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The Nervous system is the master controlling and communicating system of the body.
3 step process◦Sensory-uses sensory receptors to monitor changes inside
and outside of the body.
◦ Integration-processes and interprets the information and makes decision about what to do with the information (integration)
◦Motor-activation of muscles or glands in response to the stimuli
Organization of the Nervous System
All nervous system organs are separated into two classifications by structure.
◦Central Nervous System
◦Peripheral Nervous System
Central Nervous System(CNS)
Made up of the brain and spinal cord
Main purpose is to interpret the incoming sensory information and relay tissue instructions based on past experiences or conditions.
Peripheral Nervous System(PNS)
Anything in the nervous system outside of the brain and the spinal cord.
Consists of mainly nerves◦2 types
Spinal Nerves- Carry impulses to and from the spinal cord.
Cranial Nerves- Carry impulses to and from the brain.
Functional ClassificationsThe functional classification is only
concerned with the peripheral nervous system
◦Sensory/afferent- send information from the sensory receptors to the CNS.
◦Motor/efferent-carry impulses from the CNS to the muscles and glands and initiate a response.
Motor Divisions
Somatic nervous system◦allows conscious, voluntary movement of
skeletal muscles.◦Not all muscular activity is voluntary
Skeletal muscle reflexes-stretch reflex◦When a muscle spindle is stretched, it sends a message
to the brain telling the brain to contract the muscle to prevent tearing. Patellar-tendon reflex
Motor Divisions
Autonomic Nervous System◦Regulates events that are automatic or
involuntary Activity of cardiac muscles and smooth muscle Separated into two parts
◦Sympathetic-fight or flight, produce reactions under stress
◦Parasympathetic-rest and digest, all other autonomic functions, blood vessel dilation, pupil dilation.
Nervous Tissues◦ Astrocytes-
· Abundant, star-shaped cells
· Brace neurons to their nutrient
supply
· Form barrier between capillaries and neurons
· Control the chemical environment of the brain by picking up excess ions and recapturing released
neurotransmitters.
· Microglia-· Spiderlike phagocytes that dispose of debris (dead cells)
· Ependymal· Line the cavities of the brain and spinal cord.
· Their cilia help circulate CSF that fills those cavities
· CSF forms a protective cushion for the CNS
Oligodendrocytes-◦Produce a myelin sheath around nerve fibers in
the CNS◦Insulation/protection for nerves
Nervous Tissue: Support Cells
· Satellite cells· Protect neuron cell bodies by cushioning
cells
· Schwann cells· Form myelin sheath in the peripheral
nervous system
Neuron Anatomy
· Neurons = nerve cells
· Cells specialized to transmit messages
· Major regions of neurons
· Cell body – nucleus and metabolic center of the cell
· Processes – fibers that extend from the cell body
Neuron Anatomy
· Body of the Cell· Metabolic center of the neuron· Nissl substance – specialized rough
endoplasmic reticulum· Neurofibrils – intermediate cytoskeleton
that maintains cell shape
Neuron Anatomy
· Extensions outside the cell body· Dendrites -carry messages toward the
cell body
· Axons –carry messages away from the cell body to another neuron
Axons and Nerve Impulses
Axons transmit their information at their terminal ends.
All axons branch out at their end forming thousands of axonal terminals.
Once the impulse reaches the axonal terminal it stimulates the release of neurotransmitters into the extracellular space.
Synapse
In Between each axonal terminal is a small gap called a synapse.
Neurons never touch other neurons.
Neurons
Most long nerve fibers are covered with a fatty material called myelin. ◦It protects and insulates the fibers and
increases the transmission rate.Axons outside of the CNS are insulated
(myelinated) by Schwann cells.◦Wrap themselves around axons for insulation.◦When it is wrapped around the axon, the
myelin sheath encloses the axon.
Neurons
The neurilemma is in between the myelin sheath and the Schwann cells.
The Myelin sheath is formed by many different Schwann cells, this leaves gaps of uncovered surface area that are called Nodes of Ranvier.
Multiple sclerosis
Myelin sheaths around the fibers are gradually destroyed.
Once destroyed they harden and become “scleroses”
This decreases the persons ability to control their muscles and their mobility decreases.
Central Nervous System
Clusters of neuron cell bodies and collections of nerve fibers are named nuclei when in the CNS.
They are well protected in the body within the skull or the spinal column.
These cells do not go through cell division after birth. If a cell dies, it is not replaced. Thus the need for the bony protective coverings.
CNS anatomy
Ganglia- small collection of cell bodies in the CNS.
Tracts- bundles of nerves running through
White matter-dense collections of myelinated tracts (fibers)
Gray matter-unmyelinated fibers and cell bodies
CNS
Sensory Neurons◦Afferent-go toward the brain/spinal cord for
processing◦Transmit information about outside stimuli to
the CNS◦Cutaneous receptors-skin◦Proprioceptors- muscle/tendon◦Nociceptors- pain impulses
Proprioceptors
Detect the amount of stretch or tension in skeletal muscles, tendons or joints.
Information is sent to the brain so that it can make adjustments for any changes in posture/balance.
CNS
Motor Neurons-◦Efferent, carry impulses from the CNS to the
muscles/glands for action.◦Relay the action message to the muscles
Association Neurons-◦Also known as interneurons.◦They connect the motor and sensory neurons in
neural pathways.
Sensory Receptors
Naked Nerve Endings- pain and temperature
Messner’s Corpuscles- touch receptorsPacinian Corpuscles-Deep PressureGolgi Tendon Organs (GTOs)-
proprioception (contraction)Muscle Spindle-proprioception (stretch)
Nerve Impulses-Phyisology
2 major functions◦Irritability- ability to respond to a stimuli and
convert it into a nerve impulse
◦Conductivity-ability to transmit the impulse to other neurons, muscles or glands.
Physiology
When at rest, the plasma membrane is polarized, meaning there are fewer positive ions sitting on the inner face of the membrane than on the outside.
The major positive ions on the inside of the cell are potassium (K+), and the positive ions on the outside, are sodium (Na+)
If the inside is more negative than the outside, the neuron remains inactive.
Physiology
Many types of stimuli are used to excite the neurons, to activate and create an impulse. ◦Light excites eye receptors◦Sound excited some ear receptors◦Pressure for cutaneous receptors, etc.
Physiology-depolarization
Regardless of the stimuli, the result is all the same, permeability of the cell membrane changes for a brief period.◦Once the neuron is activated, the sodium gates
of the plasma membrane open and allow the sodium (Na+) into the cell.
◦Law of diffusion- higher concentration of Na+ outside the membrane
◦Once inside the polarity of the inside of the cell changes, this process is called depolarization.
Depolarization cont.
If the stimulus is strong enough, and the rush of sodium is great enough, the neuron is activated through depolarization.◦Once depolarized the neuron will transmit the
nerve impulse (action potential).**This process is all or nothing**. The
impulse will either be sent all the way through the neuron, or not sent at all.
Repolarization
Almost immediately after the Na+ ions rush in, the membrane permeability changes◦ It returns to being impermeable to Na+, but
permeable to K+, just as before.Now the K+ ions are free to float back out
into the tissue fluid. This happens rapidly, the quick outflow of these ions restores the electrical conditions of the cell. Returning to its resting (polarized) state. This process is called repolarization
Conductivity of Neurons
Once an impulse is sent through the neuron and it reaches the axonal terminal, tiny vesicles containing the neurotransmitter chemical fuse with the axonal membrane, releasing the chemical transmitters.
These chemicals travel across the synapse and bind to the next neuron.
This will more often than not restart the action potential in that next neuron.