a) OrganiSation of CNS & PNS
To understand spinal cord injury, we need to understand the structure and function of the cord. To do that, we must first understand the organisation of the central and peripheral nervous systems
The Nervous System
•The brain and spinal cord together make the central nervous system (CNS).
•Cranial nerves grow from the brain and spinal nerves grow from the spinal cord that run everywhere in the body to make the body's neural network and control the entire body.
•Cranial and spinal nerves along with their branches make the peripheral nervous system (PNS).
•The brain acts as the control and command centre that receives all the sensory information from all over the body and sends out commands to the relevant parts of the body
•The spinal cord acts as the main conduit that carries all this information to and fro, between the brain and the rest of the body.
A Schematic of the Central Nervous System and some parts of the Peripheral Nervous System
The brain and the spinal cord are part of the central nervous system (CNS) and cranial and spinal nerves are part of the peripheral nervous system (PNS) - not shown peripheral nerve branches of the rest of the body (arms, legs etc.)
b) Body’s response to injured nerve and spinal cord
Injury to the nervous system
•The nervous system is made of very specialised cells called neurons.
•In adults, normally when a neuron dies or gets damaged, it cannot be replaced by a new neuron. The loss is permanent.
•This is why any injury to the nervous system results in permanent damage.
•However, the body responds differently to a peripheral nerve (PNS) injury that it does to a spinal cord (CNS) injury.
The Neuron
The neuron is the main excitable cellular component of all nervous tissue in the body. It is electrically excitable and communicates with adjacent neurons via a chemical synapse. The main components of a neuron are its cell body, that houses the nucleus, the axon (which can be either myelinated or unmyelinated) and the dendrites (extending from the cell body) that receive information from other neurons.
Injury to a peripheral nerve
•A peripheral nerve is made of neurons with or without myelinated axons. Myelin is a fatty sheath created by adjacent cells, which in the PNS are called Schwann cells. Hundreds of axons come together to form a bundle that we see as a nerve.
•In the case of a peripheral nerve injury, these axons are usually injured.
•Our bodies can still regrow the damaged axons and regain the function lost with the initial nerve damage, as long as the cell bodies are unharmed.
•The damaged axons are slowly degraded and removed so that the cell body can regrow a new one in its place. The details of this process are covered in Module 9.
Injury to spinal cord
•Injury to the spinal cord is much different than injury to a peripheral nerve. A spinal cord contains many axons in the white matter. Adjacent cells that create the myelin sheath in the CNS are called oligodendrocytes.
•In the core of the spinal cord, the grey matter contains numerous cell bodies which are also damaged and lost in case of a spinal cord injury.
•There are a lot of small and microscopic blood vessels in the cord which rupture and form a hematoma (bruising) in an injured cord.
•This leads to continued damage to the spinal cord in an insidious manner, known as secondary damage.