Transcript Neural Tissue

Maintaining Homeostasis
• The nervous system and the endocrine
system share the responsiblities of
maintaining the internal homeostasis of the
human body.
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Control peripheral effectors
Sense the internal and external environment
Integrate sensory information
Coordinates voluntary and involuntary activity
Nervous System
• Mass of only 2kg it makes up
about 3% of total body weight
• Smallest and most complex of
11 body systems
• Composed of brain, spinal cord,
peripheral nerves, cranial nerves,
spinal nerves, ganglia, and
sensory receptors
Functions
• Sensory function
– Detection internal and external stimuli
– PNS to CNS
– sensory or afferent neuron
af- toward
• Integrative function
– Processes sensory information received from afferent neurons by
analyzing, storing and making decisions for appropriate responses
– Interneurons
• Motor function
– Responds to integration decisions made by interneurons causing
effector cells to respond
– CNS to PNS
– motor or efferent neuron
ef- away
Two Subdivisions
CNS
• Integration, processing
and coordination of
sensory data and
motor commands
• Higher functions
• Consists of brain &
spinal cord
PNS
• Brings sensory info
to CNS
• Brings motor
commands to
peripheral tissue
• Includes all tissue
outside CNS
Cellular Organization of Neural Tissue
Neurons
• Comprised of three components
– Soma or Cell Body
• Contains a large nucleus with a prominent
nucleolus
• Most neurons cannot divide (amitotic) because they
lack the centrioles that are essential for mitosis to
occur.
• Contain clusters of rough ER and free ribosomes
called Nissle bodies, where protein synthesis occurs
• Newly synthesized substances are conveyed in the
axoplasm
– Anterograde transport
• Movement from cell body to axon terminals
– Retrograde transport
• Movement of materials from axon terminal
back to the cell body
– Dendrites
• Highly branched process that joins the soma at the cone shaped axon hillock.
• Receive information from other neurons
– Axon
• Long cytoplasmic process attached to soma at the axon hillock
– Initial segment
• First part of axon
• Contains the trigger zone, area where impulses arise at the junction of the
axon hillock and the initial segment
– Axon terminals
• End of axon
• May have bulb-shaped structures called synaptic end bulbs containing
vesicles to store neurotransmitters for communication between the neuron
and another neuron or effector cell
• Along with synaptic terminals can conduct an action potential and propagate
it toward another neuron, a muscle fiber, or a gland cell
Structural Diversity in Neurons
Bipolar
•Have one main dendrite and one axon
•Found in the retina of the eye, inner ear, olfactory
area of the brain
Unipolar
•Sensory neurons that begin in the embryo as bipolar
neurons
•During development, the axon and dendrite fuse into
a single process that divides into two branches
•Axon branch that extends into the CNS ends in
synaptic end bulbs
•Axon branch that extends into the periphery has
dendrites at its distal end
Multipolar neurons
Anaxonic neurons
Rare, small, lack features
•Usually have several dendrites and one axon
•Most abundant in brain and spinal cord
Neuroglia of CNS
• Makes up about half the volume of the CNS
• Do not generate or propagate action potentials
therefore they do not function with memory
• Can readily multiply and divide
• Support, nourish, and protect neurons and
maintain homeostasis
Astrocytes
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Largest and most numberous of glial cells in
CNS
Star-shaped with many process
Form three-dimensional framework of
CNS, guide neuron development, maintain
blood-brain barrier, respond to neural
tissue damage
Oligodendrocyte
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Smaller than astrocytes, with fewer
processes
Form supporting network around CNS
neurons by producing myelin sheath
covering many CNS axons
Microglia
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Small wandering cells with few processes
that have many small branches
Protect CNS cells from disease by engulfing
invading microbes and debris of dead cells
Ependymal cells
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Epithelial cells arranged in a single layer;
either cuboidal or columnar; many cilliated
Line ventricles of the brain and central
canal of spinal cord
Form cerebrospinal fluid and assist in its
circulation
Peripheral Nervous System Neuroglia
Schwann cells
– Flattened cells that encircle PNS axons
– Produces part of the myelin sheath or neurilemma of PNS
axons
– Participate in regeneration of PNS axon
Satellite cells
– Flattened cells arranged around the cell bodies of neurons in
ganglia
– Support neurons within PNS ganglia
Myelination
• Most mammalian neurons are surrounded
by a multilayered lipid and protein covering
or myelin sheath
– Electrically insulates the axon of a neuron
– Increases the speed of nerve impulse
conduction
– Amount of myelin increases from birth to
maturity.
White & Gray Matter
• White matter
– Consists primarily of the myelinated axons of many
neurons
– Surrounds the gray matter in the spinal cord
• Gray Matter
– Contains primarily neuronal cell bodies, dendrites,
unmyelinated axons, axon terminals, and neuroglia
– Thin shell of gray matter covers the surface of the
cerebrum and cerebellum
SensoryReceptors
Exteroceptor
Provides information about the external
environment in the form of touch,
temperature, and pressure sensations
Proprioceptor
Monitors the position and movement of
skeletal muscles and joints
Interoceptor
Monitor the digestive, respiratory,
cardiovascular, urinary, and reproductive
systems and provide sensations of taste,
deep pressure, and pain
Neurons communicate with other cells
• Neurons are electrically excitable cells that
communicate with one another
– Graded potentials are used for short-distance
communication
– Action potentials allow communication over
both short and long distances within the body
Steps for Generation of an Action Potential
• Area of an excitable membrane brought to threshold by
graded depolarization
• Sodium channel activation occurs
• Sodium ions enter the cell and depolarization occurs
• Sodium channels are inactivated
• Voltage-gated potassium channels open and potassium
moves out of the cell, initiating repolarization
• Sodium channels regain their normal properties
• Temporary hyperpolarization occurs
Action Potential consists of Depolarizing and Repolarizing Phase
Ion Channels
• Graded potentials and action potentials occur because the plasma
membranes of neurons contain many different kinds of ion channels
that open or close in response to specific stimuli
– Leakage channels
• More potassium ion leakage channels than sodium ion leakage channels are
present
• membrane has high permeability to potassium than sodium
– Voltage-gated channel
• Opens in a response to membrane potential or voltage
• Participate in action potentials
– Ligand-gated channel
• Opens and closes in response to a specific chemical stimulus
• Chemical ligands include- neurotransmitters, hormones, and particular ions
– Mechanically-gated channel
• Opens and closes in response to mechanical stimulation in the form of vibration
such as sound waves, pressure, or tissue stretching
The Synapse
• Special junction between neurons or between a
neuron and an effector.
– Presynaptic neuron
• Neuron sending the signal
– Postsynaptic neuron
• Neuron receiving the message
• Dependent upon
– Quantity, nature, and frequency of neurotransmitter
– Characteristics of the receptor
• Site of intercellular communication between neurons
Chemical Synapse vs. Electrical Synapse
• Space between two cells (synaptic cleft)
• Presynaptic nerve releases a
neurotransmitter that diffuses through the
synaptic cleft and binds to receptors in
plasma membrane of postsynaptic neuron
• Most common
• Impulse conducted directly through gap
junction of adjacent cells
• Direct signal transduction is spread from
cell to cell
• Provide faster communication and
synchronization of activity for a large
number of neurons or muscle fibers
• Rare in CNS
Damage and Repair
• PNS neurons have a greater capacity for repair and
regeneration than CNS neurons
– Axons and dendrites of PNS neurons that are associated
with a neurolemma may undergo repair if the cell body
remains intact, if the schwann cells are functions, and if
scar tissue formation does not occur too rapidly
– Axons in the CNS are myelinated by oligodendrocytes
that do not form neurolemmas; hence regeneration can
not occur
Neural Regeneration
• Very limited
• In PNS: Wallerian Degeneration
Normal function may or may not be reestablished