Structure of Neurons and Spinal Cord Gray Matter

Gray Matter of the Spinal Cord

• Gray matter is the part of the neuron that is not myelinated
• Myelin sheaths are lipid-protein structures that surround and insulate the axon, allowing for action potentials to be carried quickly
• Nodes of Ranvier are the gaps between myelin sheaths where action potentials skip along, allowing for saltatory conduction

Structure of a Neuron

• Dendrites: extensions of the neuron with receptors that respond to neurotransmitters
• Cell body (soma): contains the nucleus and the majority of the cell's organelles
• Axon: carries signals away from the cell body
• Axon terminals: end of the axon where signals are transmitted to other neurons
• Myelin sheaths: surround the axon, allowing for quick conduction

Nucleus and Ganglia

• Nucleus: a group of cell bodies in the central nervous system (CNS)
• Ganglia: a group of cell bodies in the peripheral nervous system (PNS)

Anatomical Portions of the Gray Matter

• Posterior gray horn: responsible for sensory functions
• Anterior gray horn: responsible for motor functions
• Intermediate zone: contains lateral gray horns that arise from the spinal cord between T1-L2
• Gray commissure: a sliver of tissue that connects the two sides of the horn

Functions of the Horns

• Posterior gray horn: primarily responsible for sensory functions, including somatic sensations (skin, skeletal muscles, joints, tendons, ligaments) and visceral sensations (internal organs)
• Intermediate zone: responsible for visceral motor functions (smooth muscle, cardiac muscle, glands)
• Anterior gray horn: responsible for somatic motor functions (skeletal muscles)

Rex Lamina and Associated Nuclei

• Rex lamina: a way to categorize the different areas of the gray matter in the spinal cord

• Lamina 1-6: primarily occupied by the posterior gray horn

• Lamina 7: primarily occupied by the intermediate zone

• Lamina 8-9: primarily occupied by the anterior gray horn

• Central lamina (lamina 10): connects the two sides of the horn, contains the gray commissure

• Posterior marginal nucleus: occupies lamina 1

• Substantia gelatinosa of Rolando: occupies lamina 2

• Nucleus proprius: occupies lamina 3-4

• Clark's column (clark's nucleus): occupies lamina 7

• Interomedial medial nucleus group: occupies lamina 7

• Interomedial lateral nucleus: occupies lamina 7### Lamina 9: Lateral, Central, and Medial Nuclear Groups

• Lateral Nuclear Group:

  • Present at C5-T1 and L2-S3
  • Supplies distal limbs (forearms and hands)

• Central Nuclear Group:

  • Contains three nuclei:
    • Phrenic Nucleus (C3-C5): supplies diaphragm
    • Accessory Spinal Nucleus (C1-C5): supplies sternocleidomastoid and trapezius muscles
    • Onuf's Nucleus (S2-S4): supplies external anal and urethral sphincters and muscles involved in erection and ejaculation

• Medial Nuclear Group:

  • Somatotopic arrangement:
    • Posterior portion: supplies flexors
    • Anterior portion: supplies extensors
  • Supplies axial musculature (trunk and neck)

Lamina 10: Dorsal Gray Horn

• Receives sensation from dorsal root ganglia
• Axons cross over to the other side through the gray commissure

Lamina 1: Posterior Marginal Nucleus

• Receives pain and temperature sensations
• Primarily carried by A-delta fibers
• Synapses with the anterior lateral spinothalamic tract

Lamina 2: Substantia Gelatinosa of Rolando

• Receives pain and temperature sensations
• Carried by C-fibers
• Becomes the spinal trigeminal nucleus in the medulla
• Involved in pain modulation

Lamina 3-4: Nucleus Proprius

• Receives pain, temperature, touch, vibration, and proprioception
• Carried by A-beta, A-delta, and C-fibers
• Information is transmitted to the spinal thalamic tract or the dorsal column-medial lemniscus pathway

Lamina 5

• Carries pain and temperature sensations from viscera
• Information is transmitted to the anterior lateral spinothalamic tract

Lamina 6

• Involved in the ventral spino-cerebellar tract (L2-Co1)
• Carries proprioceptive sensations from the lower extremities
• Involved in limb reflexes

Lamina 7

• Intermediolateral Nuclear Group (T1-L2): cell bodies of the sympathetic nervous system
• Intermediomedial Medial Nucleus (S2-S4): cell bodies of the parasympathetic nervous system
• Clark's Nucleus (C8-L3): relay station for dorsal spino-cerebellar tract, carries proprioceptive sensations

Lamina 8

• Relay station for subcortical tracts (rubrospinal, vestibulospinal, reticulospinal, and tectospinal)
• Cell bodies synapse with various tracts to control skeletal muscles responsible for distal flexion, extension, and head and neck movement

Gray Matter of the Spinal Cord

• Gray matter is composed of unmyelinated neurons
• Myelin sheaths surround and insulate the axon, enabling fast conduction
• Nodes of Ranvier allow action potentials to jump, facilitating saltatory conduction

Structure of a Neuron

• Dendrites receive neurotransmitters
• Cell body (soma) contains the nucleus and most organelles
• Axon transmits signals away from the cell body
• Axon terminals transmit signals to other neurons
• Myelin sheaths surround the axon for fast conduction

Nucleus and Ganglia

• Nucleus: a group of cell bodies in the central nervous system (CNS)
• Ganglia: a group of cell bodies in the peripheral nervous system (PNS)

Anatomical Portions of the Gray Matter

• Posterior gray horn: sensory functions
• Anterior gray horn: motor functions
• Intermediate zone: contains lateral gray horns between T1-L2
• Gray commissure: connects the two sides of the horn

Functions of the Horns

• Posterior gray horn: somatic and visceral sensations
• Intermediate zone: visceral motor functions
• Anterior gray horn: somatic motor functions

Rex Lamina and Associated Nuclei

• Rex lamina: categorizes gray matter areas
• Lamina 1-6: posterior gray horn
• Lamina 7: intermediate zone
• Lamina 8-9: anterior gray horn
• Central lamina (lamina 10): gray commissure
• Posterior marginal nucleus: lamina 1
• Substantia gelatinosa of Rolando: lamina 2
• Nucleus proprius: lamina 3-4
• Clark's column: lamina 7
• Interomedial medial nucleus group: lamina 7
• Interomedial lateral nucleus: lamina 7

Lamina 9: Lateral, Central, and Medial Nuclear Groups

• Lateral Nuclear Group: C5-T1 and L2-S3, supplies distal limbs
• Central Nuclear Group:
  • Phrenic Nucleus (C3-C5): diaphragm
  • Accessory Spinal Nucleus (C1-C5): sternocleidomastoid and trapezius muscles
  • Onuf's Nucleus (S2-S4): external anal and urethral sphincters, and muscles involved in erection and ejaculation
• Medial Nuclear Group:
  • Somatotopic arrangement: posterior (flexors) and anterior (extensors)
  • Supplies axial musculature (trunk and neck)

Lamina 10: Dorsal Gray Horn

• Receives sensation from dorsal root ganglia
• Axons cross over through the gray commissure

Lamina 1: Posterior Marginal Nucleus

• Receives pain and temperature sensations via A-delta fibers
• Synapses with the anterior lateral spinothalamic tract

Lamina 2: Substantia Gelatinosa of Rolando

• Receives pain and temperature sensations via C-fibers
• Involved in pain modulation
• Becomes the spinal trigeminal nucleus in the medulla

Lamina 3-4: Nucleus Proprius

• Receives pain, temperature, touch, vibration, and proprioception
• Carried by A-beta, A-delta, and C-fibers
• Information is transmitted to the spinal thalamic tract or the dorsal column-medial lemniscus pathway