Download Essentials of Human Anatomy

Essentials of Human Anatomy
Nervous System I
Chapter 7
Dr Fadel Naim
Ass. Prof. Faculty of Medicine
IUG
Introduction
• The function of the nervous system,
along with the endocrine system, is to
communicate
• The nervous system is made up of the
brain, the spinal cord, and the nerves
Functions of Nervous System
Sensory Function
• sensory receptors gather
information
• information is carried to the
CNS
Integrative Function
• sensory information used to
create
• sensations
• memory
• thoughts
• decisions
Motor Function
• decisions are acted
upon
• impulses are
carried to effectors
Organization of the Nervous System
• Organized to detect changes in internal and external
environments, evaluate the information, and initiate an
appropriate response
• Subdivided into smaller “systems” by location:
– 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
Divisions of Peripheral Nervous System
Sensory Division
• picks up sensory information and delivers it to the CNS
Motor Division
• carries information to muscles and glands
Divisions of the Motor Division
• Somatic – carries information to skeletal muscle
• Autonomic – carries information to smooth muscle,
cardiac muscle, and glands
Divisions Nervous System
Organization of the Nervous System
• “Systems” according to the types of
organs they innervate
– 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
Organization of the Nervous System
• Autonomic nervous system (ANS)
• Efferent division of ANS—carries information to
the autonomic or visceral effectors (smooth and
cardiac muscles and glands)
– Sympathetic division—prepares the body to deal with
immediate threats to the internal environment
– Parasympathetic division—coordinates the body’s
normal resting activities
• Visceral sensory division—carries feedback
information to autonomic integrating centers in the
CNS
Organization of the Nervous System
• Afferent and efferent divisions
– Afferent division—consists of all incoming
sensory pathways
– Efferent division—consists of all outgoing
motor pathways
Sensory Division
• Somatic sensory components:
– General somatic senses:
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touch
pain
pressure
vibration,
temperature
proprioception.
– Special senses:
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Taste
Vision
Hearing
Balance
smell
Sensory Division
• Visceral sensory components
– transmit nerve impulses from blood vessels and
viscera to the CNS
– visceral senses primarily include:
• temperature
• stretch (of the organ wall).
Motor Division
 The somatic motor component (somatic nervous
system; SNS):
– conducts nerve impulses from the CNS to skeletal muscles
– also known as the voluntary nervous system
• The autonomic motor component (autonomic nervous
system; ANS): internal organs, regulates smooth muscle,
cardiac muscle, and glands.
– Innervates
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Internal organs
Regulates smooth muscle
Regulates cardiac muscle
Regulates glands
– also known as the visceral motor system or involuntary nervous
system
Nerve Cells
• Nervous Tissue
– Two distinct cell types
• Neurons
– excitable cells
– initiate and transmit nerve impulses
• Glial cells
– nonexcitable cells
– support and protect the neurons
Characteristics of Neurons
• Neurons have a high metabolic rate.
• Neurons have extreme longevity.
• Neurons typically are non-mitotic.
Neuron Structure
• Neurons come in all shapes and sizes
• All neurons share certain basic structural
features.
• typical neuron:
– Cell body (soma)
– Dendrites
– Axon
Neuron Structure – Cell Body
• The cell body
– the neuron’s control center
• responsible for:
– receiving
– integrating
– sending nerve impulses.
– Consists of:
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Plasma membrane
Cytoplasm
Nucleus with prominent nucleolus
Chromatophobic substance (Nissil bodies): RER
Free ribosomes
Cells of the Nervous System
– Components of
neurons
• Dendrites
– Each neuron has one or
more dendrites, which
branch from
the cell body
– Conduct nerve signals to
the cell body of the
neuron
– Distal ends of dendrites
of sensory neurons are
receptors
Cells of the Nervous System
– Components of
neurons
• Axon
– A single process
extending from the axon
hillock, sometimes
covered by a fatty layer
called a myelin sheath
– Conducts nerve
impulses away from the
cell body of the neuron
Neuron Structure – Axon
• Structures
– Collaterals
– Telodendria (axon terminals)
– Synaptic knobs (terminal boutons)
• The axon transmits a nerve impulse away
from the cell body toward another cell.
Classifications of Neurons
• Neurons vary widely in morphology and location.
– classified based on
• structure
• function.
• Structural classification: number of processes
extending from the cell body.
– unipolar neuron has a single process
– bipolar neurons have two processes
– multipolar neurons have three or more processes
Classification of Neurons – Structural
Differences
Unipolar
• one process
• ganglia
Bipolar
• two processes
• eyes, ears, nose
Multipolar
• many processes
• most neurons of
CNS
Classification of Neurons –
Functional Differences
Sensory Neurons
• afferent
• carry impulse to CNS
• most are unipolar
• some are bipolar
Interneurons
• link neurons
• multipolar
• in CNS
Motor Neurons
• multipolar
• carry impulses away
from CNS
• carry impulses to
effectors
Nerves
• Nerves are organs of the PNS.
• Sensory (afferent) nerves convey sensory information to
the CNS.
• Motor (efferent) nerves convey motor impulses from the
CNS to the muscles and glands.
• Mixed nerves: both sensory and motor
• Axons terminate as they contact other neurons, muscle
cells, or gland cells.
• An axon transmits a nerve impulse at a specialized
junction with another neuron called synapse.
Peripheral
Nerves
Organization – coverings:
•Epineurium
wraps entire nerve
•Perineurium
wraps fascicles of tracts
•Endoneurium
wraps individual axons
Repair of Nerve Fibers
• Mature neurons are incapable of cell
division; therefore, damage to nervous
tissue can be permanent
• Neurons have limited capacity to repair
themselves
• Nerve fibers can be repaired if the damage
is not extensive, the cell body and
neurilemma are intact, and scarring has
not occurred
Regeneration of PNS Axons
• PNS axons are vulnerable to cuts and trauma.
• A damaged axon can regenerate
– if some neurilemma remains.
• PNS axon regeneration depends upon three
factors.
– amount of damage
– neurolemmocyte secretion of nerve growth factors
• stimulates outgrowth of severed axons
– distance between the site of the damaged axon and the
effector organ
Repair of Nerve Fibers
• Wallerian degeneration:
• Stages of repair of an axon in a peripheral motor neuron
– Following injury, distal portion of axon and myelin sheath degenerates
– Macrophages remove the debris
– Remaining neurilemma and endoneurium form a tunnel from the point of injury
to the effector
– New Schwann cells grow in the tunnel to maintain a path for regrowth of the axon
– Cell body reorganizes its Nissl bodies to provide the needed proteins to extend the
remaining healthy portion of the axon
– Axon “sprouts” appear
– When “sprout” reaches tunnel, its growth rate increases
– The skeletal muscle cell atrophies until the nervous connection is reestablished
• In CNS, similar repair of damaged nerve fibers is unlikely
THE END