Human Neuroanatomy: Spinal Cord Overview

Human Neuroanatomy

• This presentation covers human neuroanatomy, focusing on the spinal cord.
• The central nervous system (CNS) is represented by the neuraxis, composed of the encephalon and spinal cord.
• A cross-section of the neuraxis shows grey matter (neuronal cell bodies, dendrites, and unmyelinated axons) and white matter (myelinated nerve fibers).
• The spinal cord (SC) has a segmental organization, mirroring its embryonic and phylogenetic origin.

Spinal Cord

• The spinal cord is not a straight structure; it has curvatures similar to the vertebral column.

• The spinal cord's features include two enlargements: cervical and lumbosacral.

  • The cervical enlargement starts at the 3rd cervical vertebra and ends at the 2nd thoracic vertebra (C3-T2)
  • The lumbosacral enlargement starts at the 9th thoracic vertebra and ends at the 1st lumbar vertebra (T9-L1).

• The conus medullaris is the tapered end of the spinal cord.

• The filum terminale extends from the conus medullaris to the coccyx.

• The cauda equina consists of the nerve roots from the lumbar, sacral, and coccygeal segments that extend beyond the conus medullaris.

• The spinal cord is surrounded by meninges (Pia mater, Arachnoid, and Dura mater).

• The meninges surround the spinal cord & contain cerebrospinal fluid.

  • The pia mater is a delicate innermost layer.
  • The arachnoid is a middle layer.
  • The dura mater is the outermost, toughest layer.

• The spinal cord is suspended in the meningeal sac by its continuity with the medulla oblongata, the nerves passing through the intervertebral foramina, and denticulate ligaments.

• Spinal cord segments are not matched to the numbered vertebrae.

Spinal Nerves

• 31 pairs of spinal nerves connect to the spinal cord.
  • 8 pairs of cervical nerves
  • 12 pairs of thoracic nerves
  • 5 pairs of lumbar nerves
  • 5 pairs of sacral nerves
  • 3 pairs of coccygeal nerves
• Spinal nerves are made of both dorsal (posterior/afferent/sensory) and ventral (anterior/efferent/motor) roots.
  • Dorsal roots contain sensory neurons.
  • Ventral roots contain motor neurons.
• The dorsal root ganglion (DRG) contains the cell bodies of sensory neurons.
• Somatic sensory fibers carry information from the skin, muscles and joints.
• Visceral sensory fibers carry information from the internal viscera.
• Somatic motor fibers stimulate skeletal muscles (voluntary actions).
• Visceral motor fibers stimulate smooth muscle (involuntary actions).
• The peripheral nervous system (PNS) includes nerve fibers from both the spinal and cranial nerves.

Grey Matter

• The grey matter is more developed at the level of enlargements, less in the tract between the enlargements.

• Grey matter cells are divided into Golgi type I neurons (long axons) and Golgi type II neurons (short axons, interneurons).

  • Radicular neurons (Golgi type I): located in anterior and lateral horns, responsible for somatic and visceral motor functions. They are divided into motoneurons or somatic motor cells, and visceromotor neurons or visceral effector cells.
  • Funicular neurons (Golgi type I): interneurons with axons confined to the grey matter.

• Motor neurons are further classified into a-motor neurons and y-motor neurons

• Visceromotor neurons or visceral effector cells are located in the base of the anterior horn and lateral horn. They innervate involuntary musculature, but do not directly stimulate smooth muscle.

White matter

• White matter bundles the ascending and descending pathways.
• The funiculi are divided into the posterior, lateral, and anterior.
  • Association bundles ensure intersegmental connections.
• Projection Bundles carry signals from the sensory cortex to the spinal cord, and from the spinal cord to the motor cortex. 
• General (somatic and visceral) sensitivity and specific sensitivity (vision, olfaction, taste, balance, hearing) sensory systems are recognized

Spinal Cord Nuclei

• The spinal cord is organized into motor nuclei and sensory nuclei.

  • Motor nuclei (somatomotor and visceromotor) are located in the anterior horn and lateral horn, control voluntary and involuntary movements.
  • Sensory nuclei (somatosensitive and viscerosensitive) are located in the posterior horn; relay sensory information to the brain.

• Specific spinal cord columns include, antero-medial, posters-medial, lateral.

• Various nuclei of the spinal cord control body functions, like maintaining posture, and the nerves in the spinal cord are involved in muscle function and control.

Spinal Cord Reflexes

• Reflexes are involuntary, immediate motor responses.

  • Innate reflexes are genetically determined; learned reflexes are acquired.

  • Spinal reflexes are processed in the spinal cord; cranial reflexes in the brain.

  • Monosynaptic reflexes involve one synapse; polysynaptic reflexes involve many.

• The reflex arc is the pathway of a given reflex; it has sensory receptors, sensory neuron, interneuron(s), motor neuron, and effector (muscle/gland) .

This quiz focuses on the human spinal cord, emphasizing its anatomy and segmental organization. Learn about the structure, curvatures, and key features such as the cervical and lumbosacral enlargements, as well as the conus medullaris. Test your understanding of the neurological components of the central nervous system.