Nervous System Flashcards
Control is determined through:
A. The nervous system
B. and through the endocrine system.
Nervous System (What does it do)
A. This system is vital for every function accomplished within the human body regardless of whether it is a conscious or unconscious action.
B. Even the body’s other control system, the endocrine system is regulated by the nervous system and cannot function without it.
C. Our body is in a constant battle to maintain hemostasis. The nervous system senses significant changes and regulates appropriate responses.
Organization of the Nervous System (systems)
Two principal divisions:
A. Central Nervous System (CNS)
1. The central control center.
2. Consists of brain and spinal cord.
3. Receives information from the body, interprets it, and then directs action in response.
B. Peripheral Nervous System (PNS)
1. “Wiring” which transmits information both to and from the central nervous system.
a. afferent - toward CNS.
b. efferent - away from CNS.
2. Therefore, PNS is both afferent and efferent.
3. The efferent portion of the PNS can be further subdivided into 2 parts:
a. Somatic nervous system (soma = body) - directs the control of skeletal muscles in the body. It is, therefore, under conscious and voluntary control.
b. Autonomic Nervous System (ANS) - (visceral nervous system). This system sends impulses to smooth muscle, cardiac muscle, and glands. Therefore it is under unconscious and involuntary control. This system can be further divided into 2 more divisions:
i. Sympathetic Division
ii. Parasympathetic Division
Histology of the Nervous System
A. Made up of 2 basic cell types:
1. Neurons - actually conduct the impulses and are involved in movement, sensation, and integration (thinking) of information.
2. Neuroglia - supportive type cells that support and protect the neurons.
3. In spite of its size, the CNS has very little connective tissue. Nerve cells and processes are supported by neuroglia.
B. Types of Neuroglia - These cells account for about 90% of the cells found in the brain.
1. Astrocytes (star-shaped cell)
a. Possess many long processes. At the end of these processes are expanded areas called pedicles which attach to capillaries.
b. Two types:
i. Protoplasmic Astrocytes - found in gray matter of CNS.
ii. Fibrous Astrocytes - found in white matter of CNS.
c. Astrocytes appear to be most important - because of their association with blood vessels it is believed that they play a role in transport of substances from blood to CNS and back. They are believed to be a factor in “Blood-Brain Barrier”.
2. Oligodendrocytes (means few branches) - similar to astrocytes but much smaller and having shorter and fewer processes.
a. These help to form an insulating sheath over axons of nerve cells (myelin). Analogous to Schwann cells of PNS.
b. Oligodendrocytes appear to have a metabolic interdependence with neurons. Are located usually closer to nerve cell bodies.
3. Microglia (micro = small) - very small cells with short processes giving them a spiny or thorny appearance. Found in both white and gray matter.
a. Phagocytic and functionally similar to macrophages in connective tissue.
b. Microglia - protect CNS by phagocytosis of invading microbes, cellular debris, or harmful compounds.
4.. Ependymal Cells are not always included in texts as neuroglial cells, but these are the cells that line the ventricles of the brain containing cerebrospinal fluid.
D. Nerve Cells = Neurons
1. The classical neuron
2. Composed of 3 distinct portions
a. Cell body - perikaryon
b. dendrites - incoming information
c. axon - outgoing message
3. Cell Body
a. Composed of well-defined nucleus and nucleolus with granular cytoplasm.
b. Contains “Nissl substance” (Nissl Bodies) which stain dark blue - this is due to a large amount of rough endoplasmic reticulum and free ribosomes functioning in protein synthesis.
4. Dendrites
a. Highly branched extensions of the cytoplasm.
b. Neuron usually has several.
c. Generally considered to function in conducting charge differences toward the nerve cell body.
d. Often divide like branches of a tree and receive numerous synaptic inputs from other neurons (one cell type - Purkinje cell - receives up to 200,000 axon terminations on a single cell).
5. Axons
a. Normally each neuron has only one axon.
b. Vary a great deal in size depending on nerve cell type - may be several feet long in some cases.
c. Contains bundles of microtubules (Neurofibrils) or microfilaments which transport materials up and down the axon (axonal transport).
d. In contract to dendrites, axons very rarely branch. However, sometimes it will = axon collaterals.
e. Originates from cell body as cone-shaped axon hillock.
f. Cytoplasm is specifically called axoplasm and surrounding membrane = axolemma.
E. Schwann Cells
1. This cell type is found associated with axons in the PNS.
2. Forms myelin sheath.
3. Functionally similar to oligodendrocytes in that it provides an insulating phospholipid and protein sheath around the nerve called myelin.
4. Thus there are myelinated and unmyelinated axons depending on their relation to Schwann cells.
5. A Schwann cell myelinates an axon by winding itself around and around like a window shade pushing the cytoplasm and nucleus to the outer layer. This sheath is then referred to as the neurilemma. Thickness of the sheath varies between neurons. Unmyelinated axons may have Schwann cells around them, but not in this layered arrangement.
6. The myelin sheath shows gaps between cells called nodes of Ranvier.
7. An axon and its myelin sheath are often referred to as a nerve fiber
Classification of Neurons
(may be classified due to both structure and function.)
A. Structural Classification - classified by shape
1. Multipolar neurons - several dendrites and one axon - most neurons are of this type.
2. Bipolar neurons- have one dendrite and one axon (examples: in eyes, inner ear, and olfactory area)
3. Pseudounipolar neurons - these show a single process very close to the cell body, but split forming a “T” shape with one direction functioning as a dendrite and one as an axon.
B. Functional Classification - classified by direction of impulse transmission.
1. Sensory Neurons - Afferent Neurons - transmit from peripheral receptors to CNS.
2. Motor Neurons = Efferent Neurons - convey information from CNS to peripheral “effectors”. (muscles or glands).
3. Interneurons = Association Neurons - establish interrelationships among other neurons - form complex functional chains or circuits.
C. Based on Neurotransmitter type - we will study this later in course
- example - all motor neurons (efferent neurons) which leave the CNS use acetylcholine as their neurotransmitter and are referred to as cholinergic
Neuron Regeneration
A. Neurons lose their ability to divide - it is generally believed that around the time of birth we lose all ability to form new neurons.
B. Therefore, a destroyed neuron is lost forever.
C. Regeneration of axons has some limited success.
1. If axon is myelinated, it will regenerate if destroyed as long as the myelin sheath has not been destroyed and the axon does not have too great of a distance to travel. If the axon is too long, the sheath will degenerate some time after the axon if a new axon has not yet grown into it. If the sheath degenerates, the axon will not find its way back to the original site of innervation.
2. If entire nerve is severed (including myelin sheath), the degree of reinnervation will depend greatly on the skill of the surgeon who reconnects the severed ends. If not reconnected, no reinnervation will occur.
3. In CNS, regeneration will not occur if axon is destroyed or broken. Neuroglia in the CNS tend to proliferate and block the passageway following injury. This makes injury to axons in the CNS permanent.
Classification of Nerve Fibers
Based on Axon Diameter – Type a, b, or c fibers or based on speed of conduction - Type I, II, III, or IV fibers
A. A-Fibers – Type I or Type II fibers
1. Largest diameter nerve fibers.
2. Always myelinated, therefore have saltatory conduction.
3. Conduct impulses most rapidly (130 meters/sec).
4. Form axons of large sensory neurons (transmit touch, pressure, joint position, heat, cold).
5. Form axons in motor nerves to skeletal muscle.
6. Usually found in nerves which would enable us to detect and react to hazardous things in our environment.
B. B-Fibers – Type II or Type III fibers
1. "middle-sized" diameter in relation to other neurons. 2. Usually myelinated and have saltatory conduction, but usually conduct impulses at a slower rate than A-Fibers (about 10 meters/second). 3. Normally found in nerves where immediate response is not necessary for life or death. Often convey physiologic information between skin or organs and CNS.
C. C-Fibers – Type IV fibers
1. Have smallest diameter of all.
2. Slowest rate of conduction (0.5 meters/second).
3. Unmyelinated (therefore, no saltatory conduction).
4. Found in nerves transmitting touch, pressure, heat, cold from skin or pain from viscera - not involved in reflex responses however - also in motor nerves to smooth muscle and glands. * note - Both B-Fibers and C-Fibers often associated with functions of the autonomic nervous system.
D. A separate system uses roman numerals I, II, III and IV which identify speed of conduction. Type C fibers are basically the same as type IV fibers, but type II fibers may be either A or B fibers. Type III fibers are B fibers.
Synapses
A. Signals are transmitted from neuron to neuron via specialized structures called synapses at their junctions (“synapse” means “connection”).
B. Most often these are between an axon terminal of one neuron, and a dendrite or cell body of a second neuron (may be on axon however) (in fact, may be about any combination imaginable, but less likely) - Axoaxonic - Axosomatic - Axodendritic, etc.
C. Synaptic cleft - very small space between the two neurons within a synapse (about 20 nanometers or 200 Å).
D. Presynaptic Neuron - Neuron “before” synaptic cleft. This usually forms a presynaptic knob, bouton or presynaptic terminal.
E. Postsynaptic Neuron - Neuron located “after” the synaptic cleft.
F. Neurotransmitters - chemical substances formed by the neuron (usually from amino acids) which alter the permeability of the postsynaptic neuronal membrane.
1. Arrival of impulse in terminal of presynaptic will cause a release of neurotransmitter into the synaptic cleft where it will stimulate or inhibit depolarization of the postsynaptic neuron.
2. It has traditionally been believed that one neuron makes only one transmitter. However, more recent research is suggesting that neurotransmitters of different types may be present in the same neurons. Perhaps being co-released but having different effects… (e.g. possible one acting presynaptically?)
3. Neurotransmitters are normally found in the terminals of presynaptic neurons and are stored in synaptic vesicles. These number in the thousands.
Afferent Nerve Receptors
A. Information about one’s surrounding environment which reaches the CNS is ultimately the result of receptors.
B. Receptor = peripheral ending of a sensory neuron that is especially sensitive to a given kind of stimulus.
C. These sensory neurons then transmit the information to the CNS where it will be interpreted as a specific sensation (e.g. pain, touch, sound)
D. Since each neuron itself is capable of transmitting no information other than a simple impulse, it is apparent that identification of the sensation depends on the specific connections of the sensory pathways in the brain.
E. General Afferents - those from the body.
F. Special Afferents - those from the head.
General - pain, touch, pressure, cold, warmth, propriception
Special - sight, hearing, taste, smell, equilibrium.
*Receptors usually appear to be free nerve endings. These are undifferentiable anatomically but apparently have different functional roles like: pain, crude touch, pressure, itch, temperature, etc.
G. Meissner’s Corpuscles - receptors for discrete touch. Especially numerous in hands, feet, lips, nipples, and the tip of the tongue.
H. Pacinian Corpuscles - receptors sensitive to deep pressure and vibration. Usually found in deeper layers of the skin.
I. Free Nerve Endings - monitor things like temperature and pain.
J. Muscle Spindle Fibers - stretch receptors in skeletal muscles monitoring stretch of the muscle
* note there are also stretch receptors in smooth muscle of hollow organs and vessels
K. Golgi Tendon Organs - in tendons of skeletal muscles - monitor tension.
