Download Nervous System – Ch 7

Nervous System – Ch 7
Introduction
Neurons – nerve cells
 Transmit information in the form of electrochemical
changes (nerve impulses)
 Neuron structure
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Cell body
Dendrites – numerous; receive messages
Axons – send information; usually one per neuron
Nerves – bundles of axons
Neuroglial cells – supporting cells that provide neurons
with physiological requirements and function
Two groups of nervous system:
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Central nervous system (CNS) – brain and spinal cord
Peripheral Nervous System (PNS) – nerves and other body parts
General Functions of Nervous
System
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Sensory Function
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Sensory receptors gather information by detecting changes
inside and outside the body (light, sound, temp, O2
concentration)
• Convert information into nerve impulses that are integrated, added
to memory, or translate to sensations
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Integrative Function
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Conscious or subconscious decisions
Motor Functions
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Use peripheral neurons to carry impulses to effectors
(responsive structure)
Two categories
• Somatic nervous system – consciously controlled; skeletal muscle
• Autonomic nervous System – involuntary; heart muscle
Neuroglial Cells
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Neuroglia
Functions:
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Fill spaces
 provide structural framework
 produce myelin
 carry on phagocytosis
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Types of Neuroglial Cells:
Microglial cells: scattered through CNS; support neurons and
phagocytize bacterial cells and cell debris
Oligodendrocytes: occur in nerve fibers; provide myelin around
axons in brain and spinal cord
Astrocytes: found between neurons and blood vessels; provide
structural support, join parts, regulate nutrient and ion concentration
in tissues, form scar tissue
Ependymal cells: form epithelial-like membrane to cover specialized
brain parts (choroid plexuses) and form inner linings that enclose
brain spaces (ventricles) and spinal cord (central canal)
The PNS contains neuroglial called Schwann cells that form the
myelin sheath around axons.
Neuron Structure
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Vary in size and shape
 Structure:
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Neuron cell body
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Consist of cytoplasm, cell membrane, and organelles
Chromatophilic substance (Nissl bodies) similar to rough ER
also contain ribosomes
Contain a nucleus and nucleolus
Do not divide
Dendrites
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Short, highly branched
Main receptive surface
Axons
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Arise from elevation of cell body (axonal hillock) to conduct nerve
impulses away from the cell body
Many mitochondria, microtubules, neurofibrils
Single structure that may have many branches
Large ones are covered by sheaths of Schwann cells (membrane
covering). Membrane is composed of myelin (lipoprotein)
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Myelin sheath - higher proportion of lipids in cell membrane
 Neurilemmal sheath – surrounds myelin sheath; contains cytoplasm and
nuclei
 Nodes of Ranvier – gaps between Schwann cells
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With myelin sheath – myelinated
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Also found in CNS
Appear white – white matter
Unmyelinated axons and neurons form the gray matter
When damaged, the PNS regenerate due to neurilemma; the CNS
do not because of lack of neurilemma
Classifying Neurons
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Differ in size, structure, shape of cell bodies
 Vary in length and size of axons and dendrites
 Vary in function
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Some carry impulses to brain or spinal cord
Some transmit impulses out of brain or spinal cord
Some conduct impulses from neuron to neuron
Bipolar Neurons
 Cell
body has two processes, one from
each end
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One is axon, one is dendrite
 Eyes,
nose, ear neurons
Unipolar Neurons
 Single
process from body that
branches into two that
function as a single axon
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 In
Peripheral process – dendrite
association near peripheral
body parts
Central process – enters the
brain or spinal cord
specialized masses called
ganglia
Multipolar Neurons
 Many
processes from cell
body
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One is an axon
All the others are dendrites
 Most
neurons in brain and
spinal cord are multipolar
Sensory Neurons
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Also called afferent neurons
Carry nerve impulses from peripheral body to
brain or spinal cord
Have specialized receptor ends on dendrites OR
dendrites are closely related with receptor cells
in skin and sensory organs
Changes stimulate receptors, triggering a nerve
impulse
Most are unipolar, some are bipolar
Interneurons
 Also
called association or internuncial
neurons
 Lie within brain or spinal cord
 Multipolar and link other neurons
 Transmit impulses from one part of brain
or spinal cord to another
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Direct incoming sensory impulses
Motor Neurons
 Efferent
Neurons
 Multipolar
 Carry nerve impulses out of the brain or
spinal cord to effectors
 Stimulate muscles to contract and glands
to release secretions
Synapse
Nerve impulses travel from neuron to neuron
along pathways.
The junction between communicating
neurons is a synapse.
Gap between neurons is synaptic cleft.
Neurotransmitters
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Chemical that reacts with specific
receptors to create a nerve impulse
Acetylcholine – controls skeletal muscle actions
Norepinephrine – “good” feeling, low levels may
lead to depression
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Dopamine – “good” feeling
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Serotonin – sleep
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Histamine – alertness
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Endorphins – reduce pain
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Nitric oxide – vasodilation, memory
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pg. 218 chart of neurotransmitters
Types of Nerves
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Nerves are cordlike bundles of nerve
fibers held together by connective tissue
layers that conduct impulses
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Sensory nerves – impulses into the brain
or spinal cord
Motor nerves – carry impulses to
muscles or glands
Mixed nerves – include both of the above
Nerve Pathways
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Routes that nerve impulses follow in the nervous
system
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Reflex arcs are the simplest pathways that constitute
reflexes.
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Reflexes are subconscious responses to stimuli within
or outside the body. Help maintain involuntary actions
such as heart rate, breathing rate, blood pressure and
digestion
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Knee-jerk reflex – simple, only using two
neurons; helps maintain upright posture
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Withdrawal reflex – unexpected touch to
something painful; protective by limiting tissue
damage
Meninges
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Membranes beneath the bony coverings of the skull and
vertebral column for protection
Three layers
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Dura mater – outermost layer of connective tissue,
blood vessels, and nerves; interior periosteum of
skull bones and between lobes of brain; surround
spinal cord for protection.
Arachnoid mater – thin membrane lacking blood
vessels in the middle
Pia Mater – thin layer of nerves and blood vessels
that nourish the cells of the brain and spinal cord;
hugs surfaces of organs
Cerebrospinal fluid (CSF) – in between arachnoid
and pia maters.
Spinal Cord
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Nerve column passing from brain into
vertebral canal
Begins at the foramen magnum and
terminates at the first and second lumbar
vertebrae.
Structure of Spinal Cord
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Consists of 31 segments that give rise to spinal
nerves which branch to various body parts
Cervical enlargement is the thickening of the
spinal cord in the neck region that supplies nerves
to upper limbs
The lumbar enlargement is in the lower back
giving nerves to lower limbs.
Divided into right and left halves by the anterior
median fissure and posterior median sulcus
grooves.
Functions of Spinal Cord
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Two major functions: conducting nerve impulses
and center for spinal reflexes
Axons of the spinal cord provide two way
communication between the brain and the body
parts
Ascending tracts carry sensory information to the
brain
Descending tracts conduct motor impulses from
the brain to the muscles
Knee-jerk and withdrawal reflexes are spinal
reflexes because they pass through the spinal
cord