The nervous system relies on specialized cells to transmit information efficiently throughout the body. Among these, Schwann cells play a significant role, particularly within the peripheral nervous system. They are active participants in nerve function, protection, and repair, contributing to how our bodies move, sense, and respond to the environment.
Defining Schwann Cells
Schwann cells, also known as neurolemmocytes, are a type of glial cell found exclusively in the peripheral nervous system. They originate from neural crest precursors during embryonic development. Schwann cells are categorized into two primary types: myelinating Schwann cells and non-myelinating Schwann cells.
Myelinating Schwann cells form a protective layer called the myelin sheath around individual axons of motor and sensory neurons. This differs from the central nervous system, where oligodendrocytes myelinate multiple axons. Non-myelinating Schwann cells do not form a myelin sheath. Instead, they ensheath multiple small-diameter axons, forming structures known as Remak bundles.
Myelination: The Primary Role
Myelination by Schwann cells involves these cells wrapping tightly around an axon, creating a multi-layered insulating structure called the myelin sheath. The outermost layer, containing the Schwann cell nucleus, is called the neurilemma.
The primary function of this myelin sheath is to insulate the axon, similar to how plastic coating insulates an electrical wire. This insulation prevents ion leakage, allowing the electrical signal, or action potential, to spread passively along myelinated regions. The myelin sheath is not continuous; there are regular gaps called nodes of Ranvier, which are rich in voltage-gated sodium channels.
When an action potential reaches a node of Ranvier, it is regenerated, effectively “jumping” from one node to the next. This mode of transmission is known as saltatory conduction. Saltatory conduction significantly increases the speed of nerve impulse transmission, by a factor of up to 100-fold, compared to unmyelinated axons, where the impulse travels continuously. This rapid signal propagation is important for fast reflexes and coordinated muscle movements.
Schwann Cells in Nerve Repair and Regeneration
Schwann cells play a significant role in the repair and regeneration of damaged peripheral nerves. Following an injury, Schwann cells distal to the damage transform into a repair-supportive state.
These activated Schwann cells participate in clearing cellular debris, including myelin, from the injury site. They also form aligned tubular guidance structures known as Büngner bands. These bands act as physical scaffolds, guiding regenerating axons from the proximal nerve stump towards their original target tissues. Schwann cells further support axonal regrowth by secreting various growth factors, such as nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF), that promote neuronal survival and axonal elongation. This regenerative capacity is largely attributed to the adaptable nature of Schwann cells, in contrast to the limited regeneration observed in the central nervous system.
Beyond Myelination: Supporting Neuron Health
Beyond their roles in myelination and nerve repair, Schwann cells provide broader support to neuron health. Non-myelinating Schwann cells ensheath and protect small, unmyelinated axons. They organize these axons into Remak bundles, providing a physical barrier and contributing to the structural integrity of the nerve.
Both myelinating and non-myelinating Schwann cells offer trophic support to neurons. This involves providing nutrients and secreting various growth factors important for neuronal survival, growth, and proper function. Schwann cells also contribute to the production of the extracellular matrix within the peripheral nerve trunk, which supports the neuronal environment. Their presence is important for the long-term maintenance of healthy axons.