Anterior Thorax & Nervous System Foundations Learning Objectives

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1. Distinguish between the structures and terminology of the central nervous system (CNS) and peripheral nervous system (PNS), and explain how their interconnectivity influences the presentation of clinical disorders.

The nervous system is divided into the central nervous system (CNS) and peripheral nervous system (PNS). Although they are described separately, these systems are functionally and anatomically continuous.

Key terminology common to entire nervous system:

• Neuron: fundamental unit of the nervous system; crucial for transmitting and processing information. Composed of three main structures:

  • Cell body (soma) - contains organelles

  • Dendrites: processes that carry impulses towards the neuronal cell body

  • Axon: long process that carries impulses away from the neuronal cell body

    • Often myelinated to speed up signal transmission

Nerve fiber: composed of an axon, neurolemma (with myelin, if present), and associated connective tissue

The central nervous system is composed of the brain and spinal cord.

Terminology:

• Gray matter: consists of mainly neuronal cell bodies with other structures (dendrites, axon terminals, neuroglia, etc.); little to no myelin

  • Nucleus: functionally discrete collections of neuronal cell bodies in CNS; typically located deep in brain

• White matter: composed of systems of aggregations of axons; typically myelinated (gives white color)

Tract: a discrete collection of axons (fibers) in the CNS that perform a common function

The peripheral nervous system is composed of nervous tissue outside of the CNS. This includes:

• 12 pairs of cranial nerves and 31 pairs of spinal nerves

  • Spinal nerves typically form complex associations, called plexuses, from which named nerves arise

Terminology specific to PNS:

• Nerve: nerve fiber bundles, associated connective tissue, and vasculature

Ganglion: collection of cell bodies of neurons outside of CNS (in PNS)

There are too many clinical correlations regarding the central & peripheral nervous systems to get into much detail here. Rest assured - we will spend considerable time here. Let’s leave this learning objective reflecting on the interconnectivity and interactions between the central & peripheral nervous systems - so a disorder affecting the CNS will impact the PNS (and vice versa). CNS pathologies, such as multiple sclerosis or stroke, can lead to altered motor and sensory functions which manifest as peripheral symptoms. This underscores the importance of comprehensive neurological exams.

2. Differentiate between the functions of various types of afferent and efferent nerve fibers, and apply this knowledge to identify potential clinical correlations.

Most named nerves contain both afferent and efferent fibers packaged together. If a nerve is not explicitly stated as being purely efferent or afferent, you can assume it has both types of fibers.

Basic definition of terms:

• Afferent: transmission of information away from an organ or sensory receptor, ultimately arriving at the central nervous system (CNS).

• Efferent: transmission of information from the CNS to an effector organ to create an effect, such as muscle contraction or glandular secretion

• Somatic: ‘of the body’; think skeletal muscles, skin, and bones

Visceral: relates to internal organs; the term viscera is often associated with just gastrointestinal (GI) organs but more generally this term should be associated with internal organs associated with main cavities of the body, such as lungs, heart, GI and pelvic organs

Afferent (Sensory) Innervation

Types of Afferent (Sensory) Fibers:

• Somatic Afferent

  • Most commonly discussed afferent fiber

  • Conveys sensation from the skin, including pain, touch, pressure, and temperature

  • Dermatome maps illustrate the specific areas of skin innervated by a single cranial or spinal nerve

• Proprioception

  • Provides the CNS with information about joint positioning and the degree of tension in muscle tendons

  • When discussing the sensory innervation of a muscle, proprioception is typically the focus

• Dysfunction in somatic afferent fibers can lead to sensory deficits, such as numbness, tingling, etc. (paresthesia), and can help localize nerve or CNS pathologies

• Visceral Afferent

  • Conveys information from hollow viscera (organs) and blood vessels

  • Carries pain or reflex sensations, such as those related to blood pressure or organ distension

  • Visceral pain is often poorly localized and can present as referred pain, which can complicate the diagnosis of conditions like appendicitis or myocardial infarction.

• Special Afferent (Special Sensory)

  • Associated with special senses such as vision, olfaction, hearing, taste, and equilibrium

  • These fibers are part of certain cranial nerves

  • Damage to special sensory pathways can lead to significant deficits, like a loss of vision (optic n.) or hearing loss (vestibulocochlear n.)

Efferent (Motor) Innervation

Types of Efferent (Motor) Fibers:

• Somatic Efferent

  • Transmits impulses from the CNS to skeletal muscles, leading to contraction

  • Loss of function in somatic efferent fibers can cause paralysis or weakness of the affected muscles, as seen in conditions such as Bells palsy

• Visceral Efferent

  • Transmits impulses from the CNS to glands and smooth muscle

  • Associated with autonomic nervous system (ANS)

  • Damage of visceral afferent fibers can result in autonomic dysfunction, such as in disorders like Horner’s syndrome

3. Distinguish between the parts of a spinal nerve, and explain how spinal nerves are named.

Spinal nerves originate from the spinal cord as ventral and dorsal rootlets, originating from the gray matter’s ventral and dorsal horns, respectively. These rootlets form ventral and dorsal roots. 

Ventral roots: solely motor (efferent)

Damage or lesions to ventral roots would lead only to motor deficits.

Dorsal roots: solely sensory (afferent)

The dorsal root houses the dorsal root ganglia (DRG), which are sensory neuron cell bodies located outside of the central nervous system.

Damage or lesions to dorsal roots or dorsal root ganglia would lead only to sensory deficits.

The ventral and dorsal roots merge to form the (trunk of the) spinal nerve, a mixed nerve containing both sensory and motor fibers. The trunk of the spinal nerve exits the vertebral column via the intervertebral foramina.

It is within or just distal to the foramina that the nerve splits into the dorsal primary ramus (DPR) and ventral primary ramus (VPR). These are the first branches of the spinal nerve and since they derive from the trunk of the spinal nerve, they are also mixed (containing sensory & motor fibers) nerves.

Damage or lesions to the trunk of the spinal nerve, VPR, or DPR would lead to deficits in both motor and sensory functions.

Of the primary rami, the ventral primary rami are of greater clinical significance due to their much greater area of coverage. The DPR provides motor & sensory innervation to the deep back muscles, some vertebral joints, and the skin of the back, with limited plexus formation. In contrast, the VPRs innervate all other body regions not served by DPRs and cranial nerves, including all parts of the limbs and anterior regions of neck, thorax, & abdomen, and frequently form plexuses (cervical, brachial, lumbar, and sacral).

Spinal nerves are named according to their spinal cord segment that they derive. There are 31 spinal nerve pairs:

• 8 cervical (C1-C8)

• 12 thoracic (T1-T12)

• 5 lumbar (L1-L5)

• 5 sacral (S1-S5)

• 1 coccygeal (Co1)

4. Describe the basic divisions of the autonomic nervous system (ANS): sympathetic & parasympathetic.

The autonomic nervous system (ANS) is an important component of the nervous system that regulates involuntary functions.

It consists of visceral efferent (motor) fibers the innervate primarily:

• Smooth muscle (involuntary type of muscle found in walls of blood vessels and many internal organs)

• Cardiac muscle (involuntary type of muscle found in the heart)

• Glands

The ANS is organized into two divisions:

• Sympathetic (‘fight or flight’)

• Parasympathetic (‘rest and digest’)

Note: The enteric nervous system (ENS) is sometimes described as part of the ANS but is also considered independently. This system is composed of a complex network of ganglia and nerves in the walls of the gastrointestinal tract. The ENS will be discussed in more detail with the gastrointestinal system.

Both divisions of the ANS system have a two-neuron chain. This is in contrast to skeletal muscle (somatic efferent), which has a one-neuron chain (from CNS directly to the skeletal muscle).

• Preganglionic neurons originate in the central nervous system (CNS) and synapse in autonomic ganglia. Recall a ganglion is a collection of cell bodies of neurons outside of CNS (in PNS).

• Postganglionic neurons originate in ganglia (outside of CNS) and innervate target tissues

Visceral afferent (sensory) fibers are not part of the ANS but travel alongside autonomic pathways. Visceral afferent fibers carry signals from organs to the CNS (e.g., stretch, pain, chemical stimuli, etc.).

We will be discussing the clinical correlations of the autonomic nervous system throughout the curriculum as there are more to account for here and can be inferred by the major effects of each division. One important note is that many pharmacologic agents can selectively activate or block receptors associated with the autonomic nervous system.

Sympathetic Division

Major Function: prepare body for action (‘fight or flight’)

Distribution: widespread throughout the body (think it is innervating smooth muscle of all blood vessels!)

Major Effects

• Constricts blood vessels to organs not involved in combating stress (kidneys & GI tract)

• Dilates blood vessels involved in increased activity (supplying heart & skeletal muscles)

• Increases heart & respiratory rates

• Bronchodilation (increase diameter of airway)

• Pupil dilation

• Increases blood glucose

• Stimulates sweat glands

Anatomy

• Origin: gray matter of T1-L2 spinal cord segments (‘thoracolumbar’)

• Preganglionic fibers: short and release acetylcholine

• Postganglionic fibers: long and release norepinephrine

Parasympathetic Division

Major Function: Conserve and restore body energy (‘rest and digest’)

Distribution: relatively limited mainly to head and viscera of thorax, abdomen, and pelvis

Major Effects

• Decrease heart and respiratory rates

• Bronchoconstriction (decrease diameter of airway)

• Increase GI motility and secretions

• Promotes glandular activity

• Pupil constriction

Anatomy

• Origin: brainstem in nuclei of specific cranial nerves (III, VII, IX, X) and gray matter of sacral spinal segments S2-S4 (‘craniosacral’)

• Preganglionic fibers: long and release acetylcholine

• Postganglionic fibers: short and release acetylcholine

5. Describe the osteological components of the thoracic cage, and explain the clinical relevance of the sternal angle and potential complications associated with rib fractures.

The thoracic (rib) cage is composed of bone and cartilage and encloses the thoracic cavity.

Sternum

The sternum in the anterior-most bone of the thorax, overlying mediastinal (region in thorax that contains heart, etc.) structures. It is composed of three parts (from superior to inferior):

• Manubrium - this is the widest portion of the sternum

  • Suprasternal (jugular) notch - superomedial notch; easily palpable

  • Clavicular notches - lateral and where the clavicles articulate (sternoclavicular joints)

  • Sternal angle - where the manubrium articulates with the body

    • This is a key palpable landmark that marks where the 2nd costal cartilage attaches. This allows one to count intercostal spaces and ribs from this point, which is clinically relevant for a variety of reasons, including placement of a stethoscope for auscultation of heart valves/sounds.

• Body - longest portion of the sternum

  • Costal notches on lateral sides for articulation with the rib’s costal cartilages

• Xiphoid process - most variable portion

  • May be bifid or curved

  • May not fully ossify

Ribs

Ribs (costae) are flat bones with a varying amount of curvature that form the bulk of the thoracic cage. There are 12 pairs of ribs. Ribs attach to thoracic vertebrae and most ribs attach to the sternum via costal cartilages. Intercostal spaces separate ribs. 

Rib fractures are among the most common thoracic injuries. Complications arise when the fractured segments damage deeper structures, such as the pleura or lungs. Injuries to these structures are often more serious than the fracture itself.

Thoracic vertebrae

There are 12 thoracic vertebrae corresponding to the 12 pairs of ribs. Each rib attaches to a thoracic vertebra via its head and tubercle. The thoracic vertebrae are distinguished by the presence of costal facets located on their vertebral bodies and transverse processes, which serve as articulation points with the ribs. The presence of costal facets help differentiate a thoracic vertebra from a cervical or lumbar.

6. Describe the attachments, actions, and innervation of the pectoralis major, pectoralis minor, and serratus anterior, and apply this knowledge to explain these muscles’ clinical significance in breast surgery, axillary procedures, and nerve injury resulting in scapular displacement.

Pectoralis major m.

“Pec. major’ is the largest and most obvious muscle of the anterior thorax. It moves the humerus at the shoulder joint. 

• Attachments:

  • Clavicular & sterncostal heads insert via a shared tendon on the proximal humerus (crossing the shoulder joint)

• Actions:

  • Will play a role in most actions as the shoulder joint (excepting abduction & lateral/internal rotation) depending on starting position

  • Particularly powerful in adduction & medial/internal rotation

• Innervation:

  • Lateral & medial pectoral nn.

• This muscle and associated fascia are frequently encountered or resected due to the proximity to breast tissue in surgeries in the area.

Pectoralis minor m.

‘Pec. minor’ is a diminutive muscle found deep to pec. major. It stabilizes the scapula.  

• Attachments:

  • Ribs 3-5 to the coracoid process of the scapula (will affect scapular movement)

• Actions: 

  • Scapular stabilization and protraction - these will serve to optimize shoulder movements

• Innervation:

  • Medial pectoral n. (the nerve pierces the muscle)

• This muscle serves as an important surgical landmark in this region, particularly in its relationship with the axillary a.

Serratus anterior m.

‘Serratus’ is an important stabilizer of the scapula.  

• Attachments:

  • Ribs 1-8 to the medial border of the scapula (will affect scapular movement)

• Actions:

  • Scapular protraction & upward rotation (essential for full range of motion of the shoulder joint)

• Innervation:

  • Long thoracic n. (unique in that it runs superficial to the muscle)

    • The nerve roots for long thoracic are easy to remember given the popular mnemonic ‘C5-6-7 keeps the wings from heaven.’

• Injury to the long thoracic n. causes a type of ‘winged scapula,’ specifically medial winging with the scapula rest more medial than typical because the serratus anterior isn’t capable of effectively protracting the scapula.