Transcript The_Nervous_Systemx

The Nervous System
Organization of the Nervous System
The Neuron
Cells of the Nervous System
2 principle cell types
1. neuroglial cells- non-conducting cells which
protect, support, provide metabolism for
neurons.
2. Neuron- functional unit of the nervous
system. Conducting cell. Produce an
electrical impulse when stimulated.
Neuroglial cells
Neurons
Types of neuroglial cells
• Oligodendroglial cells- found in the CNS, form fatty insulative
sheath around nerve fibers of the CNS
• Microglia-protect neuron by phagocytising invading
microorganisms that may cause disease.
• Astrocyte-form the blood-brain barrier which transport
materials from blood to the brain.
• Schwann cell-found in the PNS, encase nerve fibers with a
fatty insulative material (myelin)
Organization of the nervous system
Central Nervous System
Brain and spinal cord
Peripheral Nervous System
Everything outside of the CNS
Structure of a neuron
• Cell body (perikaryon)
contains neuroplam, well defined
nucleus with a nucleolus
• Shape may vary: round, diamond, star , pyramid
• Nissl bodies in the neuroplasm, ribosomes.
• Neurofibrils-tubes within the neuroplasm
• Dendrites-afferent process, carry impulses toward the cell body.
• Axons-efferent process, carry impulses away from the cell body.
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Axons may be branched or singular
Terminate into a small swollen end, called a synaptic knob. (end bulb)
Myelin sheath over an axon called a neurolemma.
Myelinated fibers conduct impulses at a faster rate than nonmyelinated fibers. Called saltatory conduction 150m/sec
• Neurilemma aids in the regeneration of nerve fibers of the PNS.
Classification of Neurons by Structure
• Classify them by the number of processes that extend from the cell
body.
• Unipolar- One single process, axon and dendrite in the same process
• Bipolar- 1 axon, 1 dendrite, found in retina or the eye, and
chemoreceptors of the nose.
• Multipolar- most numerous, many branched dendrites and a single
axon. Found in the brain and spinal cord only.
Functional Classification of Neurons
• Sensory neurons- transmit impulses from receptors toward the
CNS.
• Exteroreceptors – monitor the external environment
• Interoreceptors - monitor the internal environment
Proprioreceptors- monitor tension and position of joints
found in ligaments, tendons, muscles
• Associative neurons- found within the CNS only, connect sensory
neurons to motor neurons. (Reflex Arc)
• Motor neurons- transmit impulses away from the CNS to an
Effector ( muscle or a gland) This brings about a coordinated
response to a stimulus.
Physiology of a Neuron
• Reaction-Reflex
• Reaction time-time lapse between recognizing danger and
muscular action.
• excitability (irritability) ability to respond to a stimulus
• Conductivity- enables neurons to encode the stimulus in the form
of electrical impulses
• Integration-sorting out incoming impulses and interpreting their
meanings in order to provide proper actions.
Transmission of the Nerve Impulse
• The neuron supplies energy for the transmission of an impulse.
• The neuron has an unequal distribution of ions between the outside
and inside of the cell membrane. Electrical gradiant
• This difference is created by Na+ outside the cell and K+ ions inside
the cell. The neuron does this by the process of active transport
• This is called the resting membrane potential -70 mv (polarized)
• When an adequate stimulus is received by a polarized neuron the
cell membrane become permeable to Na+ from outside and K+ from
the inside. These 2 ions then trade places in about .001 sec.
• The membrane potential goes from -70 to +30. This is called a
depolarized membrane.
• This causes the adjacent area to do the same. This creates a chain
reaction down the entire length of the neuron.
• The depolarization of the membrane which passes over the entire
neuron is called an action potential or a nerve impulse.
Repolarization of the Membrane
• Within .001 sec the membrane is restored to its original condition by
a rapid influx of K+ ions from the outside. Na+ move back outside
the cell.
• The sodium – potassium pump (active transport) is what
accomplishes this with the expenditure of ATP.
Characteristics of Nerve Impulses
• Nerve impulses can be recorded much like a muscle twitch
• Threshold stimulus is a stimulus strong enough to cause
depolarization
• Subthreshold stimuli if produced fast enough will be additive to
cause depolarization (summation)
• Suprathreshold stimulus, one that is greater than threshold,
produces depolarization. No different than threshold.
• This indicates that the degree and duration of the impulse is
independent of the strength of the stimulus.
• How can one tell the difference between light touch and a strong
slap?
• The answer lies in the frequency of impulses sent over the neuron
and the number of neurons activated.
Speed of Impulses
• Different nerve fibers have different rates of conduction
Large, myelinated nerves- 130 m/sec (saltatory conduction)
Large, unmyelinated- 75 m/sec
Small, myelinated- approx. 10 m/sec
Small, unmyelinated- 2 - .5 m/sec
Synaptic Transmission
• Synapse- junction between the terminus of an axon and the
dendrite or cell body of the next neuron
• Axon terminates to a synaptic knob (presynaptic membrane)
• The dendrite lies opposite of the synaptic knob, and possesses
receptor sites sensitive to the neurotransmitter secreted by the
synaptic knob. (postsynaptic membrane)
• The space of the synaptic cleft is 200A
Neurotransmitters
• Neurotransmitters- chemicals which cause changes in
permeability at the receptor site of the postsynaptic membrane.
• They are secreted at the presynaptic membrane which in turn
causes an action potential (nerve impulse) to be generated at the
post synaptic membrane.
• Neurotransmitters may be excitatory or inhibitory.
• Excitatory – acetylcholine, serotonin, dopamine, norepinepherine
• Inhibitory- glycine, gamma-aminobutyric acid, GABA
neurotransmitters
The Reflex Arc
• The reflex arc is a nerve pathway from a receptor to an effector.
Only involves 2 or 3 neurons
• A reflex is an automatic, unconscious response to a stimulus or
some change in homeostasis.
• Heart rate, breathing rate, digestion, swallowing, sneezing,
coughing, and vomiting are all reflexes.
• It starts at the receptor which has been stimulated by some change
or stimulus.
• This send s an impulse down a sensory neuron toward the spinal
cord.
• The impulse follows the dorsal rami where it interconnects with an
associative neuron.
• Which in turn is connected to a motor neuron in which the impulse
is carried directly to an effector which in turn stimulates the
appropriate response.
• The reflex is a form of protection, instead of the sensory impulse
traveling all the way to the brain and back down to cause the
effector to respond, it is done automatically without utilizing the
brain. Making our response time much faster.
• The reflex arc follows this pathway: Receptor-----sensory neuron----associative neuron--- motor neuron---effector.
Disorders Involving Neurons
• Multiple Sclerosis- autoimmune disease, in which a plaque or
scarring occurs on the myelin sheaths of neurons of the CNS.
• These plaques interfere with the conduction of impulses over the
neuron and cause “short-circuiting” of the nerve pathways.
• Usually occurs between the ages of 20-40.
• Symptoms include: jerky body movements, double vision, vision
problems, parenthesia, speech difficulties and paralysis of
voluntary muscles.
• Disease can be progressive if not treated. Most are treated with
interferon B injections.
• Affects women slightly more than men.
• incurable
Poliomyelitis
• Virus attacks and destroys the cell bodies of motor neurons.
• Periodic epidemics have crippled or killed millions over the
centuries.
• Onset has flu like symptoms: headache, fever, sore throat, stiffness
and pain in muscles.
• Usually affected children during the summer months
• Vaccines developed by Jonas Salk and Albert Sabin in the early
1950’s has virtually eradicated this disease