Nervous System Cells

The Nervous System

• The Nervous system is responsible for communication

• Composed of the:• Brain • Spinal Cord • Nerves

Organization of the Nervous System • Central Nervous System

(CNS) • Brain • Spinal Cord

• Peripheral Nervous System (PNS)• Cranial Nerves • Spinal Nerves

Afferent and Efferent Divisions

• Afferent Division – consists of all of the incoming sensory pathways

• Efferent Division – Consists of all of the outgoing motor pathways

Somatic and Autonomic Divisions

• Somatic Nervous System (SNS) • Somatic Motor Division – carries

information to the somatic effectors (skeletal muscles)

• Somatic Sensory Division – carries feedback information to somatic integration centers in the CNS

Autonomic Nervous System (ANS)

• Efferent division of the ANS carries info to the autonomic or visceral effectors (smooth and cardiac muscle)• Sympathetic division – “fight-or-flight” response• Parasympathetic division – “rest-and-repair”

division

• Visceral Sensory division carries feedback info to autonomic integrating centers of the CNS

Cells of the Nervous System

• Neurons – excitable cells that conduct the impulses that make possible all nervous system functions

• Glia – (glial cells) support the function of neurons in various ways

Glia (Neuroglia)

• 900 billion in the human nervous system

• Retain their capacity for division (makes them susceptible to cancer)

Astrocytes

• Found only in the CNS

• Largest and most numerous

• Feed the neurons

• “Blood-brain barrier”

Microglia

• Small, stationary cells found in the CNS

• In inflamed tissue they enlarge and move about in order to carry out phagocytosis of microorganisms and cellular debris

Ependymal Cells

• Resemble epithelial cells

• Form thin sheets that line the fluid cavities of the brain

• Some secrete the fluid

• Cilia to circulate fluid

Oligodendrocytes

• Cell with few branches

• Help hold nerve fibers together

• Produce the fatty myelin sheath around nerve fibers in the CNS

Schwann Cells

• Found only in the PNS

• Functional equivalent of oligodendrocytes

• Wrap around nerve fiber

Nodes of Ranvier

• Short space in the myelin sheath between adjacent Schwann cells

Multiple Sclerosis

Neurons

• The human brain contains about 100 billion neurons

• All neruons consist of a cell body and at least two processes–an axon and a dendrite

Structural Classification of Neurons• Multipolar – have only one axon but several

dendrites

• Bipolar – have only one axon and only one highly branched dendrite

• Unipolar – have only a single process extending from the cell body

Structural Classification

Functional Classification

• Neurons can be classified according to the direction in which they conduct impulses• Afferent neurons – transmit to the spinal cord

or brain• Efferent neurons – transmit away from the

brain or spinal cord• Interneurons – conduct impulses toward motor

neurons (entirely within the CNS)

Reflex Arc

Nerves and Tracts

• Nerves are bundles of peripheral nerve fibers held together by several layers of connective tissue • Endoneurium – surrounds each nerve fiber• Perineurium – surrounds each fascicle • Epineurium – surround the nerve

• In the CNS bundles of nerve fibers are called tracts

Nerve Structure