5. Anatomy | Anatomical Divisions of the Peripheral Nervous System

What is the control type of the somatic nervous system?

Voluntary

What is the control type of the visceral nervous system?

Involuntary

What structures are innervated by the somatic nervous system?

Skeletal muscles, fascia, bone, cartilage, most of skin

What structures are innervated by the visceral nervous system?

Glands, smooth muscle (e.g., gut), cardiac muscle

What neuron types are in the somatic nervous system?

Somatic motor and sensory neurons

What neuron types are in the visceral nervous system?

Visceral motor (autonomic) and sensory neurons

What fiber types are found in the somatic nervous system?

GSE (General Somatic Efferent) and GSA (General Somatic Afferent)

What fiber types are found in the visceral nervous system?

GVE (General Visceral Efferent) and GVA (General Visceral Afferent)

What is the function of the somatic nervous system?

Movement and localized sensation

What is the function of the visceral nervous system?

Homeostatic organ function, poorly localized sensation

How is sensation localized in the somatic vs visceral nervous system?

Somatic = Well-localized; Visceral = Poorly localized

What does the mnemonic 'SAME DAVE' stand for?

Sensory=Afferent, Motor=Efferent; Dorsal=Afferent (sensory), Ventral=Efferent (motor)

What does afferent mean?

From body mm. + glands to brain/spinal cord

What does efferent mean?

From brain/spinal cord to body mm. + glands

What is a sensory example?

Holding hand to flame

What is a motor example?

Flex biceps brachii

What two parts contain only sensory?

Dorsal horn and dorsal root ganglion

What mm. are dorsal ramus associated with?

Epaxial mm.

What two parts contain only motor?

Ventral horn and ventral root

At the spinal nerve, what two types of fibers mix?

Sensory and motor

What do the dorsal and ventral ramii have?

Sensory and motor that go to or from the target organ

What is the neuron type in the somatic motor pathway (efferent)?

Somatic motor (GSE)

Where is the cell body of a somatic motor neuron located?

Ventral horn of spinal cord (inside CNS)

Describe the axon path of a somatic motor neuron.

Ventral horn (containing cell bodies) → ventral root (containing motor axons) → spinal nerve → dorsal or ventral ramus → synapse on target organ/mm. (depends on whether it was dorsal or ventral ramii)

What is the target of somatic motor neurons?

Skeletal muscle

Where do somatic motor neurons synapse?

Directly on target muscle

What does the dorsal ramus supply in motor innervation?

Epaxial muscles

What does the ventral ramus supply in motor innervation?

Hypaxial muscles

What is the neuron type in the somatic sensory pathway (afferent)?

Somatic sensory (GSA)

What are examples of somatic sensory receptors?

Skin, joints, fascia, skeletal muscle

Describe the peripheral process in the sensory pathway.

Carries impulse via dorsal or ventral ramus to the spinal nerve

Where is the cell body of a somatic sensory neuron located?

Dorsal root ganglion (outside CNS)

What is the path of the central process of the sensory neuron?

Travels via dorsal root to dorsal horn

Where do somatic sensory neurons synapse?

Dorsal horn of spinal cord

List the full pathway of somatic sensation from body wall to spinal cord.

Receptor → dorsal/ventral ramus → spinal nerve → dorsal root ganglion → dorsal root → dorsal horn

How many cervical spinal nerves are there?

8

How many thoracic spinal nerves are there?

12

How many lumbar spinal nerves are there?

5

How many sacral spinal nerves are there?

5

How many coccygeal spinal nerves are there?

1

How many spinal nerves are there in total?

31

Where does the embryonic spinal cord end?

At the coccyx

Where does the adult spinal cord end?

At the L1/L2 vertebral level

Where is an epidural typically inserted to avoid spinal cord injury?

Between L3 and L4

If both L3 and L4 are answer options, which is the safest for epidural?

L4 *** check

Where does the dural sac end?

At the level of S2

Why is it safe to place a catheter or needle for epidural below L1/L2?

Because the spinal cord ends above that level

Where is the spinal cord anchored in the adult?

At the coccyx

What spinal level is associated with the shoulder dermatome?

C5

What spinal level is associated with the medial forearm dermatome?

T1

What spinal level is associated with the axilla dermatome?

T2

List the epaxial mm.

Erector spinae, Transversospinalis, Splenius capitis and cervicis, Suboccipital mm.

Why are spinal cord and vertebrae aligned in the embryo?

Because they grow at the same rate‚ nerves exit horizontally

Why are spinal nerves longer in adults than embryos?

Because the vertebral column grows faster, causing nerves to descend to their exit points

Where does varicella zoster virus reside in shingles?

Dorsal root ganglion

What spinal level is associated with the nipple line dermatome?

T4

What spinal level is associated with the umbilicus dermatome?

T10

What spinal level is associated with the pubic symphysis?

Border of T12 and L1

What spinal level is associated with the thumb dermatome?

C6

What spinal level is associated with the middle finger dermatome?

C7

What spinal level is associated with the pinky finger dermatome?

C8

According to the Rule of Ones, where is the C1 spinal cord segment located?

C1 vertebra (perfectly aligned)

According to the Rule of Ones, where is the T1 spinal cord segment located?

T1 vertebra (still relatively aligned)

According to the Rule of Ones, where is the L1 spinal cord segment located?

T11 vertebra

According to the Rule of Ones, where is the S1 spinal cord segment located?

L1 vertebra

What type of fibers are in the ventral root?

Motor (efferent)→ somatic and visceral

What type of fibers are in the dorsal root?

Sensory (afferent)→ somatic and visceral

Where are sensory neuron cell bodies located?

Dorsal root ganglion

What fibers are carried in spinal nerves?

Mixed→ motor and sensory

What structures do ventral rami supply?

Hypaxial structures (limbs and anterior body)

What structures do dorsal rami supply?

Epaxial structures (deep back)

What spinal levels form the cervical plexus?

C1→C4

What spinal levels form the brachial plexus?

C5→T1

What spinal levels form the intercostal nerves?

T1→T11

What spinal level gives rise to the subcostal nerve?

T12

What spinal levels form the lumbar plexus?

L1→L4

What spinal levels form the sacral plexus?

L5→S3 (posterior thigh, leg, foot)

How are the layers of the meninges organized from superficial to deep?

A. Arachnoid mater, dura mater, pia mater
B. Arachnoid mater, pia mater, dura mater
C. Dura mater, arachnoid mater, pia mater
D. Pia mater, arachnoid mater, dura mater
E. Pia mater, dura mater, arachnoid mater

C

The correct answer is dura mater, arachnoid mater, pia mater (C).

The dura mater is the thick, outermost layer. The arachnoid mater is deep to, and fused to, the dura and sits superficially to the subarachnoid space, which contains cerebrospinal fluid. Deep to the arachnoid mater is the pia mater, which is the very thin layer that adheres tightly to the brain and spinal cord.

Which of these structures is mainly filled with long myelinated axons?

A. Central canal
B. Gray matter
C. Meninges
D. Vertebrae
E. White matter

E

The correct answer is white matter (E). White matter is mainly filled with long myelinated axons. The central canal (A) is filled with cerebrospinal fluid and ependymal cells, not long myelinated axons. Gray matter (B) does have myelinated axons in it. However, that is not what primarily composes the gray matter; it is also filled with neuronal cell bodies, glial cells, axons, and dendrites. The meninges (C) are not filled with long myelinated axons. Vertebral bodies (D) are bones; they are not filled with myelinated axons.

After a fall on his back, a 28-year-old male presents to your clinic reporting that he cannot feel pain or temperature in his lower extremities. An injury to which of the following spinal cord tracts would best account for his symptoms?

A. Corticospinal tract

B. Dorsal (posterior) columns

C. Spinocerebellar tracts

D. Spinothalamic tract

E. Ventral horn

The correct answer is spinothalamic tract (D). Injury to the spinothalamic tract would result in the loss of pain and temperature sensation. The corticospinal tract (A) is a motor pathway that begins at the cortex of the brain and terminates on motor neurons of the spinal cord. It controls movements of the limbs and trunk. The dorsal columns (B) transmit pressure, vibration, fine touch, and conscious proprioception (joint orientation) sensation toward the sensory cortex of the brain. The spinocerebellar tract (C) is responsible for communication of proprioceptive information to the cerebellum. The ventral horn (E) is not responsible for pain and temperature; rather, it contains the neuronal cell bodies of the somatic motor (efferent) neurons. Motor (efferent) axons emerge from this area and form the ventral root of each spinal nerve.

Which of the following PNS pathways connects the spinal cord to a peripheral target using two neurons that synapse in a ganglion?

A. General visceral afferent

B. Parasympathetic afferent

C. Somatic afferent

D. Somatic efferent

E. Sympathetic efferent

E

The correct answer is sympathetic efferent (E). Sympathetic pathways have preganglionic and postganglionic neurons that synapse in sympathetic ganglia near the spinal cord. There are no parasympathetic afferent pathways (B). The other listed pathways only consist of one neuron, with cell bodies either in the spinal cord—somatic efferent (D), or in the dorsal root ganglion just adjacent to the spinal cord, general visceral afferent (A) and somatic afferent (C).

Which of the following structures contains somatic motor, somatic sensory, and autonomic fibers?

A. Dorsal root

B. Lateral horn of the spinal cord

C. Spinal nerve

D. Sympathetic ganglion

E. Ventral root

C

The correct answer is spinal nerve (C). Spinal nerves contain all three nerve fiber types as they exit the vertebral column. The dorsal root (A) contains only sensory fibers. The lateral horn of the spinal cord (B) contains autonomic nerve cell bodies. The sympathetic ganglion (D) is where preganglionic and postganglionic sympathetic neurons synapse. The ventral root (E) is where motor fibers exit the spinal cord.