The Nervous System: Neural Tissue

Introduction –Nervous system = control center & communications network –Functions  Stimulates movements  Maintains homeostasis (with endocrine system)

Organization of the Nervous System –Central nervous system (CNS)  Brain & spinal cord –Peripheral Nervous System (PNS)  Afferent (Sensory) System  Efferent (Motor) System –Somatic Nervous System –Autonomic Nervous System  Sympathetic Nervous System  Parasympathetic Nervous System

The Nervous System

Organization of the Nervous System

Histology of Nervous Tissue –2 types of cells  Neurons –Structural & functional part of nervous system –Specialized functions  Neuroglia (glial cells) –Support & protection of nervous system

Neuroglia of CNS

Neurons –Function  Conduct electrical impulses –Structure  Cell body –Nucleus with nucleolus –Cytoplasm  Cytoplasmic processes –Dendrites –Axon

Review of Neuron Structure

Anatomy of a Neuron

Nerve Fibers of the PNS –An axon and its sheaths  Myelinated axon –Axon is surrounded by a myelin sheath  Unmyelinated axon –Axon has no myelin sheath

Myelin –White matter of nerves, brain, spinal cord –Composed primarily of phospholipids –Production  Developing Schwann cells wind around axon –Function  Increases speed of impulse conduction  Insulation and maintenance of axon

Schwann Cells and Peripheral Axons

Myelin –Nodes of Ranvier  Unmyelinated gaps between segments of myelin  Impulses “jump” from node to node

A Myelinated Axon

Nerve Fibers of the CNS –Umyelinated –Myelinated  Production of myelin is from oligodendrocytes  Nodes of Ranvier are less numerous

Nerve Fibers of the CNS

Classification of Neurons –Functional – based on the direction of impulse transmission  Sensory neurons  Motor neurons  Interneurons (association)

Functional Classification of Neurons

Nerves –Bundles of cell processes of afferent and efferent neurons –Functional unit = nerve fiber

Nerve Impulse –A change in charge that travels as a wave along the membrane of a neuron –Depends on the movement of K+ and Na+ across the nerve cell membrane.

Resting Neuron –Sodium ions are in large concentration along the outside of the cell membrane –Potassium ions are in large concentration along the inside of the cell membrane

Resting Potential

Action Potential –Requires a stimulus of adequate strength –Membrane is disturbed –Sodium ions flow inward, potassium ions flow outward  Action potential –The disturbance at the point of stimulation is enough to disturb adjacent portion of membrane. –The action potential moves along the membrane.

Action Potential

Back to Resting Potential –Potassium and sodium ions are actively transported back across the membrane –Membrane is at rest again

Neurons Exhibit Conductivity –The ability to transmit an impulse to another neuron or tissue. –Action potentials can be transmitted across synapses.  Junction between 2 neurons  Uses neurotransmitters  Transmission is one way only

Synapse

Neural Regeneration after Injury –Axon & myelin sheath distal to the injury deteriorate –Scwann cells form a cord, grow into the cut, and unite the stumps. –The axon forms buds and grows along the cord of Schwann cells –The axon continues to grow into the distal stump and is enfolded by Schwann cells. –(Rate of 1mm/day)

Neural Regeneration