MSK__Unit 3- PNS_Spinal Nerves - Maya Unit 3

How many pairs of spinal nerves are there, and what is the regional breakdown?

31 pairs total:

• 8 cervical

• 12 thoracic

• 5 lumbar

• 5 sacral

• 1 coccygeal

Through what opening do spinal nerves exit the vertebral column?

The intervertebral foramen (intervertebral foramina)

Explain the cervical spinal nerve naming quirk (8 nerves, 7 vertebrae).

C1-C7 exit above their corresponding vertebra; C8 exits below C7 (above T1). From T1 downward, each nerve exits below its corresponding vertebra.

What are the three steps in the formation of a spinal nerve?

1. Rootlets come off the horn of the spinal cord
2. Rootlets join to form a root
3. Roots unite to form the spinal nerve, which branches (carrying sensory + motor) at the intervertebral foramen

Compare the fiber content of the posterior root vs. the anterior root.

Posterior (dorsal) root = sensory (afferent) only
Anterior (ventral) root = motor (efferent) only

What does the spinal (dorsal root) ganglion contain?

The cell bodies of the somatic sensory (afferent) neurons

What is the function of the denticulate ligament?

A lateral extension of pia mater that:

Anchors and suspends the spinal cord within the dural sac (stabilizes the cord)

The spinal nerve is mixed — into which two branches does it split, and are they also mixed?

Splits into the anterior (ventral) ramus and posterior (dorsal) ramus — both are mixed (carry sensory + motor)

Summarize which spinal-nerve structures carry sensory-only, motor-only, and mixed fibers.

Sensory only: posterior root
Motor only: anterior root
Mixed: spinal nerve + anterior ramus + posterior ramus

Describe the ventral (anterior) rami: what they supply, and their pattern.

Supply the anterior/lateral trunk and limbs.

• Segmental in the thorax (= intercostal nerves) but combine to form plexuses in the cervical, lumbar, and sacral regions.

Describe the dorsal (posterior) rami: what they supply, and their pattern.

Supply the vertebral column joints, muscles of the back/spine, and overlying skin.

.

• Strictly segmental and do NOT form plexuses.

What are the rami communicantes and what do they connect?

The gray and white communicating rami

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• they connect the spinal nerve/anterior ramus to the sympathetic trunk (carrying autonomic fibers)

Somatic motor neuron: cell body location, direction of travel, NT, and effector?

Cell body in the anterior/ventral horn (CNS);

• 
  travels away from CNS to periphery;
  NT = acetylcholine (ACh);

• .
  effector = skeletal (striated) muscle

Somatic sensory neuron: cell body location, direction of travel, and morphology?

Cell body in the spinal (dorsal root) ganglion (PNS);
travels toward the CNS from the periphery;
it is a pseudounipolar neuron

In the somatic motor pathway, distinguish the upper vs. lower motor neuron.

Upper motor neuron: cell body in CNS (e.g., motor cortex); runs higher CNS → lower CNS.

.
Lower motor neuron: cell body in the ventral horn / central motor nucleus; runs CNS → PNS to skeletal muscle.
They synapse in the ventral horn.

In the sensory pathway, where are the 1st, 2nd, and 3rd order neurons, and where does the 1st synapse?

1st order: cell body in the sensory (dorsal root) ganglion — does not synapse until it reaches the CNS

.
2nd order: central sensory nucleus (CNS)

.
3rd order: projects up to the cortex (via thalamus)

What modalities are carried by general sensory receptors?

Touch, pressure, pain, thermal (temperature), and proprioception

How many neurons make up an autonomic (visceral motor) pathway, and what are they called?

Two — a preganglionic and a postganglionic neuron, synapsing in an autonomic ganglion

Compare the neurotransmitters of the sympathetic vs. parasympathetic pathways.

Sympathetic: preganglionic = ACh, postganglionic = norepinephrine (NEpi/Epi/dopamine)
Parasympathetic: ACh at both synapses

What are the typical targets of the sympathetic vs. parasympathetic pathways?

Sympathetic: smooth muscle, cardiac muscle, glands
Parasympathetic: cardiac muscle, glands

Define a dermatome.

A unilateral area of skin innervated by somatic sensory fibers from a single spinal nerve (root)

Define a myotome.

A unilateral muscle or group of muscles innervated by somatic motor fibers from a single spinal nerve

How does a dermatome differ from a cutaneous nerve's territory?

A dermatome = skin from a single spinal nerve/root

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A cutaneous (peripheral) nerve supplies skin using fibers from several spinal segments (mixed in plexuses)

Explain the two-way redundancy of spinal segments and peripheral nerves.

One spinal nerve level → several named nerves, and one peripheral nerve → several spinal segments.

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This overlap creates redundancy, so a single root lesion rarely fully paralyzes a muscle.

Define referred pain and explain its mechanism.

Pain from an internal organ perceived as somatic pain elsewhere.

• Visceral afferents enter the same spinal level as somatic afferents, so the brain misinterprets the origin.

Give the classic referred pain patterns for the heart and the diaphragm/gallbladder.

Heart: left arm, chest, neck, jaw
Diaphragm / gallbladder: shoulder (C3-C5), via the phrenic nerve