What is the Spinal Cord?
The spinal cord is a long cylinder of nerves that runs from the base of your brain through the vertebral canal through the backbone. It is part of the central nervous system (CNS) along with the brain. It is divided into different segments. Each segment contains a pair roots made out of nerve fibres. The two roots in the pair are called the dorsal (towards the back) and ventral (away from the back) roots. The spinal cord is about 45 cm in length and 2 cm in diameter in adults, and is involved in many important functions of the body.
What does the spinal cord do?
The spinal cord carries out the following major functions:
The spinal cord exits through a large hole in the base of the skull and is enclosed by the protective vertebral column. Pairs of nerves known as spinal nerves emerge from the spaces between the bony arches of the vertebrae. These spinal nerves are named according to what section of the vertebral column they come from. These regions are:
A cross-section of the spinal cord
If you cut the spinal cord cross-sectionally, you would see some grey material in the shape of a butterfly in the middle of a white surround. The grey butterfly-shaped region is called grey matter, while the surrounding white material is called white matter. The grey matter forms a core which consists of four projections called “horns”. There are two dorsal horns (near the back) and two ventral horns (away from the back). The grey matter in the core is mainly made up of neurons, interneurons and glial cells, which are all cells of the central nervous system. You can learn more about these cells here.
The white consists of bundles of nerve fibres called axons that extend the length of the cord and run up and down it. This allows different levels of the CNS to communicate with one another. Each bundle of axons is known as a tract, and is specific in the type of information it transmits. Ascending tracts send signals towards the brain, while descending tracts transmit signals from the brain to neurons in various muscles and glands throughout the body.
The grey matter is also organised according to function. The central canal is filled with cerebrospinal fluid (CSF) in the centre of the grey matter. Grey matter can be divided into two halves, and each half divided into a dorsal horn (towards the back), ventral horn (away from the back) and lateral horn (towards the side). The different horns have different functions. For example, the ventral and dorsal horns supply different types of muscle – the ventral horn contains the cell bodies of neurons that supply skeletal muscle, while the lateral horn contains cell bodies that supply cardiac and smooth muscle.
Spinal nerves provide a means of communication between the spinal cord and the rest of the body. A nerve is a cordlike organ made up of several axons that are bound together. The number of axons (also known as nerve fibres) in a nerve can vary anywhere from just a handful of fibres to more than a million. Nerves tend to have a pearly white colour and can look like frayed string as they get more and more divided. There are 31 pairs of spinal nerves altogether:
Spinal nerve injuries/
There are two spinal nerves that are particularly vulnerable to injury – the radial and sciatic nerves.
Crutches that are improperly adjusted can sometimes cause crutch paralysis by compressing the radial nerve, which passes through the underarm, against the upper arm bone.
The major nerve of the hip and thigh, the sciatic nerve, is the most vulnerable nerve in the body. Damage to this nerve causes a condition known as sciatica. Patients suffering from this condition experience a sharp pain travelling from the buttock area along the back side of the thigh and leg down to the ankle. 90% of cases result from a herniated intervertebral disc or osteoarthritis of the spine. However, sciatica can also be produced by a pregnant uterus, hip dislocation, incorrect injections in the buttock, or sitting for a long time on the edge of a hard chair.
What are reflexes?
Reflex are automatic response that don’t need conscious effort – ie. you don’t need to think about it. A reflex can be simple and basic, or acquired and conditioned. Simple reflexes are built-in responses that are not learned. Eg. Pulling the hand away from a hot object. Acquired reflexes require practice and learning. Eg. Playing the piano comes naturally to a pianist, but only after a lot of time and effort has gone into training.
There are basically 5 components to a reflex:
Withdrawal reflex – Withdrawing from pain
The withdrawal reflex is an example of a simple basic spinal reflex. This reflex causes us to withdraw instinctively and quickly from sources of pain.
When a person feels pain, for example, when touching a burning hot stove, a withdrawal reflex is initiated to pull the hand away from the stove. Receptors on the skin send information to the CNS using action potentials to tell the CNS that pain is being experienced through a neuron specialised in sending information to the CNS called an afferent neuron. The stimulus must be strong enough to stimulate the receptor to a great enough level to initiated action potentials. Stronger stimuli generate action potentials of greater frequency.
The withdrawal reflex from a hot stove can be summarised as follows:
There is only one reflex that is simpler than the withdrawal reflex – the stretch reflex. This reflex involves an afferent neuron (sending signals to the CNS) at a strecth-detecting receptor in a skeletal muscle. This afferent is linked directly to the efferent neuron (sending signal from the CNS) supplying the same muscle, so that when the muscle is stretched, signals from the brain cause the muscleto contract and oppose the stretch.
Aging of the Spinal Cord
The discs between the vertebrae of the spinal cord become increasingly hard and brittle as the body ages, making them more susceptible to damage. Some parts of the spinal cord may also become overgrown. The discs are also less able to cushion the pressure from the spinal cord, resulting in large amounts of pressure being built up on the cord and nerve roots. This can reduce sensation and the sense of balance.