Lecture 3: PERIPHERAL NERVOUS SYSTEM

Question 1

Study the Diagram of the Spinal Cord Showing Distinct Regions

Answer 1

??

Question 2

Central area of the spinal cord that has defined shape

Answer 2

“butterfly” = Grey Matter. Consists of populations of neurons and glia.

Question 3

Area of the spinal cord that is lighter in color

Answer 3

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.

Question 4

Nissl Stain

Answer 4

identifies neuron cell bodies

A Nissl Stain of the spinal cord makes the grey matter darker than the white matter.

Question 5

Fiber Stain

Answer 5

identifies myelinated axons

Fiber Stains make the white matter darker than the grey matter.

Question 6

Functional Organization of the Spinal Cord

Answer 6

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.

Question 7

GANGLION

Answer 7

Collection of neurons located outside brain or spinal cord (i.e., in the periphery)

Question 8

DORSAL ROOT GANGLIA

Answer 8

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.

Question 9

STUDY Visuals of the FORMATION OF A SPINAL NERVE
and the
Functional Organization of the Spinal Cord

Answer 9

Spinal nerve&raquo_space; Ventral Root&raquo_space; Motor Neuron&raquo_space; Ventral Horn&raquo_space; Dorsal Horn&raquo_space; Dorsal Root&raquo_space; Central Process&raquo_space; DRG Cell&raquo_space; DRG&raquo_space; Peripheral Process

[Peripheral Process]&raquo_space; Sensory Receptors&raquo_space; DRG&raquo_space; [Central Process]&raquo_space; AG&raquo_space; Viscra & Skeletal Muscle

Question 10

Tract

Answer 10

a bundle of axons that connects two CNS regions

(e.g., Corticospinal Tract = axons from cortex projecting to the spinal cord

Question 11

Ascending Tract

Answer 11

Projects from spinal cord to the brain

Question 12

Descending Tract

Answer 12

Projects from brain to the spinal cord

Question 13

Fasciculus/Fasciculi

Answer 13

A bundle of anatomically defined fibers that subserve a common function

aka, a tract.

Question 14

Funiculus

Answer 14

An area containing multiple tracts (or fasciculi)

(e.g., Lateral Funiculus of the spinal cord

Question 15

Subdivisions of the Gray Matter

Answer 15

Dorsal Horn  (sensory)
Intermediate Grey  (ANS)
Ventral Horn  (motor)

Question 16

Dorsal Horn

Answer 16

Grey matter on dorsal half of spinal cord.

Question 17

Dorsal column/funiculus

Answer 17

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.

Question 18

Ventral Horn

Answer 18

Grey matter on ventral half of spinal cord.

Question 19

Ventral funiculus

Answer 19

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

Question 20

Intermediolateral Cell Column

Answer 20

Between dorsal and ventral horn. Location of preganglionic sympathetic and parasympathetic neurons.

Only found between T1 and S3.

Question 21

Lateral funiculus

Answer 21

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:

1. Ascending sensory axon tracts (pain, temperature, cerebellar)
2. Descending motor axon tracts from brain (cortex, midbrain

Question 22

Anterior White Commissure

Answer 22

Located dorsal to the the anterior median fissure

Contains axons crossing from one side of the spinal cord to the other

Question 23

NERVES

Answer 23

Made up of axons from hundreds of individual neurons

Question 24

ENDONEURIUM

Answer 24

The connective tissue that surrounds individual axons

Question 25

PERINEURIUM

Answer 25

The connective tissue sheath that groups together Multiple bundles

Question 26

EPINEURIUM

Answer 26

The common thick external connective sheath that encloses All bundles

Question 27

nerves that are connected to the spinal cord

Answer 27

spinal nerves

Question 28

nerves that are connected to the brain

Answer 28

cranial nerves

Question 29

If the sensory axons do not leave the spinal cord, how do they get sensory information from the periphery?

Answer 29

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.

Question 30

Neurons within the ventral part of the spinal cord give rise to axons that leave ______ on the _____ side.

Answer 30

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.

Question 31

DRG Neurons

Answer 31

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.

Question 32

FORMATION OF A SPINAL NERVE

Answer 32

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.

Question 33

how are tracts are named?

Answer 33

origin-destination

Question 34

Axons that enter or leave the brain and spinal cord form nerves that are part of the peripheral nervous system.

Answer 34

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

Question 35

nerve

Answer 35

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.

Question 36

Nerves Are Made Up of Morphologically and Functionally Distinct Types of Axons:

Anatomical Differences

Answer 36

Myelinated & unmyelinated axons

Large & small diameter

Question 37

Nerves Are Made Up of Morphologically and Functionally Distinct Types of Axons:

Answer 37

Efferent axons –Carry message away from CNS (Motor).

Afferent axons – Carry message toward CNS (Sensory).

Question 38

Afferent axons

Answer 38

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.

Question 39

Efferent axons

Answer 39

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.

Question 40

General Somatic Efferent Axons (GSE)

Answer 40

Outgoing axons carrying motor commands to skeletal muscles.

Question 41

General Visceral Efferent Axons (GVE; Autonomic nervous system)

Answer 41

Outgoing axons carrying motor commands to smooth muscle, cardiac muscle, and glandular tissue, i.e., involuntary tissue.

Question 42

General Somatic Afferent Axons (GSA)

Answer 42

Axons that transmit sensory information (touch, pressure, pain, temperature, muscle tension) from the body to the spinal cord.

Question 43

General Visceral Afferent Axons

GVA; Autonomic Nervous System

Answer 43

Axons that transmit sensory information (distension, pressure, pain) from organs to the spinal cord.

Question 44

Large myelinated sensory carry…..

Answer 44

touch, pressure, distension, proprioceptive information.

Question 45

Small diameter sensory axons carry

Answer 45

pain and temperature information.

Question 46

Different peripheral nerves will have varying proportions of each type of axon.

Answer 46

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.

Question 47

CRANIAL NERVES

Answer 47

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.

Question 48

vagus nerve

Answer 48

Latin for “wanderer”

The one cranial nerve that actually leaves the skull and courses all the way to the caudal abdomen.

Question 49

CRANIAL NERVES

Specifics

Answer 49

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.

Question 50

COMPONENTS OF CRANIAL NERVES

Answer 50

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.

Question 51

SPINAL NERVE

Summary

Answer 51

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

Question 52

Common Axon Types, and their Abbriviations

Answer 52

GSE – General Somatic Efferent
GVE – General Visceral Efferent
GSA – General Somatic  Afferent
GVA – General Visceral Afferent 
SSA – Special Somatic Afferent 
SVE – Special Visceral Efferent

Question 53

CRANIAL NERVE

Summary

Answer 53

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

Question 54

SUMMARY OF THE PERIPHERAL NERVOUS SYSTEM (PNS)

Answer 54

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.

Question 55

SUMMARY OF THE PERIPHERAL NERVOUS SYSTEM (CONTINUED)

Answer 55

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

Question 56

Peripheral Nerves Convey Information to and from the CNS

Answer 56

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).

Question 57

epineurium

Answer 57

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.

Question 58

perineurium

Answer 58

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.

Question 59

endoneurium

Answer 59

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.

Question 60

The Diameter of a Nerve Fiber Is Correlated with Its Function

Answer 60

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.

Question 61

A fibers

Answer 61

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.

Question 62

B fibers

Answer 62

myelinated visceral fibers, both preganglionic autonomic fibers and some visceral afferents.

Question 63

C fibers

Answer 63

unmyelinated

Question 64

Unmyelinated fibers

Answer 64

usually referred to as C fibers but may be called group IV.

Question 65

terminology for efferent fibers

Answer 65

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.

Question 66

terminology for afferent fibers

Answer 66

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.