Lecture 3: PERIPHERAL NERVOUS SYSTEM Flashcards
Study the Diagram of the Spinal Cord Showing Distinct Regions
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Central area of the spinal cord that has defined shape
“butterfly” = Grey Matter. Consists of populations of neurons and glia.
Area of the spinal cord that is lighter in color
white matter.
Consists of axons and glia.
Grey matter divides white matter into regions or columns: dorsal white columns, lateral white columns, ventral white columns.
Nissl Stain
identifies neuron cell bodies
A Nissl Stain of the spinal cord makes the grey matter darker than the white matter.
Fiber Stain
identifies myelinated axons
Fiber Stains make the white matter darker than the grey matter.
Functional Organization of the Spinal Cord
Dorsal half of the spinal cord processes sensory information (input)
Ventral half of the spinal cord gives rise to axons that innervate skeletal and smooth muscle.
GANGLION
Collection of neurons located outside brain or spinal cord (i.e., in the periphery)
DORSAL ROOT GANGLIA
Sensory (afferent) and motor (efferent) axons come together to form a spinal nerve that project to the periphery.
the small swellings located bilaterally at the level of each intervertebral foramen.
STUDY Visuals of the FORMATION OF A SPINAL NERVE
and the
Functional Organization of the Spinal Cord
Spinal nerve»_space; Ventral Root»_space; Motor Neuron»_space; Ventral Horn»_space; Dorsal Horn»_space; Dorsal Root»_space; Central Process»_space; DRG Cell»_space; DRG»_space; Peripheral Process
[Peripheral Process]»_space; Sensory Receptors»_space; DRG»_space; [Central Process]»_space; AG»_space; Viscra & Skeletal Muscle
Tract
a bundle of axons that connects two CNS regions
(e.g., Corticospinal Tract = axons from cortex projecting to the spinal cord
Ascending Tract
Projects from spinal cord to the brain
Descending Tract
Projects from brain to the spinal cord
Fasciculus/Fasciculi
A bundle of anatomically defined fibers that subserve a common function
aka, a tract.
Funiculus
An area containing multiple tracts (or fasciculi)
(e.g., Lateral Funiculus of the spinal cord
Subdivisions of the Gray Matter
Dorsal Horn (sensory) Intermediate Grey (ANS) Ventral Horn (motor)
Dorsal Horn
Grey matter on dorsal half of spinal cord.
Dorsal column/funiculus
white matter located between 2 dorsal horns; well defined.
Bilateral
Contains large diameter sensory axons of dorsal root ganglia (DRG) neurons
Cell bodies of origin for these axons are in DRG.
Function: General sensation from periphery.
Two tracts:
One from upper limb and upper trunk.
One from lower limb and lower trunk.
Axons enter the spinal cord via a dorsal root. On entering, the axons branch and give off collaterals to spinal cord and send main axon into the dorsal funiculus where they ascend to the brainstem. Therefore these are Ascending axons.
Ventral Horn
Grey matter on ventral half of spinal cord.
Ventral funiculus
white matter located between 2 ventral horns; well defined.
Extends from ventrolateral grey matter to anterior median fissure
Contains mainly descending axons from the brainstem to the spinal cord - motor tracts
Intermediolateral Cell Column
Between dorsal and ventral horn. Location of preganglionic sympathetic and parasympathetic neurons.
Only found between T1 and S3.
Lateral funiculus
Between lateral edge dorsal horn and ventral horn; blends with ventral funiculus
Extends from lateral aspect dorsal horn to ventrolateral aspect ventral horn.
Contains multiple tracts:
- Ascending sensory axon tracts (pain, temperature, cerebellar)
- Descending motor axon tracts from brain (cortex, midbrain
Anterior White Commissure
Located dorsal to the the anterior median fissure
Contains axons crossing from one side of the spinal cord to the other
NERVES
Made up of axons from hundreds of individual neurons
ENDONEURIUM
The connective tissue that surrounds individual axons
PERINEURIUM
The connective tissue sheath that groups together Multiple bundles
EPINEURIUM
The common thick external connective sheath that encloses All bundles
nerves that are connected to the spinal cord
spinal nerves
nerves that are connected to the brain
cranial nerves
If the sensory axons do not leave the spinal cord, how do they get sensory information from the periphery?
The answer is from neurons located outside the CNS that are gathered together into structures called ganglia.
Neurons located in the dorsal part of the spinal cord have axons that remain within the spinal cord itself. They do not leave. These are functionally defined as sensory neurons.
Neurons within the ventral part of the spinal cord give rise to axons that leave ______ on the _____ side.
the spinal cord on the ventral side.
These axons are functionally defined as motor axons as they innervate skeletal muscle or, indirectly, smooth muscle found in the walls of organs such as the heart, gut, bladder, etc.
DRG Neurons
Neurons within the DRGs have a unique morphology. They have a soma with no dendrites.
Arising from the soma is a single process that divides into 2 branches. One process goes out to the periphery where they detect sensory information such as touch, pressure, pain, temperature on the skin. This information is relayed centrally via the second process which is called the central process.
The central process conveys sensory information into the spinal cord. This information may contact neurons located within the dorsal part of the gray matter or continue on to the brainstem before synapsing.
The function of these neurons is to relay this information to other regions of the spinal cord and brain.
FORMATION OF A SPINAL NERVE
First, look at a schematic cross section of the spinal cord within the vertebral canal.
Anatomically, the gray matter of the spinal cord can be divided into a dorsal horn and a ventral horn . Functionally, the dorsal horn is involved in processing sensory information. The ventral horn contains neurons that controls muscles and is involved in motor output.
Two roots arise from the spinal cord – a dorsal and a ventral root.
The ventral root consists of the axons of motor neurons that leave the spinal cord to innervate muscles in the periphery. Basically, the ventral root is a one way street directed from the CNS to the PNS.
In contrast, the dorsal root is a one way street carrying information from the periphery toward the CNS. It contains the central processes of dorsal root ganglia neurons.
On the dorsal root, a swelling indicates the location of a dorsal root ganglion. Within the ganglion are DRG neurons. These are the cell bodies that give rise to the central process present in the dorsal root, as well as to a peripheral process that projects out to the body for detecting sensory inputs.
Within this space the dorsal and ventral roots fuse to form the spinal nerve which is now a two way street containing axons going out the periphery and axons coming back from the periphery. Upon emerging from the intervertebral foramen, the spinal nerve immediately divides into 2 primary rami that innervate structures on the back or the front of the body.
how are tracts are named?
origin-destination
Axons that enter or leave the brain and spinal cord form nerves that are part of the peripheral nervous system.
This includes 12 pairs of cranial nerves arising from the brainstem and 31 pairs of spinal nerves arising from the spinal cord.
Another structure found in the peripheral nervous system are ganglia
nerve
represents the bundling of hundreds or even thousands of individual axons, with each axon arising from a separate neuron
Individual nerves may contain motor, sensory, or both type axons.
Nerves Are Made Up of Morphologically and Functionally Distinct Types of Axons:
Anatomical Differences
Myelinated & unmyelinated axons
Large & small diameter
Nerves Are Made Up of Morphologically and Functionally Distinct Types of Axons:
Efferent axons –Carry message away from CNS (Motor).
Afferent axons – Carry message toward CNS (Sensory).
Afferent axons
Carry message toward CNS (Sensory). INPUT
They arise from neurons located within dorsal root ganglia and carry messages from the periphery into the CNS. These are part of the sensory system.
Efferent axons
Carry message away from CNS (Motor). OUTPUT.
These arise from neurons within the spinal cord and carry signals from the CNS to muscles in the periphery.
General Somatic Efferent Axons (GSE)
Outgoing axons carrying motor commands to skeletal muscles.
General Visceral Efferent Axons (GVE; Autonomic nervous system)
Outgoing axons carrying motor commands to smooth muscle, cardiac muscle, and glandular tissue, i.e., involuntary tissue.
General Somatic Afferent Axons (GSA)
Axons that transmit sensory information (touch, pressure, pain, temperature, muscle tension) from the body to the spinal cord.
General Visceral Afferent Axons
GVA; Autonomic Nervous System
Axons that transmit sensory information (distension, pressure, pain) from organs to the spinal cord.
Large myelinated sensory carry…..
touch, pressure, distension, proprioceptive information.
Small diameter sensory axons carry
pain and temperature information.
Different peripheral nerves will have varying proportions of each type of axon.
Example: Cutaneous nerves only contain general somatic afferent axons whereas nerves such as the median nerve or sciatic nerve will contain GSE, GVE (to blood vessels and sweat glands in the skin), and GSA axons. This becomes important when trying to understand the symptoms exhibited by patients with peripheral neuropathies, or injury to select nerves.
CRANIAL NERVES
Cranial nerves are analogous to spinal nerves. The difference is that they arise from the brainstem and innervate structures in the head and neck. Spinal nerves innervate everything below the neck.
There is one exception to this rule as there is a cranial nerve that actually leaves the skull and courses all the way to the caudal abdomen. This nerve is called the vagus nerve (latin for wanderer). Several similarities and differences can be defined in comparing cranial and spinal nerves.
vagus nerve
Latin for “wanderer”
The one cranial nerve that actually leaves the skull and courses all the way to the caudal abdomen.
CRANIAL NERVES
Specifics
Some cranial nerves are only sensory, some only motor, some both motor and sensory.
Cranial nerves are NOT formed from dorsal and ventral roots; these are not present in the brainstem.
Neurons carrying afferent (sensory) information in a cranial nerve are located in ganglia in the periphery that are analogous to the dorsal root ganglion.
Neurons giving rise to efferent (motor) axons are located in a nucleus located somewhere in the brainstem.
COMPONENTS OF CRANIAL NERVES
General Somatic Efferent Axons (GSE) -Skeletal muscles in head and neck (e.g., facial muscles, tongue muscles, muscles that move the eyes.
General Visceral Efferent Axons (GVE; ANS) –Glandular structures in the head (e.g., salivary glands, tears). Smooth muscles in thorax and abdomen, pupil of eye)
General Somatic Afferent Axons (GSA) –Axons that transmit sensory from face, ear, oral cavity, etc.
General Visceral Afferent Axons (GVA; ANS) -Axons that transmit sensory information (e.g., distension, pressure, pain from GI tract) from organs to the brainstem.
Special Somatic Afferent (SSA) -Sensory information unique to head (taste, vision, hearing, olfactory, balance).
Special Visceral Efferent – Unique voluntary muscles which have a different embryological origin.
For cranial nerves, 3 additional functional components are added. These, in part, reflect the special senses that are unique to the head and are not found anywhere else in the body.
They include SSA’s which innervate structure excited by unique physical stimuli such as light, sound, gravity, and rotational forces. These carry visual, auditory, and balance information.
Special Visceral Afferents also carry unique sensory information from structures in the head. These are activated by chemical stimuli such as odors or specific properties of taste. Also included in this group are nerves that detect blood pressure or changes in levels of carbon dioxide in the blood.
Finally, there are a group of muscles in the head that have a unique embryological origin. Rather than arising from mesoderm. They arise from structures called branchial arches. These are true skeletal muscles which are under voluntary control; they just have a unique embryological origin. These are primarily muscles that control facial movements, jaw closing, and muscles in the larynx and pharynx. Because of this unique origin, the efferent axons to these muscles are called special visceral efferent or SVE axons.
SPINAL NERVE
Summary
There are 31 pairs of spinal nerves
Primarily innervates structures below the neck; motor and sensory
Formed by union of dorsal (sensory) and ventral (motor) roots
Motor neurons giving rise to efferents are located in ventral horn of the spinal cord
Neurons giving rise to sensory components are located in dorsal root ganglion
Nerves contain multiple components including:
GSE, GVE, GSA, GVA
Common Axon Types, and their Abbriviations
GSE – General Somatic Efferent GVE – General Visceral Efferent GSA – General Somatic Afferent GVA – General Visceral Afferent SSA – Special Somatic Afferent SVE – Special Visceral Efferent
CRANIAL NERVE
Summary
There are 12 pairs of cranial nerves
Innervates structures in the head and neck; motor and sensory. A few exceptions.
Nerve arises directly from brainstem. No roots present. Motor and sensory components unite within brainstem or as they exit brainstem.
Motor neurons giving rise to efferents are located in nuclei in the brainstem
Neurons giving rise to sensory components are located in named ganglia
Nerves contain multiple components including:
GSE, GVE, GSA, GVA
SSA, SVE
SUMMARY OF THE PERIPHERAL NERVOUS SYSTEM (PNS)
The PNS contains neurons and axons.
Neurons involved in carrying SENSORY information from the periphery to the CNS are located within ganglia located outside the brain or spinal cord. Neurons carrying sensory information from the body, below the head are located in DORSAL ROOT GANGLIA. Neurons carrying sensory information from the head and neck are located in specific ganglia.
Motor neurons involved in controlling voluntary somatic skeletal muscles as well as smooth muscles related to the viscera are located within the CNS; that is, in the spinal cord or brain. Their axons extend into the PNS.
The axons of sensory and motor neurons unite to form 31 pairs of spinal nerves and 12 pairs of cranial nerves. These nerves contain hundreds of axons bundled together. Individual axons may be either afferent or efferent relative to the CNS.
SUMMARY OF THE PERIPHERAL NERVOUS SYSTEM (CONTINUED)
Individual efferent and afferent axons within a spinal or cranial nerve may be further subdivided into one of the following functional classifications.
General Somatic Efferent (GSE) - axons that innervate skeletal muscle
General Visceral Efferent (GVE) - axons that innervate smooth muscle, cardiac muscle, glands.
General Somatic Afferent (GSA - axons that carry sensory information from the body wall
General Visceral Afferent (GVA) - axons that carry sensory information from viscera
Additional functional components are related to cranial nerves including:
- Special Sensory
- Special Visceral Efferent
Peripheral Nerves Convey Information to and from the CNS
The nerve fibers innervating the receptors described thus far have their cell bodies in dorsal root ganglia adjacent to the spinal cord or, in the case of those reporting from the head, in various cranial nerve ganglia near the brainstem. The central process of each of these ganglion cells enters the CNS. Each peripheral process joins motor axons emerging from the spinal cord (or brainstem) to form spinal nerves (or cranial nerves). The formal boundary between the central and peripheral nervous systems occurs as fibers enter or leave the spinal cord or brainstem, at the point where the myelinating cells change from oligodendrocytes to Schwann cells; for now, however, we will consider only those portions distal to the sensory ganglia (i.e., the wrappings and contents of spinal and cranial nerves).
epineurium
The outermost and most substantial of the 3 layers of connective tissue investing peripheral nerves. The epineurium is continuous centrally with the dura mater.
Composed mainly of collagen and fibroblasts, it forms a covering over nerve trunks, then thins to an incomplete layer around smaller branches near their terminations. The abundant longitudinally and spirally arranged collagen fibers of the epineurium are largely responsible for the considerable tensile strength of peripheral nerves. Peripherally, it usually ends near the termination of a nerve fiber, but it also contributes to the capsule of some encapsulated endings.
perineurium
A continuation of the arachnoid, ensheathing the fascicles of nerve fibers in peripheral nerves. Like the arachnoid, the perineurium contains a layer of cells interconnected by bands of tight junctions, forming part of the anatomical basis for the blood-nerve barrier.
The perineurium, lying within the epineurium, is a layer of thin, concentrically arranged cells with interspersed collagen. Adjacent perineurial cells are connected to one another by tight junctions that effectively isolate the epineurial spaces from the endoneurial spaces around peripheral nerve fibers. In addition, the endothelial cells of capillaries within the perineurium are connected to one another by tight junctions. Thus functional equivalents of the arachnoid barrier and the blood-brain barrier persist in the (PNS) as a Blood-Nerve Barrier. The perineurium continues as the capsule of many endings, including Pacinian and Meissner corpuscles, muscle spindles, and Golgi tendon organs. However, at other places, such as near neuromuscular junctions, the perineurium is open-ended, allowing the endoneurial space around nerve fibers to communicate with the general extracellular space of the body. This is of clinical importance because certain toxins (e.g., tetanus) and viruses (e.g., polio, herpes simplex) can gain access to the nervous system at these sites.
endoneurium
Wisps of loose connective tissue that surround the individual nerve fibers within the fascicles of a peripheral nerve.
In at least some species, these individual endoneurial sheaths are compact enough that they may help direct the regrowth of nerve fibers after injury.
The Diameter of a Nerve Fiber Is Correlated with Its Function
There are 2 major classification systems, and neither is used universally for all fibers.
The first system is based on conduction velocity. Larger fibers conduct action potentials faster than do smaller fibers.
The second classification system is based on direct microscopic measurement of axonal diameters. In this system myelinated fibers are categorized as group I, II, or III in order of decreasing size. Unmyelinated fibers are group IV.
Most commonly the letter system is used for myelinated efferent fibers and the roman numeral system for myelinated afferents.
A fibers
The fibers responsible for the A deflection.
The myelinated sensory and motor fibers.
A deflection is complex and was subdivided into α, β,γ, and δ peaks (α being the fastest). Although the β and γ peaks as originally described were probably recording artifacts, the terminology has become established and is still commonly used. Thus Aα fibers are the largest and most rapidly conducting myelinated fibers, and Aδ are the smallest and slowest of the A group.
B fibers
myelinated visceral fibers, both preganglionic autonomic fibers and some visceral afferents.
C fibers
unmyelinated
Unmyelinated fibers
usually referred to as C fibers but may be called group IV.
terminology for efferent fibers
The terminology for efferent fibers is fairly simple. The large axons innervating the extrafusal fibers of skeletal muscle are in the Aα category, and the smaller axons innervating intrafusal muscle fibers are in the Aγ category. The “A” is commonly dropped, and these are simply called α and γ motor neurons.
Preganglionic autonomic axons are usually called just that but are occasionally referred to as B fibers.
terminology for afferent fibers
Myelinated afferents are more complicated. The largest fibers, group I, are found only in muscle nerves; some form the primary endings of muscle spindles, and others innervate Golgi tendon organs. To distinguish between them, spindle primary fibers are called Ia and tendon organ fibers are called Ib.
Group II, corresponding to Aβ fibers, is quite diverse and includes the fibers that form the secondary endings of muscle spindles and those that form all the encapsulated receptors of skin and joints.
Group III consists of small myelinated afferents that form free nerve endings and includes mechanoreceptors, cold-sensitive thermoreceptors, and the nociceptors responsible for fast pain. Group III corresponds to Aδ, so these fibers are often referred to as δ fibers.
