Spinal Cord

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Spinal Cord

The spinal cord is the communication gateway between the brain and spinal nerves, which innervate the trunk and limbs. The cord is a long, tube-like band of nervous tissue. It is enclosed in 3 membranes of the meninges which in turn, are protected within the bones of the vertebral column. It connects the brain to the lower back.

The 31 pairs of the spinal nerves arise from the cord and emerge from the vertebrae. The spinal cord extends from the brain stem to the level of upper lumbar vertebrae. In the lower lumbar and sacral regions, nerve roots disembark within the spinal canal before exiting, forming the cauda equina. In cross section, two types of nervous tissue can be seen in the cord. A butterfly- shaped central core of grey matter, and a surrounding white matter.

Functions of spinal cord:

·        Control body movements and functions.                                                               It carries nerve signals from brain to body and vice versa.

·      Report senses to brain.                                                   

     Signals from other parts of the body help the brain record and process sensations such as pressure or pain.

·        Manage Reflexes.  Spinal cord controls certain reflexes with involving the brain.

Grey matter:

Grey matter is a darker hue. The grey matter contains cell bodies and dendrites of neurons. This is where neurons synapse and transmit information to each other.

White matter:

White matter appears pinkish- white. The white matter, on the other hand, is made of bundles of axons, and serves to conduct information up and down the cord. The white matter of the spinal cord contains a mixture of ascending and descending tracts.

Ascending tract   - sensory or afferent                                                                    Descending tract - motor or efferent.

Spinal tracts:

Bundles of axons are organized into specific groups with specific functions, forming the so-called spinal tracts. Spinal tracts are pathways that carry certain type of information, in a one-way traffic up and down the spinal cord between brain and body.

Ascending tracts conduct sensory information up to the brain, while descending tracts convey motor instructions down the cord.

Key facts about the ascending and descendingtracts:

·        Lateral spinothalamic - Pain and temperature.

·        Ventral spinothalamic - Pressure and crude touch.

·        Dorsal column               - Vibration, proprioception, two- point discrimination.

·        Spinocerebellar             - Proprioception in joints and muscles.

·        Cuneocerebellar            - Proprioception in joints and muscles.

·        Spinotectal                     - Tactile, painful, and thermal stimuli.

·        Spinoreticular                - Integration of stimuli from joints and muscles into the reticular formation.

·        Spino- olivary                 - Additional information to the cerebellum as an accessory pathway.

·        Corticospinal                  - Voluntary, discrete, skilled motor activities.

·        Reticulospinal                - Regulation to voluntary movements and reflexes.

·        Rubrospinal                   - Promotion of flexor and inhibition of extensor muscle activity.

·        Vestibulospinal            - Inhibition of flexor and promotion of extensor muscle activity.

·        Tectospinal                   - Postrual movements from visual stimuli.

Some tracts cross over to the other side of the cord, before they reach the brain. They convey sensory information from one side of the body to the other side of the brain. When this happens, the information is said to be transmitted contralaterally. Tracts that stay on the same side all the way are said to conduct information ipsilaterally.

Spinal nerves:

The spinal nerves. consist of 31 symmetrical pairs of nerves that connect the spinal cord to the periphery. There are 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal nerve. Spinal nerves are mixed nerves, they contain both sensory and motor fibres, they therefore have both sensory and motor functions.

Sensory fibres enter the cord via the dorsal root, while motor fibres exit via the ventral root.

Functions of spinal nerves:

·        For sensory functions, the spinal nerves receive sensory messages from the skin, internal organs and the bones.

·        For motor functions, the motor roots receive nerve messages via the front of the spinal cord and then transmits these messages to the spinal nerves.

Cervical nerves:

·        C1, C2, and C3 – spinal nerves succor to control the head and neck, including forward, backward, and sideward movements.

·        C4                    – these help to control the upper shoulder movements, as well as helping to power the diaphragm.

·        C5                    – these assist to control the deltoids and biceps, the areas of the upper arm, down to the elbows.

·        C6                   – these help to control the wrist extensions, with some supply given to the biceps.

·        C7                   – It aid and abet to control the triceps as well as the wrist extensor muscles.

·        C8                  – these help to control the hands, as well as finger flexion.

Thoracic nerves:

·        T1 and T2             – these thoracic spinal nerves furnish the top of the chest, arms, and hands.

·        T3, T4, T5             – these nerves supply into the chest wall as well as backing in breathing.

·        T6, T7, T8             – these nerves endow into the chest and down into the abdomen.

·        T9, T10, T11, T12 – these nerves supply into the abdomen and lower in the back.

Lumbar nerves:

·        L1                     – these lumbar spinal nerves bestow sensations to the groin as well as the genitals.

·        L2, L3, and L4 – these nerves assign sensations to the front of the thighs and the inner side of the lower legs.

·        L5                    – these nerves dispense sensations to the outer side of the lower legs and the upper foot. These also help to control the hips, knees, feet, and toe movements.

Sacral nerves:

·        S1             – these sacral spinal nerves affect the hips and the groin area.

·        S2             – these nerves affect the back of the thighs.

·        S3             – these nerves affect the medial buttock area.

·        S4 and S5 – these nerves affect the perineal area.

Coccygeal nerves:

·        These spinal nerves innervate the skin around the coccygeal region, including around the tailbone.

Sensory pathway of spinal cord:

Ascending tracts are sensory pathways that begin at the spinal cord and stretch all the way up to the cerebral cortex. A somatosensory pathway will typically consist of three neurons: primary (First-order), secondary (Second-order), and tertiary (Third-order).

·        First-order neurons detect stimuli and transmit signals to the spinal cord.

·        Second-order neurons – Inside the cord, first-order neurons synapse with second-order neurons, which ascend a specific tract to the brainstem, or further up to the thalamus. In some pathways first-order neurons ascend the tract to the brainstem, where they synapse with second-order neurons, which continue to the thalamus.

·        Third-order neurons conduct the information the rest of the way to the sensory cortex.

Motor pathway of spinal cord:

The motor pathway, also called the pyramidal tract or the corticospinal tract, serves as the motor pathway for upper motor neuronal signals coming from the cerebral cortex and from primitive brainstem motor nuclei. There are upper and lower motor neurons in the corticospinal tract.

·        An upper motor neuron starts in the motor cortex or brainstem, and a lower motor neuron continues from the brainstem or spinal cord.

·        The axons of lower motor neurons exit the cord via the ventral root of spinal nerve, where they continue as motor fibres to effector organs.

Somatic reflexes:

The spinal cord is also responsible for fast, involuntary responses of skeletal muscles, called somatic reflexes. A somatic reflex involves a reflex arc composed of a somatic receptor, a sensory neuron, an interneuron, a motor neuron, and an effector muscle.

Reflex	Muscle tested	Spinal nerve
Brachioradialis reflex	Brachioradialis	C5-C6
Triceps reflex	Triceps	C7-C8
Knee reflex	Quadriceps	L3-L4
Ankle reflex	Gastrocnemius	S1-S2

 

Some reflexes are however more complex, and require multiple pathways, as well as central coordination from the brain.