Introduction to Somatic and Autonomic Nervous Systems

Introduction to the Nervous System

• The nervous system is composed of the Central Nervous System (CNS) and Peripheral Nervous System (PNS).

• CNS includes the brain and spinal cord.

• PNS includes cranial, spinal, and splanchnic nerves.

• The PNS can be divided further into the Somatic Nervous System (SNS) and Visceral Nervous System (Autonomic Nervous System - ANS).

  • Visceral nervous system further broken down into visceromotor (sympathetics and parasympathetics) and viscerosensory

    • Visceromotor controls involuntary contraction of smooth muscle, cardiac muscle, and glands

    • Viscerosensory modulates internal regulation (ex/ blood pressure, chemistry, respiration)

Learning Objectives

1. Describe the function of the nervous system and distinguish between CNS and PNS.

2. Identify the components of the PNS.

3. Differentiate the tissues/organs innervated by SNS versus ANS.

4. Understand the meningeal layers surrounding the spinal cord.

5. Recognize the unique morphological features of cervical, thoracic, and lumbar spinal cord.

6. Identify the nerve components in dorsal and ventral roots and primary rami.

7. Define dermatomes and locate T4, T10, and L1 dermatomes.

8. Describe nerve fiber pathways.

Functions of the Nervous System

• Sensory: Collection of information from sensory receptors regarding internal and external environments.

• Integration: Processing sensory signals for decision-making.

• Motor: Executing responses after decisions are made.

• Combined, aims to maintain homeostatic state

Somatic vs. Visceral Nervous Systems

Somatic Nervous System (SNS)

• Innervation: Skeletal muscle and the body wall.

• Components

  • Somatomotor (GSE): Controls voluntary reflex contractions of skeletal muscle.

    • Axons that innervate all skeletal muscles of the head and neck regions + limbs

  • Somatosensory (GSA): Provides sensory innervation to skin, muscles, and joints.

    • Pain/temp/touch to skin, cornea, and certain mucosa (most notably the oral mucosa)

• Features of SNS

  • Very precise temporally and spatially

  • One motor neuron acts on one striated muscle motor unit

  • No divergence of stimuli

  • Precise: touch vibration, proprioception, temperature, pain

Visceral Nervous System/ANS

• Innervation: Internal organs, smooth muscle, cardiac muscle, and glands.

  • Controls responses that are not under voluntary or cognitive control

  • Most (not all) ANS axons innervate structures in the viscera

• Components:

  • Visceromotor (GVE): Involuntary contraction of smooth muscle and glands.

    • Will mainly focus on pathways for the cranial nerves

  • Viscerosensory (GVA): Modulates internal conditions (e.g., blood pressure, respiration).

    • Send messages from organ to CNS within splanchnic nerves

    • Viscerosensory pain: pain from an organ (travel mostly along sympathetic pathway - cell bodies in DRG along T1-L2)

      • Pathway: Splanchnic n., Sympathetic trunk, White communicating ramus, Ventral ramus, Spinal n., DRG

    • Viscerosensory reflex: Vomiting, cough, chemo receptors, baroreceptors (travel along parasympathetic pathways - cell bodies in vagal sensory ganglia)

• Divisions:

  • Sympathetic: Prepares body for 'fight-or-flight' responses.

  • Parasympathetic: Promotes 'rest and digest' activities.

  • Parasympathetic and sympathetic systems act in concert with each other and the somatic motor system to regular normal or emergency behavior.

    • Aim to maintain homeostasis in the face of challenging external conditions

• Features of visceral nervous system

  • Imprecise

  • Many postsynaptic neurons, many muscle cells

  • Lots of divergent stimuli

  • Diffuse and dull: pain, distension, ischemia, and inflammation

    • Because pain signals pass through multiple pathways

Morphology of the Spinal Cord

• General Features:

  • Major conduit by which motor information travels from the cortex to the peripheral nerves (efferent) and sensory information travels from the peripheral nerves to the cortex (afferent)

• Meningeal Layers: Three layers surround the spinal cord:

  • Dura mater: Outermost protective layer.

  • Arachnoid mater: Middle layer with web-like structures.

  • Pia mater: Innermost layer directly adhering to the nervous tissue.

• Denticulate ligaments: Derived from pia mater and divides ventral and dorsal side.

• Conus medullaris: Sacral spinal cord segments that terminate at L1-2 vertebral level.

• Filum terminale: Derived from pia mater. Extension of conus medullaris that anchors spinal cord to sacrum.

• Cauda equina: Dosal and ventral roots and filum terminale

  • Dorsal and ventral nerve roots concentrate then spread out into sacral region and lower limbs.

• Dural sleeve: Extension of dura mater pass out through sacrum and allows dorsal and ventral roots to exit sacrum

  • Present in thoracic, cervical and lumbar regions as well

• Epidural Space: Space between the dura mater and vertebral column, containing fat and blood vessels.

Spinal Cord Structure

• White matter: primarily myelinated axons

• Grey matter: primarily cell bodies

• Dorsal Horn: contains sensory neurons.

• Ventral Horn: contains motor neurons.

• Unique features of cervical and lumbar spinal cord: have very large ventral horns that contain cell bodies of motor neurons that innervate the upper and lower limbs respectively.

• Unique features of thoracic spinal cord: Have lateral horns that house the cell bodies for sympathetic (preganglionic) neurons with relatively small ventral horns

• Anatomic relationship between vertebral column and spinal cord; vertebral column protects spinal cord; dorsal and ventral roots exit via intervertebral foramen

  • Both made up of segments

• Trajectories of spinal cord segments:

  • 31 segments contribute to spinal nerves (8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal).

    • Spinal cord segment: region of the spinal cord that contributes to the formation of an individual spinal nerve

  • Spinal nerves exit at different levels based on vertebral correspondences (e.g., C8 exits above T1).

Clinical Correlation: Lumbar Puncture

• Typically performed between L3 and L4 vertebrae in adults at the level of the cauda equina to avoid damaging the spinal cord (containing CSF).

  • Lumbar puncture can only be performed in adults, else puncture spinal cord because position of spinal nerve and corresponding vertebra remain constant.

Spinal nerves

• Spinal nerves have…

  • A sensory component which transmits sensory information from the body back to the CNS

  • A motor component to execute motor commands from the CNS and ANS

• Main plexuses (superior to inferior): cervical plexus, brachial plexus, intercostal nerves, lumbar plexus, sacral plexus.

• Path of motor neuron from ventral horn to ventral ramus: Ventral horn, Ventral root, Spinal nerve, Ventral ramus

• Path of sensory receptor from dorsal ramus to dorsal horn: Dorsal ramus, Spinal nerve, Dorsal root ganglion, Dorsal root, Dorsal horn

  • Dorsal root ganglion: holds cell bodies

• Spinal nerves can be divided into different parts:

  • Roots: extend from spinal cord and combined to form the spinal nerve

    • Dorsal roots carry sensory information back into the spinal cord (afferent)

    • Ventral roots carry motor information out of the spinal cord (efferent)

  • Rami: spinal nerve branch that travels out to the periphery.

    • Dorsal rami carry sensory and motor information to and from the midline muscles and skin of the back

      • Innervates deep back muscles and overlying skin

    • Ventral rami carry sensory and motor info of the remainder of the muscles and skin of the trunk and limbs

      • Innervates all other parts of “body wall” - remaining musculature of the neck, trunk, limbs, and overlying skin

Dermatome Mapping

• Dermatomes: Regions of the body surface that are innervated by specific spinal nerves.

• Key Dermatomes:

• T4: Nipple area.

• T10: Umbilicus.

• L1: Pubic region.

• Dermatomes overlap, meaning loss of one spinal nerve typically does not produce anesthesia (need to provide anesthesia to multiple spinal nerves)

Pathway of Sympathetic and Parasympathetic Nerves (Visceromotor/GVE)

• Sympathetic Nerves:

  • Cell bodies in lateral horn at T1 to L2.

  • Leave CNS through ventral roots of T1 to L2

  • Go to all areas of the body (including body walls)

  • 2nd neuron cell bodies in ganglia not associated with target organ (where signal is amplified).

    • Target skin and thoracic viscera,: in paravertebral ganglia

    • Target abdominopelvic viscera: prevertebral ganglia

  • Sympathetic trunk: paired conduits for sympathetic axons with ganglia spread along the trunks (sites for synapses) that runs from the base of the skull to the tip of the coccyx.

    • Superior cervical ganglion: most superior near C2; larger and longer than all other ganglia because all synapses for head and neck are here

  • General pathway of sympathetics: Lateral horn of T1-L2 segments, Ventral root, Spinal n., Ventral ramus, White communicating ramus, Sympathetic trunk, …. ultimately innervate sweat glands, smooth muscles in bv, and arrector pili muscles in skill and body wall vessels

  • Pathway to body wall structure at same level: Lateral horn of T1-L2 segments, Ventral root, Spinal n., Ventral ramus, White communicating ramus, Sympathetic trunk, Chain ganglia, Gray ramus (at same level) dorsal/ventral rami (depending on target)

  • Pathway to body wall structures in the head, neck, and limbs: Lateral horn of T1-4, Ventral root, Spinal n., Ventral ramus, White communicating ramus, Sympathetic trunk (ascends), Synapse at cervical chain ganglia, gray rami, dorsal/ventral rami

  • Pathway to body wall structures in lumbar and sacral regions: Lateral horn of T12-L2, Ventral root, Spinal n., Ventral ramus, White communicating ramus, Sympathetic trunk (descends), Synapses at lumbar and sacral chain ganglia, gray rami, dorsal/ventral rami

  • Pathway to thoracic viscera: Lateral horn of T1-4, Ventral root, Spinal n., Ventral ramus, White communicating ramus, Sympathetic trunk, Synapses at thoracic chain ganglia, Exit sympathetic trunk through splanchnic nerves (connected to thoracic organ)

    • Innervate cardiac m. in heart and smooth muscle in the airway

  • Pathway to abdominal viscera: Lateral horn of T5-12, Ventral root, Spinal n., Ventral ramus, White communicating ramus, sympathetic trunk, Enter thoracic splanchnic nerve, Preaortic ganglia, Synapse, Connect to target organ

    • Innervate smooth muscle in abdomen

• Parasympathetic Nerves:

  • Cell bodies in brain and lateral horn of spinal cord at S2, 3, 4.

  • Leave CNS through cranial nerves and ventral roots

  • 2nd neuron cell bodies are in ganglia associated with target organ

  • DOES NOT go to body wall structures

  • Paired parasympathetic ganglia of the head: Otic, ciliary, submandibular, pterygopalatine

    • Parasympathetic ganglia for the thorax and abdomen located near target organ

• Body wall: Limbs, thoracic and abdominal walls, bones and muscles, the skin of the face and neck, the external genitalia, the spine

Summary of Nerve Components

• Dorsal and ventral rami: Somatosensory, somatomotor, visceromotor fibers.

• Cutaneous Nerves: Somatosensory and visceromotor fibers.

• Motor Nerve (skeletal muscle): Somatomotor, somatosensory, visceromotor fibers.

Special Senses

• 5 special senses: Smell (Olfactory epithelium), Vision (Retina), Hearing (Chochlea), Balance (vestibular apparatus), Taste (taste buds)

• CN with ONLY special sensory components: Olfactory (I) - Smell; Optic (II) - Vision; Vestibulocochlear (VIII) - Hearing and balance

• CN with ONLY somatomotor: Trochlear (IV); Abducent (VI); Accessory (XI); Hypoglossal (XII)

• CN with 2 nerve components: Oculomotor (III) - Somatomotor and parasympathetic/pre

• Remaining CN:

  • Trigeminal (V) - SS and SM (only V3 has somatomotor)

  • Facial (VII): SS, SM, Para/pre, Taste

  • Glossopharyngeal (IX) - VS, SS, SM, Para/pre, Taste

  • Vagus (X) - VS, SS, SM, Para/pre, Taste

Conclusion

Understanding the structure and functional dynamics of the somatic and autonomic nervous systems is essential for interpreting how the nervous system controls both voluntary and involuntary body functions.