Lecture 2: SOMATIC vs. AUTONOMIC NERVOUS SYSTEM

Question 1

SOMATIC NERVOUS SYSTEM

Answer 1

The somatic system is the part of the PNS that is responsible for carrying motor and sensory information both to and from the CNS. This system is made up of nerves that connect to the skin, sensory organs and all skeletal muscles.

It is responsible for producing contractions of skeletal (voluntary) muscle and for processing sensory information that arrives via external stimuli including hearing, touch and sight.

Question 2

ventral root

Answer 2

Information destined for muscles is carried via axons that leave the spinal cord in a ventral root.

Question 3

dorsal root

Answer 3

Information carrying sensory information into the spinal cord enters via a dorsal root.

Question 4

AUTONOMOUS means

Answer 4

INDEPENDENT

Question 5

AUTONOMIC NERVOUS SYSTEM

Answer 5

PORTION OF THE NERVOUS SYSTEM THAT SUPPLIES MOTOR AND SENSORY INNERVATION TO STRUCTURES THAT ARE NOT UNDER VOLUNTARY CONTROL.

Question 6

Areas that the ANS controls:

Answer 6

SMOOTH MUSCLE IN THE
WALLS OF ORGANS, BLOOD
VESSELS, EYE, BRONCHI, ETC.

CARDIAC MUSCLE
(i.e., the heart)

GLANDS
(e.g., salivary, digestive)

Question 7

The ANS HAS TWO FUNCTIONALLY, CHEMICALLY, AND ANATOMICALLY DISTINCT DIVISIONS:
CLASSIC VIEW

Answer 7

Sympathetic – Prepares the body for an emergency

Parasympathetic – Conserves and restores energy; maintains homeostasis

Question 8

Sympathetic

Answer 8

Prepares the body for an emergency

Raises the rate and strength of the heart beat.
Dilates the eye pupil.
Constricts the blood vessels in the skin

Question 9

Parasympathetic

Answer 9

Conserves and restores energy; maintains homeostasis

Slows the rate and strength of the heart beat.
Constricts the eye pupil.
Stimulates secretion in the glands of the Digestive tract.

Question 10

The ANS HAS TWO FUNCTIONALLY, CHEMICALLY, AND ANATOMICALLY DISTINCT DIVISIONS:
NEW CONCEPT

Answer 10

In general, the Sympathetic and Parasympathetic systems represent antagonist elements that regulate the activity of involuntary structures.

2 systems are active at all times, not only in emergency (i.e., “Fight or flight”) mode

Question 11

BASIC AUTONOMIC MOTOR CIRCUIT

Answer 11

The ANS (sympathetic and parasympathetic) requires 2 neurons to effect a response in a target organ.

1ST neuron (PREGANGLIONIC) IS ALWAYS LOCATED WITHIN THE CNS – (i.e., either brain or spinal cord).

2nd neuron (POSTGANGLIONIC) is ALWAYS in a ganglia (not a DRG) located somewhere in the periphery.

Question 12

PARASYMPATHETIC NERVOUS SYSTEM NEUROTRANSMITTERS OF PREGANGLIONIC AND POSTGANGLIONIC NEURONS

Answer 12

The preganglionic neuron (in the CNS) send ACh to the postganglionic neuron (in the peripheral ganglia), which then sends more ACh to the target cell.

Relative Distance:
PreN&raquo_space;»»» PostN&raquo_space; Target

Question 13

SYMPATHETIC NERVOUS SYSTEM NEUROTRANSMITTERS OF PREGANGLIONIC AND POSTGANGLIONIC NEURONS

Answer 13

The preganglionic neuron (in the CNS) send ACh to the postganglionic neuron (in the peripheral ganglia), which then sends NE to the target cell.

Relative Distance:
PreN&raquo_space;» PostN&raquo_space;»»> Target

Question 14

NE

Answer 14

Norepinephrine

Question 15

ACh

Answer 15

Acetylcholine

Question 16

Somatic Neurons

Answer 16

Motor neurons in spinal cord project directly to skeletal muscles they innervate

Question 17

Autonomic Neurons

Answer 17

Motor neurons in spinal cord project to an synapse on another neuron located in a ganglion in the periphery. This neuron then innervates target structure.

Question 18

LOCATION OF GANGLIA:

SYMPATHETIC CHAIN

Answer 18

PARAVERTEBRAL GANGLIA

Bilateral; located adjacent to vertebral column

GANGLIA ARE ATTACHED TO ADJACENT SPINAL NERVE BY WHITE AND GRAY RAMI COMMUNICANTE.

EXTENT OF SYMPATHETIC CHAIN: Skull to Coccyx

Sympathetic chain of ganglia is located at all levels. Note connectors (rami communicante) between ganglia and adjacent nerves.

Question 19

LOCATION OF GANGLIA:

COLLATERAL (PREVERTEBRAL) SYMPATHETIC GANGLIA

Answer 19

COLLATERAL GANGLIA LOCATED IN FRONT OF THE ABDOMINAL AORTA

in between the sympathetic chain ganglia lines

Question 20

LOCATION OF PARASYMPATHETIC GANGLIA

Answer 20

In most areas, postganglionic parasympathetic neurons are located in walls of organ being innervated.

The degree of divergence is considered to be smaller in parasympathetic than in sympathetic ganglia. This, together with the location of parasympathetic ganglia in individual organs, makes it possible for the parasympathetic system to exert restricted, localized control. Sympathetic activation, in contrast, can be more widespread.

Question 21

SYMPATHETIC NERVOUS SYSTEM:

THORACOLUMBAR DIVISION

Answer 21

Preganglionic sympathetic cell bodies are located in the spinal cord between T1 and L2.

Question 22

The axons of ______ neurons leave the spinal cord via the ventral root at their level of origin.

Answer 22

ALL preganglionic sympathetic

Question 23

PREGANGLIONIC PARASYMPATHETIC NEURONS ARE LOCATED IN:

Answer 23

CRANIO-SACRAL

1. THE BRAINSTEM (CRANIAL)
   Preganglionic neurons are located in nuclei associated with specific cranial nerves (III, VII, IX, and X).
2. SACRAL PART OF SPINAL CORD BETWEEN S2 AND S4 (SACRAL)

RANIO-SACRAL

1. THE BRAINSTEM (CRANIAL)
   Preganglionic neurons are located in nuclei associated with specific cranial nerves (III, VII, IX, and X).
2. SACRAL PART OF SPINAL CORD BETWEEN S2 AND S4 (SACRAL)

Postganglionic parasympathetic neurons are located in ganglia that are located in or near the target structure.

Question 24

SUMMARY:

Somatic Nervous System

Answer 24

The somatic nervous system consists of neurons in the CNS that innervate skeletal, voluntary muscles.

The somatic nervous system receives sensory information from receptors located in the body wall.

There is a direct projection from the neuron in the brainstem/spinal cord to the muscle.

Question 25

SUMMARY:

Autonomic Nervous System

Answer 25

The autonomic nervous system provides motor innervation to smooth muscle, cardiac muscle, and glands.

The ANS requires 2 neurons to elicit of a motor response. The first neuron, called the preganglionic neuron, is located within the CNS and synapses on the second neurons called the postganglionic neurons that are located in ganglia found in the periphery. The postganglionic neuron projects to the target organ/gland.

There are 2 functionally distinct divisions of the autonomic nervous system: Sympathetic and Parasympathetic.

Most, but not all organs/glands receive innervation from both divisions. However, functionally they have opposite effects on these targets.

Preganglionic sympathetic neurons are located in the thoracic and upper lumbar part of the spinal cord (thoracolumbar).

Preganglionic parasympathetic neurons are located in the brainstem and sacral part of the spinal cord (craniosacral).

Postganglionic sympathetic and parasympathetic neurons are located within distinct ganglia located in the periphery or within the wall of the organ being innervated.

NOTE: There is a sensory component to the ANS we’ll discuss later.

Question 26

Intermediate Gray Matter Contains ____.

Answer 26

Autonomic Neurons

Question 27

Each Spinal Cord Segment Innervates a _______

Answer 27

Dermatome

Question 28

The Spinal Cord is ______ than the Vertebral Canal.

Answer 28

The Spinal Cord is Shorter than the Vertebral Canal.

Question 29

The Posterior Horn contains ____ and _____.

Answer 29

Sensory Interneurons and Projection Neurons.

Question 30

The Anterior Horn contains _____ .

Answer 30

Motor Neurons

Question 31

An adult human spinal cord appears small, being only about 42-45 cm long and about 1 cm in diameter at its widest point.

Answer 31

The adult spinal cord weighs only about 35 g, so one could be mailed for just 2 stamps.

Question 32

dorsal rootlets

Answer 32

dorsal (i.e., posterior) rootlets enter the cord in a shallow longitudinal groove (the posterolateral sulcus) on its posterolateral surface

Question 33

ventral rootlets

Answer 33

ventral (i.e., anterior) rootlets leaves from the poorly defined anterolateral sulcus.

Question 34

dorsal and ventral rootlets

Answer 34

The dorsal and ventral rootlets from discrete sections of the cord coalesce to form dorsal and ventral roots, which in turn join to form spinal nerves.

Question 35

dorsal root

Answer 35

The posterior, sensory root of a spinal nerve, which divides into a variable number of regularly spaced rootlets that enter the spinal cord along its posterolateral sulcus.

Each dorsal root bears a dorsal root ganglion just proximal to the junction between dorsal and ventral roots.

Question 36

dorsal root ganglion

Answer 36

contains the cell bodies of the primary sensory neurons whose processes travel through that particular spinal nerve.

Question 37

31 segments in a human spinal cord

Answer 37

8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal.

Question 38

segment

Answer 38

A portion of the cord that gives rise to a spinal nerve

Question 39

conus medullaris

Answer 39

The pointed tapering caudal end of the spinal cord, at about vertebral level L1-L2.

Question 40

cervical enlargement

Answer 40

Supplies the upper extremities, is conventionally considered to extend from the fifth cervical to the first thoracic segment (C5 to T1), inclusive.

contains increased numbers of motor neurons and interneurons.

Question 41

lumbar enlargement

Answer 41

The lumbar (or lumbosacral) enlargement, which supplies the lower extremities, extends from the second lumbar to the third sacral segment (L2 to S3).

contains increased numbers of motor neurons and interneurons.

Question 42

dermatome

Answer 42

The area of skin innervated by a single spinal cord segment.

Each spinal nerve (except C1, which typically has only a rudimentary dorsal root) innervates a single dermatome.

Knowledge of the segmental innervation of muscles and cutaneous areas can be extremely helpful in diagnosing the site of damage in or near the spinal cord.

Example: compression of a dorsal root can cause pain in its dermatome, allowing pain caused by root compression to be differentiated from pain caused by peripheral nerve damage. In addition, the highest level of a sensory or motor deficit may allow deductions about the segmental level of a suspected spinal cord lesion.

Question 43

The spinal cord approaches its adult length _____ the vertebral canal does.

Answer 43

The spinal cord approaches its adult length before the vertebral canal does.

The spinal nerves still exit through the same intervertebral foramina as they did early in development, and each dorsal root ganglion remains at the level of the appropriate foramen. Proceeding from cervical to sacral levels, the dorsal and ventral roots become progressively longer because they have longer and longer distances to travel before reaching their sites of exit from the vertebral canal.

Question 44

lumbar cistern

Answer 44

The CSF-filled subarachnoid cistern extending from the conus medullaris, at about vertebral level L1-L2, to the end of the spinal dural sac, at about vertebral level S2. Nerve roots of the cauda equina travel through the lumbar cistern.

Question 45

cauda equina

Answer 45

Latin for “horse’s tail”.

The collection of spinal nerve roots traversing the lumbar cistern on their way to the intervertebral foramina through which they leave the vertebral canal.

A needle carefully inserted into the lumbar cistern will pass harmlessly among nerve roots, allowing safe sampling of cerebrospinal fluid (CSF).

Question 46

filum terminale

Answer 46

Literally, the “terminal thread.”

The pial prolongation extending through the cauda equina, from the caudal end of the spinal cord (the conus medullaris) to the end of the dural sac surrounding the spinal cord.

the caudal end of the cord is anchored to the end of the dural tube by the filum terminale, an extension of the pial covering of the conus medullaris. The filum terminale then acquires a dural outer layer and in turn is anchored to the coccyx.

Question 47

autonomic nervous system

Answer 47

The parts of the PNS (extending slightly into the CNS, to the cell bodies of preganglionic neurons) that control smooth and cardiac muscle and glands. The autonomic systemincludes parasympathetic, sympathetic, and enteric subdivisions.

Question 48

enteric nervous system

Answer 48

The subdivision of the ANS contained within the walls of the gut. The enteric system includes sensory and motor neurons and interneurons entirely outside the CNS and is able to function independently, although normally it is modulated by other autonomic elements connected to the CNS.

Although less well known than the other two, the enteric nervous system perhaps best exemplifies the concept of automatic, self-regulating function. It consists of 2 interconnected plexuses (the myenteric plexus [of Auerbach] and the submucosal plexus [of Meissner]), including sensory neurons, interneurons, and visceral motor neurons, in the walls of the alimentary canal; all these neurons and their processes lie entirely outside the CNS. The plexuses are quite extensive, containing a number comparable to the number of neurons in the entire spinal cord! The enteric nervous system accounts for the observation that near-normal, coordinated gut motility persists even in the total absence of connections between the gut and the CNS. The normally present sympathetic and parasympathetic connections between the gut and the CNS allow for modulation of this motility.

Question 49

sympathetic and parasympathetic efferents originating in the CNS do not reach their targets directly….

Answer 49

A two-neuron chain is involved.

The first neuron, a preganglionic neuron, has its cell body in the CNS. Its axon terminates in a peripheral ganglion on the second neuron, a postganglionic neuron.

Question 50

preganglionic neuron

Answer 50

Has its cell body in the CNS. Its axon terminates in a peripheral ganglion on a second neuron, a postganglionic neuron.

The preganglionic neurons of sympathetic AND parasympathetic systems liberate acetylcholine onto the postganglionic neurons, where it acts on nicotinic receptors.

Preganglionic parasympathetic fibers originate from neurons in two widely separated parts of the CNS—the brainstem and the sacral spinal cord.
They travel in sacral spinal nerves or in certain cranial nerves (III, VII, IX, and, most important, X) to ganglia in or near their targets.

Question 51

postganglionic neuron

Answer 51

Postganglionic parasympathetic neurons also release acetylcholine onto their targets, in this case affecting muscarinic receptors.

Most postganglionic sympathetic neurons, in contrast, release norepinephrine (a prominent exception is the sympathetic innervation of sweat glands, which is cholinergic).

Postganglionic neurons in these peripheral parasympathetic ganglia then innervate the target organ.

Question 52

craniosacral outflow

Answer 52

The parasympathetic (or craniosacral) outflow goes almost exclusively to thoracic, abdominal, and pelvic viscera and to some important cranial targets. There are, for example, no parasympathetic fibers to the limbs.

In a general sense, the parasympathetic system enhances energy storage. Activation of parasympathetic nerves causes decreased cardiac output and blood pressure, increased peristalsis in the gut, and salivation, as well as pupillary constriction and bladder contraction. Visceral afferents traveling with sacral spinal nerves and with cranial nerves IX and X are appropriate for these functions, carrying information about blood pressure and chemistry and fullness of the bladder and gastrointestinal tract. Cranial nerves VII, IX, and X also carry information from taste buds, obviously relevant to energy intake.

Question 53

Preganglionic Sympathetic Neurons Are Located in ____ and ____ Spinal Segments

Answer 53

Preganglionic Sympathetic Neurons Are Located in Thoracic and Lumbar Spinal Segments

Question 54

thoracolumbar outflow

Answer 54

The sympathetic division has thoracolumbar “outflow”, meaning that the neurons begin at the thoracic and lumbar (T1-L2/3) portions of the spinal cord.

catabolic system involved in preparing the body for emergencies. Also called the sympathetic nervous system.

Question 55

sympathetic chain

Answer 55

The preganglionic fibers travel in spinal nerves to ganglia relatively close to the spinal cord. Some of these ganglia form an interconnected sympathetic chain just distal to the dorsal root ganglia. The prevertebral ganglia, are a little farther away.

The major exception is the adrenal medulla, which is directly innervated by preganglionic sympathetic fibers. The adrenal medulla develops from neural crest cells and, similar to postganglionic sympathetic neurons, its cells secrete norepinephrine (and epinephrine as well); it can therefore be thought of as a displaced sympathetic ganglion.

Question 56

Sympathetic fibers are _____ than parasympathetic fibers, reaching all parts of the body.

Answer 56

Sympathetic fibers are more widely distributed than parasympathetic fibers, reaching all parts of the body.

Question 57

white communicating rami

Answer 57

Preganglionic fibers exit the spinal cord in thoracic and upper lumbar ventral roots and then travel from the spinal nerves to the sympathetic chain via white communicating rami, so called because the preganglionic fibers are myelinated and hence white; only spinal nerves T1 to L2-L3 have white rami.

Question 58

gray communicating rami

Answer 58

unmyelinated postganglionic fibers rejoin spinal nerves via gray communicating rami; all spinal nerves include gray rami and postganglionic sympathetic fibers.

Question 59

Visceral Distortion or Damage Causes Pain That Is Referred to _____.

Answer 59

Visceral Distortion or Damage Causes Pain That Is Referred to Predictable Dermatomes.

Question 60

Visceral pain

Answer 60

Visceral pain is different from somatic pain in that it is poorly localized to the diseased organ and commonly is Referred to an area of the body surface.

The area to which the pain is referred corresponds to the dermatome innervated by the spinal segment to which the visceral afferents project. Thus the heart is supplied by visceral afferents that enter the cord in upper thoracic segments, and coronary artery disease is associated with pain referred to the left side of the chest and part of the left arm (angina pectoris). The basis of this referred pain is the convergence of visceral and somatic pain fibers in a given dorsal root on the same spinothalamic tract cells reached by somatic pain fibers, leading to the brain interpreting spinothalamic tract impulses as pain in the somatic region. The functional utility of such an arrangement is not clear, but knowledge of typical patterns of referred pain is important clinically.

Question 61

Referred Pain

Answer 61

Pain seeming to originate from some predictable part of the body surface as a result of a damaged internal organ. A common example is angina pectoris, pain in the left side of the chest and the left arm accompanying coronary artery disease.

Question 62

radicular arteries

Answer 62

The arterial supply of the spinal cord comes from the vertebral arteries and from branches, ultimately from the thoracic and abdominal aorta, called radicular arteries.

Question 63

anterior spinal artery

Answer 63

A single midline vessel that originates rostrally as two arteries (one from each vertebral) which shortly join and then course within theanterior median fissure along the entire spinal cord. It receives additional blood from the thoracic/abdominal aorta through numerous anastomoses with radicular arteries below the upper cervical region, and gives rise to hundreds of central and circumferential branches that supply the anterior two-thirds of the cord.

Question 64

posterior spinal artery

Answer 64

A small branch of each vertebral artery that travels near the line of attachment of dorsal roots, supplying the posterior third of the spinal cord. Like the anterior spinal artery, it receives additional blood from the thoracic/abdominal aorta through numerous anastomoses with radicular arteries below the upper cervical region.

Question 65

??

Answer 65

The arterial supply of the spinal cord comes from the vertebral arteries and from branches, ultimately from the thoracic and abdominal aorta, called radicular arteries. Each vertebral artery gives rise to an anterior spinal artery; the two anterior spinal arteries fuse to form a single midline vessel—the longest artery in the body—that courses along the anterior median fissure of the spinal cord. The vertebral or posterior inferior cerebellar artery of each side also gives rise to a posterior spinal artery, which proceeds along the line of attachment of the dorsal rootlets. The posterior spinal arteries and the midline anterior spinal artery supply upper cervical levels with blood from the vertebral arteries. Below this, all 3 spinal arteries form a more or less continuous series of anastomoses with radicular arteries for the length of the cord. The spinal arteries are too small to convey blood from the vertebral arteries to all spinal levels; beginning with lower cervical segments, the cord depends on these radicular arteries for its survival. One particular radicular artery, present at about spinal cord level T12 in most individuals, is called the great radicular artery (or artery of Adamkiewicz) and may provide the entire arterial supply for the lumbosacral spinal cord.

The anterior spinal artery, which is usually a continuous vessel for the length of the spinal cord, gives rise to a series of hundreds of central and circumferential branches that supply the anterior two thirds of the spinal cord, including the base of the posterior horn and a variable portion of the lateral corticospinal tract. The posterior spinal arteries may subdivide into longitudinal branches medial and lateral to the dorsal roots and are really more of a plexiform network of small arteries. Collectively they supply the posterior columns, substantia gelatinosa, dorsal root entry zone, and a variable portion of the lateral corticospinal tract.

Venous drainage is by a series of six irregular, plexiform channels: one each along the anterior and posterior midlines, and one along the line of attachment of the dorsal and ventral rootlets of each side. These are drained by radicular veins, which in turn empty into the epidural venous plexus.