Central Nervous System Structures and the Brachial Plexus: Study Guide

Central Nervous System Structures

Brain Models: Locations and Functions

The brain is composed of several distinct regions, each with specialized functions. Understanding the anatomy and physiology of these structures is essential for interpreting neurological function and pathology.

• Pineal gland: Produces melatonin, regulating sleep-wake cycles.

• Cerebrum: Largest part of the brain, responsible for higher cognitive functions.

• Cerebellum: Coordinates voluntary movements and balance.

• Corpus callosum: Connects the left and right cerebral hemispheres, facilitating interhemispheric communication.

• Precentral gyrus: Primary motor cortex, controls voluntary movements.

• Postcentral gyrus: Primary somatosensory cortex, processes sensory information.

• Thalamus: Relay station for sensory and motor signals to the cerebral cortex.

• Hypothalamus: Regulates autonomic functions, hormone release, and homeostasis.

• Superior/inferior colliculi: Involved in visual and auditory reflexes.

• Optic tract/nerve/chiasm: Transmits visual information from the retina to the brain.

• Olfactory bulb: Processes smell information.

• Fornix: Connects hippocampus to other brain regions, involved in memory.

• Pons: Relays signals between cerebrum and cerebellum; involved in sleep and respiration.

• Medulla: Controls autonomic functions such as breathing and heart rate.

• Arbor vitae: Tree-like arrangement of white matter in the cerebellum.

• Midbrain: Controls eye movement, auditory and visual processing.

• Foramen magnum: Large opening at the base of the skull for the spinal cord.

• Cerebral aqueduct: Connects third and fourth ventricles, allows flow of cerebrospinal fluid (CSF).

• Pituitary gland: Master endocrine gland, regulates other glands.

Ventricles

• Lateral ventricles: Paired cavities within the cerebral hemispheres, produce and circulate CSF.

Cranial Foramina: Locations and Nerves

Cranial foramina are openings in the skull that allow passage of nerves and blood vessels. Knowing their locations and associated nerves is crucial for clinical neuroanatomy.

• Superior orbital fissure: Transmits cranial nerves III, IV, V1, and VI.

• Optic foramen: Passage for optic nerve (CN II).

• Jugular foramen: Passage for cranial nerves IX, X, XI and jugular vein.

• Hypoglossal canal: Passage for hypoglossal nerve (CN XII).

Spinal Cord Model: Key Structures

The spinal cord is organized into distinct regions and layers, each with specific functions in sensory and motor transmission.

• White columns: Bundles of myelinated axons transmitting signals up and down the cord.

• Fissure and sulcus: Deep and shallow grooves dividing the cord.

• Horns: Gray matter regions containing neuron cell bodies.

• Dorsal root ganglion: Contains sensory neuron cell bodies.

• Meningeal layers: Three protective membranes: dura mater, arachnoid mater, pia mater.

• Cervical ganglions: Superior, middle, and inferior ganglia involved in autonomic regulation.

Long Spinal Cord Structures

Several anatomical features are important for understanding spinal cord function and clinical conditions.

• Enlargements: Cervical and lumbar regions where nerves for limbs arise.

• Cauda equina: Bundle of spinal nerves below the end of the spinal cord.

• Spinal nerves/Sympathetic chain: Peripheral nerves and autonomic ganglia.

Spinal Reflex Arc

The spinal reflex arc is a fundamental neural circuit that mediates rapid, involuntary responses to stimuli.

• From the foot/leg: Example: withdrawal reflex from pain.

• From the hand/arm: Example: stretch reflex in biceps.

Cord Conditions

Several pathological conditions can affect the spinal cord and its function.

• Spinal stenosis: Narrowing of the spinal canal, compressing nerves.

• Spinal cord fusion surgery: Surgical procedure to stabilize the spine.

• Herniated discs: Displacement of intervertebral disc material, causing nerve compression.

Brachial Plexus

Overview and Organization

The brachial plexus is a network of nerves that supplies the upper limb. It is organized into roots, trunks, divisions, cords, and branches.

• Roots: Anterior rami of spinal nerves C5, C6, C7, C8, T1 (C4 & T2 make minor contributions).

• Trunks: Upper, middle, and lower trunks formed by merging roots.

• Divisions: Each trunk splits into anterior and posterior divisions.

• Cords: Lateral, medial, and posterior cords formed by divisions.

• Branches: Terminal nerves that innervate muscles and skin of the upper limb.

Mnemonic: "Randy Travis Drinks Cold Beer" (Roots, Trunks, Divisions, Cords, Branches)

Brachial Plexus: Key Nerves and Muscles Innervated

The following table summarizes the main nerves of the brachial plexus, their origins, and the muscles they innervate.

Key Nerve/Nerve Branch	Origin	Distribution (Innervation)
Median Nerve	Lateral Cord of C6 & C7; Median Cord of root C8 & T1	Flexor muscles in forearm (except Carpi Ulnaris), Flexor Digitorum Profundus
Axillary Nerve	Posterior of C5 & C6	Deltoid, Teres Minor, Shoulder Joint
Musculocutaneous Nerve	Lateral Cord of C5, C6 & C7	Biceps Brachii, Brachialis, forms lateral cutaneous nerve of forearm
Medial Cord	Anterior Ramus of C7 & T1	Forms Ulnar Nerve, and Medial Root to Median Nerve
Posterior Cord	Posterior rami C5 & C6 Roots; Posterior Ramus of C7& Rootlet C8	Forms radial nerve, and Axillary Nerve
Lateral Cord	Anterior Ramus of C5 & C7	Forms Lateral Root, and Musculocutaneous Nerve
C7 Root	Spinal cord between C6 & C7 vertebrae	Middle trunk, Long Thoracic Nerve
C8 Root	Spinal cord between C7 & T1 vertebrae	One portion of inferior trunk
Root T1	Spinal cord between T1 & T2 vertebrae	One portion of inferior trunk

Additional info:

• The brachial plexus is clinically important for diagnosing nerve injuries in the upper limb.

• Damage to specific nerves can result in characteristic motor and sensory deficits.

Brachial Plexus: Pathway Summary

The pathway of the brachial plexus can be summarized as:

• Spinal Cord → Roots → Trunks → Divisions → Cords → Branches (terminal ends)

• Cords: Posterior / Lateral / Medial

• Brachial Roots: Anterior rami of spinal nerves C5, C6, C7, C8, T1 (C4 & T2 make minor contributions)

Example:

Damage to the median nerve can result in loss of flexion in the forearm and impaired hand function ("ape hand").