Lecture 4a - Peripheral Nervous System

ANAT 305 - Cross Sectional Anatomy. University of Alberta

What are the two portions of the nervous system

The CNS and the PNS

What are the two portions of the CNS

the brain (cerebrum, cerebellum, and brainstem)

the spinal cord

What are the two components of the PNS

The somatic nervous system and the autonomic nervous system

what can both the somatic and autonomic nervous system be split into

motor and sensory subdivisions

What are the two portions of the motor subdivision of the ANS

the sympathetic and parasympathetic nervous systems

What regions of the spinal cord control the sympathetic motor ANS, and what type of response does it generate

The sympathetic motor ANS is primarily controlled by the thoracic and lumbar regions of the spinal cord, generating a 'fight or flight' response.

What regions of the spinal cord control the parasympathetic motor ANS, and what type of response does it generate

The parasympathetic motor ANS is primarily controlled by the brainstem and sacral regions of the spinal cord, generating a 'rest and digest' response.

What three things are sensory autonomic nerves (visceral afferents) sensitive to?

stretch, ischemia, and chemoreception

• They detect changes in the internal environment, such as blood pressure, oxygen levels, and chemical composition.

Is the pain sensation from sensory autonomic nerves localized well or poorly

Pain sensation from sensory autonomic nerves is poorly localized, often resulting in diffuse discomfort that is not easily traced to a specific source.

visceral afferent pain fibres in the sensory autonomic nervous sytem travel back to the spinal cord via what two nerves?

1. sympathetics

2. vagus nerve

What three types of stimulation are visceral afferent fibres sensitive to?

1. stretch

2. ischemia

3. chemoreception

what is the origin of somatic structures

somatic structures arise from the somatic mesoderm

• from somites

Examples of somatic structures

skin, muscles, bone of the body wall, and parital membranes

• pleura, pericardium, peritoneum, periosteum

Where do somatic structures get their motor nerve supply from

spinal nerves (both segmental and non-segmental)

• all dorsal spinal nerves = segmental

• ventral spinal nerves only segment T1-T12

where do somatic structures get their sensory nerve supply from

spinal nerves (both segmental and non-segmental)

• all dorsal spinal nerves = segmental

• ventral spinal nerves only segmented T1-T12

What modalities do sensory spinal nerves respond to?

all modalities  - pain, touch, temperature, chemoreception

Describe somatic pain

it is sharp and well localized

what is the origin of the visceral / splanchnic structures

the splanchnic mesoderm

Examples of visceral/splanchnic structures

viscera

• heart, lungs, liver, glands, smooth muscle, gut, uterus

Visceral membranes

where do splanchnic structures get their motor supply from?

the autonomic nervous system

• sympathetic = thoracolumbar (T1-L2)

• parasympathetic = craniosacral (III, VII, IX, X, and S2-4)

where do splanchnic structures get their sensory nerve supply from?

Visceral afferents in the autonomic nervous system that travel with the sympathetics or vagus nerve (CN X)

What two sensations do visceral afferents respond to?

stretch and ischemia

describe splanchnic structure pain

Splanchnic structure pain is often poorly localized and can be described as deep, aching sensations.

what do the branchial arches structures include

head and neck structures

where do branchial arch structures get both their motor and sensory nerve supply

Cranial nerves

What two portions of the peripheral nervous system are important for understanding the concept of pain?

1. spinal nerves

2. autonomic nervous system

Spinal nerves supply _____ structures

somatic (body wall, extremeties) structures

• skeletal muscle, bone, parietal membranes and then become cutaneous to supply their dermatomes

what are dermatomes?

Areas of skin supplied with it’s own sensory innervation from specific spinal nerves, which correlate to segments of the spinal cord.

Describe what is going on in the sensory neuron of this diagram

A sensory input (cold) travels along a sensory neuron (yellow) in the skin

• this can be dorsal (posterior) or vental (anterior)

the sensory axon runs along the dorsal root and reaches the dorsal root ganglion and passes to spinal cord where it synapses with a second order neuron that travels up to the thalamus and cerbral cortex via the spinothalamic tract where the brain activates a response

Describe what is going on in the sympathetic neuron of this diagram

the preganglionc sympathetic neuron originates in the intermediolateral cell column (lateral) horn at T1-L2 (thoracolumbar since sympathetic)

• its axon exits via the ventral root and joins the spinal nerve entering the white ramus (myelinated) communicans and synapses in the sympathetic ganglion with a postganglionic neuron

the post ganglionic neuron leaves the ganglion via the gray ramus communicans (non-myelinated) where it rejoins the spinal nerve then follows the dorsal and ventral rami to reach the skin to innervate muscles

Why do spinal nerves needs sympathetic innervation from the ANS?

because sympathetic fibers supply the sweat and sebaceous glands, arrector pili muscles, and smooth muscle in the blood vessels (which are all visceral) within the somatic strucutres

describe what is going on in the red motor neuron

If instead the sensory input was pain, instead of going to the brain first, an internuron excites a motor neuron in the ventral horn for a reflex arc

• the motor neuron sends out it’s axon through the ventral root, passing into the spinal nerve and then out into the primary ramus to reach skeletal muscles to move

what are primary rami?

The primary rami are the major branches of spinal nerves that split into the dorsal ramus and ventral ramus. They carry both sensory and motor fibers to innervate muscles and skin in their respective regions.

how many pairs of spinal nerves do we have and where do they arise from?

There are 31 pairs of spinal nerves, which arise from the spinal cord and are organized into cervical, thoracic, lumbar, sacral, and coccygeal regions.

After supplying skeletal muscles, parietal membranes, and periosteum, what do the spinal nerves innervate?

the spinal nerves will become cutaneous and supply dermatomes of the skin, providing sensation

Where are dermatomes arranged in a segmental ladder-like fashion?

the thorax and abdomen

Why are dermatomes in the extremities and head/neck non-segmental?

Because innervation there is mixed through plexuses, not simple segmental nerves.

• this makes the dermatomes to be more complicated looking, not just segmented lines

What is the dermatome level of the sternal angle of Louis?

T2

what is the dermatome level of the nipple?

T4

what is the dermatome level of the xiphoid process

T7

what is the dermatome level of the umbilicus?

T10

what is the dermatome level of the suprapubic region

L1

why don’t we need to know the levels in between landmark dermatome levels?

because the dermatome levels are segmental from T1-L2, we can fill in the dermatomes in a ladder-like fashion

Are all dorsal primary rami from C1 to Coccygeal 1 arranged segmentally?

yes

are all ventral primary rami arranged segmentally?

no

what are the only segmentally arranged ventral rami

the ventral primary rami of the thoracic spinals nerves T1-T12

What are the T1-T11 ventral primary rami of thoracic spinal nerves called?

intercostal nerves

what do the intercostal nerves supply?

the intercostal muscles, parietal pleura, periosteum of ribs, diaphragm of superficial fascia, and skin of the thoracic and abdominal walls

what is the T12 ventral primary rami of thoracic spinal nerve called?

the subcostal nerve

what happens to the other ventral spinal nerves that are non-segmental

they form nerve plexuses that innervate various regions of the body.

What spinal cord regions does the cervical plexus innervate?

C1-C4

what spinal cord regions does the brachial plexus innervate?

C5-T1

what spinal cord region does the lumbar plexus innervate?

L1-L4

What does it mean for a nerve to be segmental

segmental spinal nerves are nerves that arise from certain spine levels and innervate somatic structures derived from the somites of that same level.

• for example: spinal nerve T1 will supply somatic structures derived from the T1 somites and T1 somatic mesoderm

What does it mean to be a non-segmental spinal nerve?

Non-segmental spinal nerves are those that arise from multiple spinal cord levels and innervate structures not limited to a single somite level, often providing broader functional areas of innervation.

• these ventral primary rami join up to form nerve plexus

nerve plexus definition

a network of interjoining nerves

What fibres supply motor innervation to the the thoracic and abdominal viscera?

Sympathetic fibers arising from spinal cord via the sympathetic trunk

what spine segments have the sympathetic nerves to provide motor innervation to the thorax

T1-T5

what spine segments have the symptathetic nerves to provide motor innervation to the abdomen

T6 - L2

where are ALL preganglionic sympathetic neuron bodies located?

In the lateral horn of the spinal cord segments T1-L2

Once the preganglionic sympathetic neuron arives at the sympathetic trunk it has five options, what is the first option?

Synapses in the sympathetic ganglion at the same level, and the postganglionic neuron leaves with the spinal nerve at that same level to be distributed to smooth muscle in somatic structures

Once the preganglionic neuron arives at the sympathetic trunk it has five options, what is the second option?

it synapses in the sympathetic ganglion at the same level and the postganglionic neuron tracks up or down the trunk to exit with the spinal nerves above or below T1 or L2 levels

Once the preganglionic neuron arives at the sympathetic trunk it has five options, what is the third option?

it synpases with the sympathetic ganglion at the same level and the postganglionic neuron tracks up or down the trunk until it find a direct nerve branch going to the thorax

• cardiac nerves, pulmonary plexus

Once the preganglionic neuron arives at the sympathetic trunk it has five options, what is the fourth option?

It DOES NOT synapse at the same level, but tracks up or down the trunk to synapse to reach the ganglia of the cervical or sacral region.

• example: tracks up to superior cervical ganglion to reach structures in the head and neck via the internal carotid plexus

Once the preganglionic neuron arives at the sympathetic trunk it has five options, what is the fifth option?

it DOES NOT synapse at the same level, but tracks up or down the trunk until it finds a splanchnic nerve the synpase in the abdominal prevertebral ganglion to reach abdominal organs via blood vessels

• example: tracks down to the celiac ganglion to reach the foregut via the celiac artery

What are splanchnic nerves?

Nerves that arise from the sympathetic nervous system and innervate the abdominal and pelvic organs. They carry preganglionic sympathetic fibers to various ganglia in the abdominal cavity.

• do not synapse at sympathetic chain ganglia, they pass through and synapse in prevertebral ganglia near abdominal aorta

what are visceral afferent fibres

sensory fibres from the viscera that respond to stretch, ischemia and chemoreception

What are white ramus communicans, and where are they found?

White rami communicantes are the myelinated preganglionic fibers that connect the spinal nerves to the sympathetic trunk

• since they are only associated with the sympathetic trunk, they are only found through levels T1-L2

What are grey ramus communicans, and where are they found?

Grey rami communicantes are unmyelinated postganglionic fibers that connect the sympathetic trunk to the spinal nerves. They are found at all spinal levels

Describe the pathway visceral afferent fibres take from the GI tract to get to the spinal cord, and then to the brain?

visceral afferent fibres follow the pathway created by sympathetics from the GI tract to the sympathetic trunk, where they can travel through gray or white ramus communicans to get to the dorsal root leading to the dorsal horn. These visceral afferents will then synapse with second order neurons and ascend to the thalamus via the spinothalamic pathway, and then on to the sensory cortex for processing

Describe the pathway visceral afferent fibres can take from the heart to get to the brainstem?

Visceral afferent fibres can either

a. travel along the sympathetic’s route (cardiac nerves) to get to the sympathetic trunk, and then to the dorsal root to get to the dorsal horn. the visceral afferents then will synpase with second order neurons to get to brain 

b. the viceral afferents can travel with the vagus nerve to the brainstem directly

what levels of the spinal cord does the greater splanchnic nerve arise from, and what ganglion/plexus will it synapse at?

The greater splanchnic nerve arises from the sympathetic chain at spinal levels T5 to T9.

• synapses at celiac ganglion to innervate foregut

what levels of the spinal cord does the lesser splanchnic nerve arise from, and what two ganglion/plexus will it synapse at?

The lesser splanchnic nerve arises from the sympathetic chain at spinal levels T10 to T11.

1. Superior Mesenteric Ganglion (innervate midgut)

2. Aorticorenal Ganglion (innervate kidneys, adrenal glands, and upper ureter)

What levels of the spinal cord does the least splanchnic nerve arise from, and what ganglion will it synpase at?

The least splanchnic nerve arises from the sympathetic chain at spinal levels T12

• synapses with the renal ganglion to innervate the kidneys.

what levels of the spinal cord does the lumbar splanchnic nerve arise from, and what ganglion will it synpase at?

The lumbar splanchnic nerve arises from the sympathetic chain at spinal levels L1 to L4

• synapses with the inferior mesenteric ganglion to innervate the hindgut in addition to the bladder, external genitalia, and uterus

where does parasympathetic outflow arise from

CN III, VII, IX, X

pelvic splanchnic nerves

What is CN III, what parasympathetic ganglion does it synapse at, and what structures does it interact with

• CN III = Oculomotor nerve

• Synapse: Ciliary ganglion

• Structures: Pupillary constrictor muscles (pupil constriction), Ciliary muscle (lens accommodation)

What is CN VII, what two parasympathetic ganglion does it synapse at, and what structures do they innervate?

• CN VII = Facial nerve

• Synapse:

  • Pterygopalatine ganglion → Lacrimal gland, nasal mucosa, palatine glands

  • Submandibular ganglion → Submandibular and sublingual salivary glands

What is CN IX, what parasympathetic ganglion does it synapse at, and what structure does it innervate?

• CN IX = Glossopharyngeal nerve

• Synapse: Otic ganglion

• Structures: Parotid salivary gland

What is CN X, what parasympathetic ganglion does it synapse at, and what structures does it innervate?

• CN X = Vagus nerve

• Synapse: Terminal ganglia in or near organ walls

• Structures: Thoracic and abdominal viscera up to the proximal 2/3 of the transverse colon (heart, lungs, stomach, intestines, etc.)

Where do pelvic splanchnic nerves arise from, what parasympathetic ganglion does it synapse at, and what structures does it innervate?

• Pelvic splanchnics = S2–S4

• Synapse: Terminal (intramural) ganglia in or near pelvic organ walls

• Structures: Distal 1/3 of transverse colon, sigmoid colon, rectum, bladder, reproductive organs

what are the two types of pain?

1. direct

2. referred

what are the two types of refered pain?

1. somatic

2. visceral

Somatic referred pain is due to what two reasons?

embryological displacement or the phenomenon of projection

what is visceral referred pain due to?

the brain does not have a map of visceral structures like it does for somatic structures so it misinterprets where the pain is coming from

where does direct pain arise from

it arises from the skin of body walls and limbs, and is sharp, well-focuses and easily localized

• perceived as arising directly from the site of injury

what nerves carry direct pain

spinal nerves

what 6 (all) sensory modalities do spinal nerves respond to

1. pain

2. temperature

3. touch

4. stretch

5. ischemia

6. proprioception

What is referred pain?

Pain that arises from an injured or inflamed structure in the body, but is perceived as arising from another structure some distance away

where is pain generally referred to?

the dermatome that is supplied by the same spinal segments that are supplying the injured structure

two types of referred pain

1. somatic

2. visceral

What nerves carry away somatic referred pain, and how would the pain be described?

Somatic referred pain is carried by spinal nerves 

pain is sharp, focused, and easily localized

what are the two reasons somatic referred pain is referred

1. embryological displacement

2. projected referred pain

embryological displacement

The diaphragm develops embryologically from the cervical region hence its nerve supply from the cervical cord segments, which also supply the skin of the shoulder so that irritation of the diaphragm can be referred to the shoulder

• C3-5

Projected referred pain

anytime a nerve is injured anywhere along it’s length, pain is projected or referred to the end of that nerve

• funny bone —> ulnar nerve

• sinusitis —> toothache

what nerve fibres carry visceral referred pain and how would the pain be described?

carried by visceral afferent fibres and the pain is vague and dull

what three signals to visceral afferent fibres respond to

stetch, ischemia, and chemoreception

How long is the neuron chain for somatic sensory (afferent) pathways

3 neurons that are intercepted at the thalamus

describe the three neuron pathway for somatic sensory pathway

1. peripheral receptor —> spinal cord/medulla

2. Spinal cord/medulla —> thalamus

3. thalamus —> cerebral cortex

how long is the motor neuron chain for somatic motor (efferent) pathway?

2 neurons that directly influence muscles