What is the anatomy of the neuro system?

Anatomy of the Nervous System

The nervous system is divided into the central nervous system (CNS) consisting of the brain, brainstem, cerebellum, and spinal cord, and the peripheral nervous system (PNS) comprising cranial nerves and spinal nerves that provide sensory and motor innervation to the body. 1

Central Nervous System Structure

• The CNS contains millions of neurons and glial cells (astrocytes, oligodendrocytes, and microglia) that process information, analyze signals, and activate effector organs 2
• Glial cells create the blood-brain barrier, protect and nourish neurons, provide antioxidant activity, and maintain immunological control 2
• The CNS is protected by specialized barriers including the blood-brain barrier (BBB) and blood-spinal cord barrier (BSCB) that regulate the passage of substances between blood and neural tissue 3
• Key tight junction proteins in these barriers include claudin-1, claudin-3, claudin-5, claudin-11, claudin-12, claudin-19, occludin, Zona occludens-1, and tricellulin 3

Peripheral Nervous System Organization

• The PNS is divided into cranial nerves (CN III-XII) supplying the head and neck, and spinal nerves that supply the upper and lower limbs 1
• The blood-nerve barrier (BNB) protects peripheral nerves from potentially harmful blood-borne substances 3
• Motor nerves arise from the ventral neural tube under the influence of morphogenetic molecules like sonic hedgehog 1
• Sensory elements of the PNS originate from neural crest cells, which give rise to dorsal root ganglia neurons, autonomic ganglia, and paraganglia 1
• Supporting Schwann cells that provide myelination also originate from neural crest cells 1

Cranial Nerve Anatomy

• The 12 pairs of cranial nerves (CN I-XII) emerge from the rostral portion of the embryologically developing neural tube 4
• Cranial nerves include: olfactory (I), optic (II), oculomotor (III), trochlear (IV), trigeminal (V), abducens (VI), facial (VII), vestibulocochlear (VIII), glossopharyngeal (IX), vagus (X), spinal accessory (XI), and hypoglossal (XII) 4
• The olfactory (CN I) and optic (CN II) nerves are actually tracts formed from the telencephalon and diencephalon, respectively, and are not considered true nerves 4
• Functionally, cranial nerves contain sensory (afferent) and motor (efferent) components, including sensory visceral, general, and special, as well as motor somatic, branchial, and visceral fibers 5

Autonomic Nervous System

• The autonomic nervous system interacts dynamically with the central nervous system to control visceral functions, including heart rate regulation 4
• It consists of sympathetic and parasympathetic branches that work in concert rather than independently 4
• The cardiac nervous system has three major components: the central nervous system, intrathoracic extracardiac neuronal pools, and the intrinsic cardiac nervous system 4
• Afferent (sensory) neurons relay information about mechanical and chemical changes from organs to higher structures 4
• Efferent (motor) neurons provide control over target organs, with sympathetic neurons originating from the reticular formation and parasympathetic neurons from the nucleus ambiguus and dorsal motor nucleus 4
• Local circuit neurons integrate signals between afferent and efferent neurons and are found in intrathoracic ganglia and ganglionated plexi 4

Clinical Considerations in Cranial Neuropathy

• Cranial neuropathy can result from pathology affecting nerve fibers at any point from the CN nucleus to the end organ 4
• Due to the proximity of many CN nuclei, segments, and exit sites, some lesions may result in multiple cranial neuropathies 5
• MRI is the standard modality for imaging cranial nerves, with 3.0T preferred over 1.5T for better signal-to-noise ratio and spatial resolution 4
• Fundamental imaging techniques include thin-cut T1-weighted, T2-weighted, and contrast-enhanced T1-weighted sequences 4
• The use of intravenous contrast is imperative for the evaluation of cranial neuropathy with MRI 4

Specific Neural Pathways

• The hypoglossal nucleus contains somatic motor neurons that provide motor innervation to the intrinsic and extrinsic muscles of the tongue 6
• These neurons function as lower motor neurons of the corticobulbar pathway, receiving input from upper motor neurons that originate in the motor cortex 6
• In threatening situations, a complex circuitry involving the central and autonomic nervous systems is activated, with the prefrontal cortex and amygdala regulating heart rate through a pathway involving the nucleus ambiguus, dorsal nucleus of the vagus nerve, and the vagus nerve itself 4
• This circuit highlights the intertwined relationship between central and peripheral nervous systems, which supports autonomic regulation in the brain-heart axis 4