Download The Nervous System: Neural Tissue

The Nervous System: Neural Tissue
I.
Organization of the nervous system
A. Functions of the nervous system
1. gathers sensory information
2. makes decisions of what should be done at each moment (integration)
3. effects a response by activating effector organs
B. Two divisions
1. Central Nervous System (CNS) – brain & spinal cord
2. Peripheral Nervous System (PNS) – part of the nervous system outside the CNS
a. cranial nerves – carry impulses to & from the brain
b. spinal nerves – carry impulses to & from the spinal cord
C. Functional Classification of Neurons
1. Sensory (afferent) neurons – carry impulses from receptors towards the CNS
a. Exteroceptors – sensitive to stimuli that occur outside the body (sense of touch, sight,
taste, etc.)
b. Interoceptors – respond to stimuli inside the body & monitor changes in the blood, body
temp., etc.)
2. Association neurons (interneurons) – lie between sensory & motor neurons in the CNS and
integrate incoming & outgoing signals.
3. Motor (efferent) neurons – carry impulses away from the CNS and to effectors (muscles or
glands)
D. Two types of Neural Cells
1. Neurons – carry impulses
2. Neuroglial cells – support cells for the neurons
a. Microglia – phagocytic cells that protect neurons from invading microorganisms.
b. Astrocytes – star-shaped cells that connect neurons to capillaries and help exchange
nutrients between capillaries and neurons.
c. Oligodendrocytes – cover the fibers of the CNS neurons with myelin sheaths.
d. Schwann Cells – cover the fibers of the PNS neurons with myelin sheaths.
E. Neuron Structure
1. Soma - cell body
a. contains all organelles except centrioles (thus neurons are not able to reproduce)
b. Nissl bodies – complex rough ER network with many ribosomes.
2. Dendrites – typically shorter processes that convey impulses toward the soma.
3. Axons – longer process (up to 3-4 ft) that conveys signals away from the soma.
a. Each neuron will have only one axon, however it may be branched.
b. At the end of the axon will be axon terminal (AKA - synaptic knobs or synaptic boutons).
c. Some axons will be covered by white, fatty insulation called a myelin sheath. Myelin
sheaths increase the speed of impulse transmission.
1) White matter in the brain consists of myelinated fibers
2) Gray matter consists mainly of unmyelinated fibers.
d. The spaces between the myelin sheaths are called Nodes of Ranvier.
F. Structural Classification of Neurons
1. Unipolar – have a single process that emerges from the cell body. Functions mainly as an
axon for the PNS.
2. Bipolar – have two processes- one axon, one dendrite. Found in the retina of the eye and
the olfactory mucosa.
3. Multipolar – have many (at least 2) dendrites and one axon. Most common neuron in the
body.
II.
Neurophysiology
A. Nerve cells are able to transmit signals due to their properties of
1. Irritability – responding to a stimuli
2. Conductivity – able to carry an impulse down its length
B. The Electrochemical gradient & the Resting Potential
1. The energy for an impulse is supplied by the neuron.
2. Neurons create a chemical imbalance between the inside and the outside of the cell
membrane (Potential difference).
3. This chemical imbalance allows the electrical charge to travel down the length of the axon.
4. Proteins along the cell membrane, called Sodium-Potassium Pumps, actively transport Na
out of the cell & K into the cell.
5. The build-up of Na outside the cell & K inside the cell makes the Electrochemical Gradient.
6. Because the Na & K ions have different voltages, an electrical differential exists between the
inside & outside of the cell membrane.
7. The Resting Potential of a “charged” or Polarized axon is –70 mV. (The inside is negative
relative to the outside)
C. The Acton Potential
1. The transmission of an impulse down the length of an axon is called the Action Potential.
2. A stimulus is applied to a polarized membrane, which causes the membrane to become
permeable to Na.
3. Since there is a high concentration of Na outside the cell, the Na rushes in and changes to
ionic differential to a more positive level (about +30 mV) called Depolarization.
4. The depolarization of a membrane at a certain area stimulates neighboring region of the
membrane and causes it to depolarize. (This causes a chain reaction down the length of the
membrane).
5. The impulse is always the same strength and only moves one way.
D. Repolarization
1. Within 1 ms, the Na gates have closed and K gates have opened to allow for K to flow out of
the cell.
2. The loss of K helps to restore the axon to the Resting Potential of –70 mV. (Repolarization)
3. The Na/K pumps are then activated to restore normal Na & K levels.
E. Stimuli for Depolarization
1. The minimum stimulus needed to trigger a depolarization and an impulse is called a
Threshold stimulus.
2. The All – or – None Law says that action potentials will happen completely or not at all.
3. All action potentials are alike no matter the strength of the stimulus.
4. Frequency of impulse transmission is what separates strong from weak stimulus.
5. Stimuli not strong enough to trigger depolarization are called Subthreshold stimuli.
6. Subthreshold stimuli can be summated to cause depolarization.
7. Suprathreshold stimuli trigger only a normal action potential.
8. The Absolute Refractory Period is the period of time when the Na gates are open & a
second stimulus can NOT come down the axon – no matter how strong it is.
9. The Relative Refractory Period is the time immediately after the Na gates close and
repolarization is still occurring that a exceptionally strong stimulus may cause depolarization.
F. Impulses
1. Impulses travel at different speeds based upon the
a. diameter of the axon
b. degree of myelination
2. Thicker axons (which are usually longer as well) carry impulses faster than smaller axons.
3. Myelinated axons carry impulses faster than unmyelinated axons.
a. Myelin sheaths have spaces between them (about 1mm apart) called Nodes of Ranvier.
b. The current can only be carried at these nodes, thus the impulse has to “leap” the myelin
sheath. This is called Saltatory Conduction.
4. Speed of Impulse Conduction
a. Large, myelinated = 15 – 150 mps
b. Medium, lightly myelinated = 3 – 15 mps
c. Small, unmyelinated = 1 mps
III.
The Synapse
A. Synapse structure
1. Synapses occur between the axon of one cell and the dendrite of the next cell
2. Presynaptic neuron – neuron conducting impulses towards the synapse
a. The end of the axon terminates (or ends) in a Knob or Bouton.
b. The knob contains many tiny sacs called synaptic vesicles that contain a
neurotransmitter.
3. Postsynaptic neuron – neuron conducting impulses away from the synapse
a. The end of the dendrite has a receptor membrane.
b. The membrane bears neurotransmitter receptors .
4. Synaptic Cleft – space between the synaptic knob & the receptor membrane. 30 – 50 nm
wide
B. Synaptic transmission
1. When the impulse reaches the axon terminal (knob), depolarization of the presynaptic
membrane causes exocytosis of the neurotransmitter into the synaptic cleft.
2. The neurotransmitter binds to the postsynaptic receptors, which causes Na ion gates to
open and local depolarization to occur.
3. Neurotransmitters – two classes
1. Excitatory: Acetylcholine (ACh), Norepinephrine, Dopamine, etc.
2. Inhibitory: Gamma-aminobutyric acid (GABA), Glycine, etc.
C. Termination & reabsorption of neurotransmitters
1. The neurotransmitter will continue to effect the receptors as long as they are present. The
neurotransmitters must be removed in one of three ways.
a. Chemical breakdown (Acetylcholinesterase [AChE])
b. Reabsorption of the neurotransmitter by the presynaptic membrane
c. Diffusion away from the synapse
IV.
Neural Disorders
A. Multiple Sclerosis (MS)
1. Autoimmune disorder that destroys the myelin sheaths of the axons of the optic nerve, brain,
&/or spinal cord.
2. Characterized by balance, speech, vision, and motor coordination problems.
3. It starts at age 30-40, is progressive & there is no treatment
B. Poliomyelitis
1. Viral infection that kills the motor neurons of the spinal cord & brain
2. The dependant motor units become paralyzed and atrophy.
3. Immunization programs have all but eliminated this disease.
C. Rabies
1. Viral infection of the CNS that is transmitted by infected animals. The virus is absorbed
through the synaptic knob and is carried by the axons up to the CNS.
2. Primary phase symptoms include headache, nausea & muscle pain. Secondary phase
symptoms include hallucinations, muscle spasms, and difficulty swallowing that cause saliva
buildup in the mouth (foaming at the mouth). Coma & death soon follow.
3. Treatment consists of a series of injections that contain antibodies, which must begin almost
immediately. W/o treatment, rabies is always fatal.