or nerve impulses, are: Electrical impulses carried along the length of axons Always the same regardless of stimulus

Types of plasma membrane ion channels: Found all over the neuron – maintain resting membrane potential Passive, or leakage, channels – always open Normally found at Synapse to regulate neurotransmission Chemically gated channels (ligand gated)– open with binding of a specific neurotransmitter Normally found in Axon for conduction of an Action Potential Voltage-gated channels – open and close in response to membrane potential PLAY

Voltage Ion Channels and Action Potential Return

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