Grey Matter

Gray Matter of the Spinal Cord

• Definition and Overview

  • Gray matter is a critical component of the spinal cord and is composed of various types of neurons that perform distinct functions.

  • In the central nervous system, there are two primary types of neurons found: projection cells and interneurons.

Types of Neurons

• Projection Cells

  • Generally larger in size.

  • Characteristic feature: Their axons extend outside the spinal cord.

  • Destinations of axons:

  • Brainstem

  • Thalamus

  • Peripheral tissues, including muscles

• Characteristics of Projection Cells

  • Examples of projection cells in the spinal cord include:

  • Track Neurons: Ascend to the brain (thalamus, brainstem).

  • Motor Neurons: Innervate muscles.

  • Function: Projection cells are essential for sending messages about sensory inputs or initiating motor actions.

  • Nature: Typically excitatory, characterized by neurotransmitters such as glutamate or acetylcholine.

• Interneurons

  • Smaller than projection neurons.

  • Characteristic feature: Their axons remain within the spinal cord, allowing for local communication.

  • Abundance: Interneurons outnumber projection neurons by approximately 7 to 1.

  • Function:

  • Many are inhibitory and use GABA as a neurotransmitter.

  • Some perform excitatory functions as part of a multi-synaptic network; thus, they are not exclusively inhibitory.

Subdivisions of the Gray Matter

• Dorsal Horn

  • Function: Specializes in processing somatic sensory inputs.

  • Inputs include:

  • Tactile signals

  • Pain and temperature afferents

  • Some proprioceptive inputs

  • Composition: Contains numerous interneurons, along with some projection cells (track cells).

• Intermediate Gray Matter

  • Function: Acts as the autonomic part of the spinal cord gray matter.

  • Composition includes:

  • Preganglionic motor neurons associated with the autonomic nervous system

  • Fewer visceral sensory inputs compared to somatic sensory inputs.

  • Distinction:

  • In thoracic/lumbar regions, mainly preganglionic sympathetic neurons.

  • In sacral regions, mainly preganglionic parasympathetic neurons.

• Ventral Horn

  • Function: Responsible for somatic motor control.

  • Contains:

  • Large alpha motor neurons, which are among the largest neurons in the central nervous system.

  • Smaller gamma motor neurons that innervate muscle spindles.

  • Mapping of motor neurons:

  • Flexor muscles located deeper, while extensor muscles are more superficial.

  • Medial locations innervate trunk muscles, while lateral locations innervate distal limb muscles (hands and feet).

Degeneration and Regeneration

• Regeneration Challenges

  • Central nervous system (CNS) regeneration is suboptimal compared to peripheral nervous system (PNS) regeneration.

  • Evidence indicates that damaged CNS neurons often do not regrow effectively due to barriers created by glial cells.

  • Glial cells release a protein termed NoGo, which physically prevents regeneration of axons.

  • Following lesions, the lack of molecular signposts that guide axonal growth contributes to poor regeneration.

• Neurogenesis in Adults

  • While previously thought that no new neurons developed after birth, recent findings indicate the presence of adult neurogenesis.

  • Sites of neurogenesis include:

  • Subventricular zone (next to ventricles)

  • Hippocampus (important for memory formation).

  • Factors that enhance neurogenesis include cognitive tasks and exercise.

• Peripheral Nervous System Regeneration

  • Peripheral nerves recover better than CNS due to active regeneration mechanisms.

  • Following damage, the distal part of the neuron undergoes Wallerian degeneration.

  • The soma of the neuron may begin to swell, indicating distress.

  • Successful regeneration involves the formation of a Schwann cord, which can guide axons back to their target if present. However, many may form a neuroma instead, leading to dysfunction.

• Causes of Limited Regrowth

  • The brain maintains highly ordered networks; inappropriate connections might be detrimental, favoring the maintenance of no connection over a wrong one.

  • Stress negatively impacts neurogenesis and overall neuron health through its detrimental physiological effects.

• Exercise and its Impact

  • Exercise induces beneficial changes in the brain by releasing trophic factors that enhance neuron health and improve homeostasis.

  • Elevated levels of neurogenesis have been documented in exercise-engaged populations, such as Parkinson's disease patients, who show larger hippocampal volumes as a result of increased physical activity.

  • Stress, on the other hand, has been found to inhibit the survival of stem cells and reduce neurogenesis.