Spinal Cord Anatomy PDF

***Spinal Cord*** - Each **segment** of the **spinal cord** gives rise to a pair of **spinal nerves**. - In humans- **31 pairs** of spinal nerves, each side of the midline. - Spinal nerve made up of **dorsal** and **ventral root** merged containing both **sensory** and **motor fibre...

***Spinal Cord*** - Each **segment** of the **spinal cord** gives rise to a pair of **spinal nerves**. - In humans- **31 pairs** of spinal nerves, each side of the midline. - Spinal nerve made up of **dorsal** and **ventral root** merged containing both **sensory** and **motor fibres**, allowing communication between the CNS and the body. - **Dorsal root** has a **dorsal root ganglion** on it which contains cell bodies of sensory neurones, which have two axons (pseudo unipolar). - The sensory neurons have a **Peripherally projecting axon**: This originates from the sensory neuron\'s cell body in the dorsal root ganglion, passes through the dorsal root into the spinal nerve, and projects to sensory targets such as the skin, skeletal muscle, or other tissues. It collects sensory information from these peripheral targets. **Centrally projecting axon**: This axon extends from the same sensory neuron and enters the spinal cord via the dorsal root. It then synapses with interneurons or ascending neurons within the dorsal horn of the spinal cord to transmit sensory information to higher levels of the central nervous system. - **Ventral root** carries the axons of motor neurons which are found in the ventral part of the spinal cord. These axons travel through ventral root into the spinal nerve to skeletal muscles. Each vertebra is associated with a pair of spinal nerves projected from the spinal cord, except the **C1 vertebra**, which is associated with two pairs of spinal nerves. **C1 spinal nerves** only contain motor axons and emerge above the C1 vertebra. They are responsible for motor control of specific neck muscles. **C2 spinal nerves** contain both motor and sensory axons and lie below the C1 vertebra. They provide sensory input and motor control to the scalp and upper neck regions. This pattern continues along the spinal column, with each spinal nerve emerging at specific vertebral levels to innervate corresponding regions of the body. **Vertebral column** is made of bones. **Vertebral body** and **vertebral arch** connected by a small bone called the **pedicle**. **Spinous process** is where muscles attach. Spinal cord is surrounded by 3 **meninges**. **Dura** on outside which is tough and provides protection. **Arachnoid** is a finer layer. **Pia** is even finer and is wrapped tightly around the spinal cord. The **subarachnoid space** is in between the pia and arachnoid and contains csf and small blood vessels. Above meninges is the **epidural space** containing small veins and fat. **Rootlets, Root and Nerves:** **Dorsal Roots** carry **sensory** **projection axons** into the spinal cord. Dorsal roots further split into dorsal rootlets before connecting to the spinal cord. **Ventral Roots** and Rootlet carry **motor neuron axons** and sympathetic neuron axons from the spinal cord. Ventral roots are formed by merging ventral rootlets and exit to join the spinal nerve. **Sympathetic Chain Ganglion** is part of the sympathetic nervous system, it serves as a hub for sympathetic neuron synapses and signal distribution. Once the **spinal nerve** exits the vertebral column, it splits into: **Dorsal (Posterior) Ramus:** Supplies sensory, motor, and sympathetic innervation to the back (skin and intrinsic muscles). And **Ventral (Anterior) Ramus**: Supplies sensory, motor, and sympathetic innervation to the anterolateral trunk and limbs.  **Neuron types associated with the spinal cord:** **Grey Matter:** Contains neurons and glia. Divided into: **Dorsal Horn**: Processes sensory input. **Ventral Horn**: Houses motor neurons. **Intermediate Zone**: Contains interneurons and other specialized neurons. **Lateral Horn**: Present in the thoracic region (T1-L2), containing preganglionic sympathetic neurons. **White Matter**: Myelinated axon tracts for signal transmission. **Dorsal Rootlets/Root**: Carry sensory signals (via dorsal root ganglia). **Ventral Rootlets/Root**: Carry motor and sympathetic signals. **Spinal Nerve**: Mixed sensory, motor, and sympathetic fibres. **Sensory neurons**: Transmit signals to the dorsal horn. **Motor neurons**: Send commands to muscles from the ventral horn. **Sympathetic neurons**: Regulate autonomic responses from the lateral horn. **Interneurons**: Coordinate reflexes and internal communication. **The lumbosacral enlargement** (thickening of the spinal cord) gives rise to: L1 to L5 lumbar spinal nerves, S1 to S5 sacral spinal nerves and the coccygeal nerve. Spinal cord segments in the lumbar enlargement are compressed in the **rostral caudal plane** **(compressed lengthwise)**. compared to cervical and thoracic segments. After spinal cord ends (After L1), L2-S5 spinal nerve roots + coccygeal nerve run caudally (downwards) in the cauda equina until they emerge at their appropriate vertebral level as spinal nerves. - The spinal cord terminates at the **conus medullaris**, typically around the L1/L2 vertebral level. - Below the conus medullaris, the **cauda equina** is a bundle of spinal nerve roots (from L2 to S5 and the coccygeal nerve) that extends down through the **lumbar cistern** to their exit points at various vertebral levels. - The **lumbar cistern** is an enlarged **subarachnoid space** filled with cerebrospinal fluid (**CSF**), located between vertebral levels L1/L2 and S2/S3. - The **pia mater** (the innermost meningeal layer) surrounding the spinal cord extends into a thin structure known as the **filum terminale**, which anchors the spinal cord to the **coccyx**. - **Dura mater** and **arachnoid mater** (outer protective coverings) extend down to S2/S3, after which they merge with the **filum terminale**. **Epidural: An epidural injection** is administered into the **epidural space**, which is located just outside the **dura mater**. This space contains fat and blood vessels, and the injection provides anaesthesia or pain relief by blocking nerve signals. **Lumbar Puncture (Spinal Tap)**: A **lumbar puncture** involves inserting a needle into the **subarachnoid space** (within the lumbar cistern) between the **L3/L4 or L4/L5 vertebrae** to collect CSF. This procedure helps diagnose conditions affecting the brain, spinal cord, or meninges (e.g., meningitis) by analysing the CSF. **As we move from the top (cervical) to the bottom (sacral) of the spinal cord, the shape of the grey matter (butterfly-shaped) changes.** **Lumbar levels (L5):** Grey matter increases again, showing a \"well-proportioned butterfly\" due to the need for motor control and sensory input in the lower limbs. **Sacral levels (S3, S4):** There is a higher proportion of grey matter, leading to a \"fat butterfly\" appearance. Fewer ascending and descending axons are present, as many have already terminated or originated at higher levels. **White matter** contains **ascending and descending axons** (nerve fibres). It is more abundant at the **cervical levels** (rostral) because many tracts are either starting or ending in the brain. As you move caudally (toward the sacral region), the **grey matter** increases because there are fewer long tracts, and more local innervation is required for limbs and organs. **Major Ascending and Descending tracts of the spinal cord:** **Ascending Tracts (Sensory Pathways)** - **Spinothalamic tract**: Carries signals related to pain, temperature, itch, and simple touch. - **Dorsal column-medial lemniscus pathway**: Transmits information for discriminative touch (fine touch) and conscious proprioception (body position awareness). - **Spinocerebellar pathway**: Responsible for non-conscious proprioception, which helps with coordination and balance without requiring conscious awareness. **Descending Tracts (Motor Pathways)** - **Pyramidal motor pathway:** Controls conscious movement and includes the lateral and anterior corticospinal tracts. These tracts are involved in precise voluntary movement. - **Non-pyramidal motor pathways:** Involved in non-conscious muscle control for balance, posture, and reflexes. Examples include: *Reticulospinal tract*: Regulates muscle tone, posture, and stereotyped movements (e.g., walking). *Vestibulospinal tract*: Helps maintain balance and keeps the head and eyes stable during movement. **Spinothalamic Tract and the Dorsal Column Pathway.** These pathways transmit sensory information from the body to the brain but carry different types of sensory modalities. **Spinothalamic Tract** (Pain, temperature, itch and simple touch)**:** - **Spinal Cord**: Sensory information from pain and temperature receptors enters the spinal cord via dorsal root ganglion (DRG) neurons. - **Interneuron Projection**: These sensory neurons synapse with projection interneurons in laminae I or V of the spinal cord. - **Decussation (Crossing):** The axons of these interneurons cross to the opposite side of the spinal cord (decussation). - **Ascending Pathway**: The axons ascend through the medulla, pons, and midbrain. - **Ventral Posterior Thalamic Nuclei**: The axons synapse in the ventral posterior thalamic nuclei, which are somatotopically mapped (organized according to the part of the body they represent). - **Primary Sensory Cortex**: Finally, the sensory information is relayed to the primary sensory cortex for conscious perception of pain, temperature, and other stimuli. **The Dorsal Column Pathway** (discriminative (fine) touch and conscious proprioception): - **Spinal Cord**: Sensory input enters the spinal cord through low threshold mechanoreceptive DRG neurons. - **Dorsal Columns**: These neurons ascend ipsilaterally (on the same side) through the dorsal column of the spinal cord. - The **fasciculus gracilis** (from lower body) and **fasciculus cuneatus** (from upper body) carry the sensory information. - **Dorsal Column Nuclei**: In the medulla, these fibres synapse in the nucleus gracilis and nucleus cuneatus. - **Medial Lemniscus**: The second-order neurons cross over (decussate) at the medulla and form the medial lemniscus pathway on the contralateral (opposite) side. - **Ventral Posterior Thalamic Nuclei**: These neurons then ascend to the ventral posterior thalamic nuclei, which are also somatotopically mapped. - **Primary Sensory Cortex**: From the thalamus, the information is projected to the primary sensory cortex, where conscious perception of touch and body position occurs. **Pyramidal Motor Pathway** (Corticospinal Tracts): - Upper motor neurons travel from the primary motor cortex to the spinal cord. Upper motor neuron axons pass through the internal capsule (i.c.) on their way to the midbrain. - 90% of fibres decussate (cross) at the medulla-spinal cord junction, traveling in the lateral corticospinal tract (l.c.t.) to control limb muscles. - 10% do not decussate and travel in the anterior corticospinal tract (a.c.t.) to control trunk muscles. **Spinocerebellar pathway** (non-conscious proprioception): - Proprioceptive signals from muscles and joints travel through dorsal root ganglion (DRG) neurons. - They synapse in the nucleus of Clarke and lateral cuneate nucleus, then travel to the ipsilateral cerebellum to coordinate unconscious movement. - Minimal decussation occurs in this pathway. **Laminae of the spinal cord grey matter are divided into regions based off functions:** **Lamina I**: Processes pain signals from nociceptive sensory neurons and relays them to the thalamus. **Lamina II**: Receives input from nociceptive neurons and modulates sensory information using local interneurons. **Laminae III-V**: Processes mechanoreceptive signals (touch, pressure) and relays to interneurons. **Lamina V**: Integrates signals from various sensory neurons (pain, temperature, touch). **Lamina VI-VII**: Processes proprioceptive signals, relays non-conscious proprioception to the cerebellum. **Laminae VII**: Contains neurons for sympathetic and parasympathetic pathways. **Laminae VIII-IX**: Contains motor neurons controlling skeletal muscles (limb and trunk). **Lamina X**: Surrounds the central canal; processes visceral sensory inputs.  **Cervical lesions** (C1-C8): Affect breathing, head/neck movement, shoulder, wrist, and hand functions. **Thoracic lesions** (T1-T12): Impact trunk stability, sympathetic tone, and ejaculation. **Lumbar and Sacral lesions** (L1-S5): Affect hip, knee, ankle, and foot movement, as well as bladder/bowel control and sexual functions.