Spinal Cord /Spinal Medulla (Medulla spinalis L.)
- 2/3rd of vertebral canal.
- Upper border of C1 to lower border of L1.
- The lowest part is conical and is called conus medullaris.
- The conus is continuous, below, with a fibrous cord called filum terminale, which is prolongation of pia mater and is attached to posterior surface of first piece of coccyx.
- 45 cm in length.
- Two enlargements;
- (a) Cervical (C3 to T2) - Supply to Upper Limb
- (b) Lumbar (L1 to S3) - enlargement Supply Lower limb.
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Age-wise changes:-
- In early stages (3rd month) both are same size.
- In Adult, spinal cord at L1 of vertebral column.
- Due to this difference in growth, the spinal nerve roots become longer and thus constitute the Cauda Equina.
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FUNCTIONS OF SPINAL CORD
The spinal cord has three major functions:
- It acts as a pathway for motor information, which travels down the spinal cord.
- It serves as a passage for sensory information in the reverse direction.
- It is a centre for coordinating simple reflexes.
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External features of Spinal Cord
- The anterior surface is marked by deep anterior median fissure, which contains anterior spinal artery.
- The posterior surface is marked by shallow posterior median fissure.
- Each half of cord is further subdivided into posterior, lateral and anterior regions by anterolateral and posterolateral sulci.
- Rootlets of dorsal / sensory roots enter via posterolateral sulcus
- Rootlets of ventral/motor roots emerge through anterolateral sulcus
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Spinal Nerves:
- 31 pairs
- 8 Cervical
- 12 Thoracic
- 5 Lumbar
- 5 Sacral
- 1 Coccygeal
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- Each spinal nerve arises by two roots:
- (1) anterior motor root.
- (2) posterior sensory root.
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Spinal Segments
- Spinal Cord- 45cm
- Vertebral column - 65 cm
- The spinal segments as a rule always lie above their numerically corresponding vertebral bodies.
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TABLE 2.1: Relation between vertebral levels and spinal segments
| Vertebral level | Formula-(to get the number of spinal segment underlying, add the numeral to the number of vertebra to get the underlying spinal segment | Example |
|---|---|---|
| Upper cervical C1-C4 | Add 0 to the number of vertebra to get the underlying spinal segment | Third cervical vertebra overlies the third cervical segment |
Segmental Innervation:-
Dermatomes: Areas of skin supplied by individual spinal nerves are called dermatomes.
The following dermatomes are of clinical significance:
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- Spinal nerve C1 does not supply any area of skin (C1 has no dermatome).
- Spinal nerve C4 supplies the tip of shoulder.
- Spinal nerves C6, C7, C8 supplies the skin of the hand.
- Spinal nerve T4 supplies the skin over the nipple.
- Spinal nerve T10 supplies the skin over the umbilicus.
- Spinal nerves L5, S1 supplies the skin of the sole.
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Myotomes:-
- Group of muscle supplied by a single spinal nerve is called Myotomes.
Features:-
- Generally a muscle is supplied by many segments.
- Some exception, Deltoid (C5), Intrinsic muscle
- The segment supplying a joint also supply the joint itself and also the skin over it (Hilton's Law)
- Muscle with some function supplied by same spinal segment.
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Clinical Anatomy
- Injury of spinal cord above C3 causes paralysis of all respiratory muscles and death due to paralysis of diaphragm.
- Injury of spinal cord at C4-C5 level paralyses all four limbs- quadriplegia.
- Injury to spinal nerves C5 and C6 (Erb's paralysis) causes loss of abduction of shoulder, loss of flexion of elbow, loss of supination and loss of extension of wrist. Unopposed action of antagonists produces policeman's tip deformity.
- Injury to spinal nerves C8 and T1 (Klumpke's paralysis) causes paralysis of intrinsic muscles of the hand causing claw hand.
- Injury of spinal cord between T2 and L1 paralyses both the lower limbs-paraplegia.
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TABLE 2.2: Segmental innervation of muscles producing various movements
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| Movements | Spinal segments (segments in brackets signify minor contribution) |
|---|---|
| Movements of the head | C1 to C4 |
| Movements of the diaphragm | (C3) C4 (C5) |
| Movements of the upper limb | C5 to T1 |
| Abduction of shoulder | C5 |
| Adduction of shoulder | C6, C7, C8 |
| Flexion of elbow | C5, C6 |
| Extension of elbow | C7, C8 |
| Supination of forearm | C6 (C7) |
| Pronation of forearm | (C6) C7 |
| Flexion of wrist | (C6) C7 |
| Extension of wrist | C6 (C7) |
| Movements of fingers | C8, T1 |
| Movements of lower limb | L2 to S3 |
| Flexion of hip | L2, L3 |
| Extension of hip | L4, L5 |
| Extension of knee | L3, L4 |
| Flexion of knee | L5, S1 |
| Dorsiflexion of ankle | L4, L5 |
| Plantar flexion of ankle | S1, S2 |
| Inversion of foot | L4 |
| Eversion of foot | (L5) S1 |
| Movements of toes | S2, S3 |
| Evacuation of bladder and bowel | S2 to S4 |
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