10. The Spinal Cord

10.1 Overview of the Spinal Cord

• The spinal cord is segmented into 31 segments: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal.

• Each spinal segment innervates a specific dermatome, contributing to skin sensitivity and muscle movement in corresponding body areas.

• Anatomically, the spinal cord is shorter than the vertebral canal, averaging 42-45 cm in adults.

• All spinal cord levels feature a similar cross-sectional structure including gray and white matter.

Innervation of Major Muscles by Spinal Segments

The spinal cord segments are responsible for the innervation of various major muscles throughout the body, categorized by region. Below are detailed functionalities:

1. Cervical Spine (C1-C8)

• C1-C4: Innervate neck muscles (e.g., sternocleidomastoid), and some diaphragm fibers (C3-C5), important for breathing.

• C5: Controls the arm (i.e., biceps, deltoid), allowing shoulder abduction and elbow flexion.

• C6: Innervates wrist extensors, facilitating wrist movements.

• C7: Innervates triceps and finger extensors, crucial for elbow extension and gripping.

• C8: Controls finger flexors, particularly affecting fine motor movements in the hand.

2. Thoracic Spine (T1-T12)

• T1-T3: Innervate upper intercostal muscles, vital for breathing mechanics and support to the upper limbs.

• T4-T6: Primarily innervate the trunk and upper abdominal muscles, aiding posture and trunk stability.

• T7-T12: Innervate lower intercostal muscles and abdominal muscles (rectus abdominis, external obliques), crucial for trunk flexion and respiratory functions.

3. Lumbar Spine (L1-L5)

• L1: Innervates hip flexors (iliopsoas).

• L2-L3: Responsible for the innervation of the quadriceps, critical for knee extension.

• L4: Innervates muscles like the tibialis anterior, important for dorsiflexion of the foot.

• L5: Innervates additional muscles involved in foot movements, including toe extension.

4. Sacral Spine (S1-S5)

• S1: Controls the calf muscles (gastrocnemius and soleus), important for plantar flexion and walking.

• S2-S4: Responsible for pelvic floor muscles, governing bladder control and sexual function, and innervate the hamstrings, facilitating knee flexion.

Clinical Implications

Damage to specific spinal segments can result in predictable motor and sensory deficits affecting muscle groups innervated by those segments. For example:

• Injury to C5 may result in weakness of shoulder abduction and elbow flexion.

• Damage to L2 can impair the function of the quadriceps, affecting the ability to extend the knee.

10.2 Functions of the Spinal Cord

• Involved in sensory processing, voluntary motor control, and reflex actions.

• Sensory Processing: The spinal cord receives sensory information through dorsal roots and processes it.

  • Ascending Pathways: Key pathways include the posterior column-medial lemniscus (for touch, proprioception) and spinothalamic tract (for pain and temperature).

• Motor Function: Sends motor impulses through ventral roots to muscles. Controlled by upper motor neurons in the brain.

• Reflex Actions: Reflexes are automatic responses to stimuli, often involving interneurons within the spinal cord.

10.3 Gray Matter Specialization

• spinal gray matter is structurally arranged into layers, with distinct functional regions:

  • Posterior Horn: Contains sensory interneurons (modulate pain, temperature).

  • Anterior Horn: Contains alpha motor neurons (control skeletal muscles).

  • Intermediate Gray Matter: Contains autonomic neurons, including preganglionic sympathetic and parasympathetic neurons.

• Layer Arrangement (Rexed's Laminae): Divided into laminae I-X, with specific functions such as pain reception and reflex integration.

10.4 Reflex Circuits in the Spinal Cord

• Muscle Stretch Reflex: Involves muscle spindles, leading to excitation of motor neurons (e.g., knee-jerk reflex).

• Golgi Tendon Organ Reflex: Inhibition of motor neurons during excessive muscle tension—autogenic inhibition occurs via interneurons.

• Withdrawal Reflex: Activation of flexor muscles following painful stimuli; involves polysynaptic pathways and coordinated activity across several spinal segments.

• Reciprocal Inhibition: When a muscle contracts, its antagonist muscle is inhibited, aiding efficient movement.

• Crossed Extension Reflex: Increases stability while withdrawing a limb from a painful stimulus by extending the opposite limb.

10.5 White Matter and Pathways

• White matter divided into ascending (sensory) and descending (motor) pathways.

  • Ascending: Posterior column-medial lemniscus system conveys touch and proprioception signals; spinothalamic tract conveys pain and temperature signals.

  • Descending: Corticospinal tracts mediate voluntary movement and influence lower motor neuron activity.

• Somatotopic Organization: Fibers are organized by body part, maintaining location-specific conduction within pathways.

10.6 Damage and Clinical Implications

• Damage to the spinal cord leads to predictable symptoms based on location.

  • Spinal Shock: Initial withdrawal reflexes may be lost, followed by flaccid paralysis.

  • Brown-Séquard Syndrome: Hemisection leads to contralateral pain/temperature loss and ipsilateral muscle weakness and loss of fine touch/proprioception.

• Syringomyelia: Central canal enlarges, damaging crossing fibers, resulting in bilateral pain/temperature loss and muscle weakness.

• Longitudinal Artery Systems: Spinal cord supplied by anterior/posterior spinal arteries along with additional radicular arteries; damage leads to various deficits.

10.7 The Autonomic Nervous System

The Autonomic Nervous System (ANS) regulates involuntary body functions, including heart rate, blood pressure, respiration, digestion, and sexual arousal. It comprises two main divisions: the sympathetic and parasympathetic nervous systems, which often have opposing effects on target organs.

Sympathetic Division:

• Preganglionic Neurons:

  • Origin: Preganglionic neurons of the sympathetic division are located in the lateral horns of the spinal cord segments from T1 to L3 (thoracolumbar region).

  • Characteristics: These neurons are relatively short due to the location of ganglia being close to the spinal cord. The preganglionic axons exit the spinal cord through the ventral roots and enter the sympathetic trunk (chain of ganglia running alongside the vertebral column).

  • Neurotransmitter: The primary neurotransmitter released by these neurons is acetylcholine (ACh), which binds to nicotinic receptors located in sympathetic ganglia. This rapid signaling initiates further responses downstream in the sympathetic pathway.

• Postganglionic Neurons:

  • Location: Postganglionic neurons are located in sympathetic ganglia, which can be either paravertebral (adjacent to the spinal cord) or prevertebral (located on the aorta). Paravertebral ganglia form a sympathetic chain that allows for widespread activation across multiple spinal levels.

  • Characteristics: These neurons are longer and innervate target tissues, such as smooth muscle, cardiac muscle, and glands across various organ systems. They can have wide-ranging influences due to the convergence and divergence of signals.

  • Neurotransmitter: Most postganglionic neurons release norepinephrine (NE), which binds to adrenergic receptors on effector organs. Specific receptor subtypes (alpha and beta) mediate different responses—alpha receptors generally cause vasoconstriction, while beta receptors often lead to increased heart rate and relaxation of smooth muscle in the lungs. However, some postganglionic neurons, especially those innervating sweat glands, release ACh, contributing to the thermoregulatory functions of the skin.

Parasympathetic Division:

• Preganglionic Neurons:

  • Origin: The parasympathetic preganglionic neurons are found in the brainstem (specifically within cranial nerves III, VII, IX, and X) and sacral spinal cord segments (S2 to S4).

  • Characteristics: Preganglionic fibers are long and travel a greater distance to ganglia located near or within target organs, allowing for localized control of organ functions. These neurons are often associated with specific functions related to rest-and-digest activities such as digestion and energy conservation.

  • Neurotransmitter: These neurons release ACh which binds to nicotinic receptors found in the ganglia, enabling synaptic transmission to the subsequent postganglionic neurons.

• Postganglionic Neurons:

  • Location: Postganglionic neurons are positioned in ganglia that are situated close to or within the walls of target organs (ganglia close to the effectors). This anatomical placement allows for precise and localized control of bodily functions.

  • Characteristics: They are shorter and project directly to the target tissues, leading to a more focused and specific response compared to the wider-reaching effects of the sympathetic division.

  • Neurotransmitter: Postganglionic neurons release ACh, which binds to muscarinic receptors on the effector organs, providing a calming effect (rest and digest response) on heart rate, promoting peristalsis in the digestive tract, and stimulating glandular secretions.

Two-Neuron Pathway:

Both the sympathetic and parasympathetic divisions utilize a two-neuron pathway. This consists of:

1. Preganglionic Neuron: Transmits impulses from the central nervous system (CNS) to autonomic ganglia, serving as the initial relay of the autonomic signal.

2. Postganglionic Neuron: Transmits impulses from ganglia to target tissues. The synaptic connection between the preganglionic and postganglionic neurons allows for integration and modulation of signals, reflecting the body's needs in real-time.

Visceral Pain:

Visceral pain signals are often referred to dermatomes, which occurs because of the convergence of visceral and somatic pathways in the spinal cord. This can lead to confusion in the brain regarding the source of pain. For instance:

• Heart Pain: Pain originating in the heart (myocardium) may be referred to the left arm or jaw due to shared spinal cord segments (C8 and T1). This anatomical overlap exemplifies how signals from different sources can activate the same neural pathways, causing misperceptions of pain location.

• Gastric Pain: Gastric pain may be felt in the mid-back, associated with the greater splanchnic nerve supply to abdominal viscera.

• Gallbladder Pain: Gallbladder pain may be referred to the right shoulder via the phrenic nerve due to connections between the diaphragm and the gallbladder innervation.

• Other Examples: Bowel and urinary tract pain can also project to lower back regions, further illustrating the intricacies of visceral pain referral patterns that can complicate diagnosis and treatment.

10.8 Spinal Pathways and Tracts

The spinal cord contains several important pathways and tracts that facilitate the transmission of sensory and motor information:

• Dorsal Column-Medial Lemniscus (DCML) Pathway:

  • Responsible for transmitting fine touch, proprioception, and vibration sensations.

  • 1st Order Neurons: These neurons have their cell bodies in the dorsal root ganglia. They enter the spinal cord and ascend in the dorsal columns (fasciculus cuneatus for upper body and fasciculus gracilis for lower body).

  • 2nd Order Neurons: Located in the medulla oblongata, these neurons receive input from 1st order neurons, decussate (cross over) and form the medial lemniscus to ascend to the thalamus.

  • 3rd Order Neurons: Located in the ventral posterolateral (VPL) nucleus of the thalamus, they project to the primary somatosensory cortex.

• Spinothalamic Tract:

  • Conveys pain and temperature sensations.

  • 1st Order Neurons: Cell bodies are in the dorsal root ganglia. They enter the spinal cord and synapse in the dorsal horn of the gray matter.

  • 2nd Order Neurons: These neurons reside in the dorsal horn, where they receive input from 1st order neurons and then decussate immediately, ascending through the spinothalamic tract toward the thalamus.

  • 3rd Order Neurons: Located in the VPL nucleus of the thalamus, they transmit signals to the primary somatosensory cortex.

• Spinocerebellar Tracts:

  • Responsible for transmitting proprioceptive information from the body to the cerebellum for coordination of movement.

  • Dorsal Spinocerebellar Tract:

    • 1st Order Neurons: Cell bodies in the dorsal root ganglia send proprioceptive signals to the dorsal horn of the spinal cord.

    • 2nd Order Neurons: Located in Clarke's column (nucleus dorsalis), these neurons send their axons ipsilaterally up to the cerebellum, entering through the inferior cerebellar peduncle.

  • Ventral Spinocerebellar Tract:

    • 1st Order Neurons: Similar to the dorsal pathway, these neurons have their cell bodies in the dorsal root ganglia.

    • 2nd Order Neurons: Located in the spinal cord, they decussate before ascending to the cerebellum via the superior cerebellar peduncle, ultimately conveying information about spinal reflex activity.

• Corticospinal Tract:

  • Major motor pathway that controls voluntary movements.

  • Upper Motor Neurons: Originate in the motor cortex (primary motor cortex), descend through the corona radiata and internal capsule, and enter the brainstem.

  • Decussation: At the junction of the medulla and spinal cord, approximately 85% of the fibers decussate to form the lateral corticospinal tract, while the remainder (about 15%) remains uncrossed as the anterior corticospinal tract.

  • Lateral Corticospinal Tract:

    • Responsible for the voluntary control of distal muscles (such as those in the limbs). It plays a crucial role in fine motor skills and skilled movements, like those involved in writing or playing an instrument.

  • Anterior Corticospinal Tract:

    • Primarily influences axial (trunk) muscles and proximal limb muscles. It aids in posture and balance by controlling the muscles that maintain stability and support during movement.

  • 2nd Order Neurons: Located in the anterior horn of the spinal cord where these upper motor neurons synapse with lower motor neurons, which project to skeletal muscles.

• Other Descending Pathways:

  • Include pathways such as the rubrospinal tract (involved in the control of flexor muscles) and reticulospinal tract (involved in regulating posture and locomotion). These also follow a similar neuron-order structure as described for the corticospinal tract.

• Somatotopic Organization:

  • Each tract demonstrates a specific organization where fibers are arranged according to the regions of the body they innervate, maintaining location-specific conduction within these pathways.

Tectospinal Tract:

The tectospinal tract is involved in reflexive head and neck movements toward visual and auditory stimuli.

• Origin: It originates in the superior colliculus, part of the midbrain, which is responsible for visual processing and reflexive head movements.

• Pathway: The neurons decussate (cross over) and descend through the brainstem to terminate in the cervical spinal cord, particularly influencing motor neurons that control neck and upper body muscles.

• Function: It facilitates rapid orientation of the head and eyes toward stimuli, providing a coordinated response to potentially dangerous or interesting sensory input.

Neuron Order Structure:

• 1st Order Neurons: These neurons arise from the superior colliculus and project down into the cervical spinal cord.

• 2nd Order Neurons: Located in the cervical spinal cord, these neurons receive input from 1st order neurons and project to motor neurons that innervate neck and upper body muscles.

• 3rd Order Neurons: These are lower motor neurons situated in the anterior horn of the spinal cord, which directly synapse and innervate the muscles responsible for neck and head movements.

10.6 Damage and Clinical Implications

• Damage to the spinal cord leads to predictable symptoms based on location.

• Spinal Shock: Initial withdrawal reflexes may be lost, followed by flaccid paralysis.

• Brown-Séquard Syndrome: Hemisection leads to contralateral pain/temperature loss and ipsilateral muscle weakness and loss of fine touch/proprioception.

• Syringomyelia: Central canal enlarges, damaging crossing fibers, resulting in bilateral pain/temperature loss and muscle weakness.

• Longitudinal Artery Systems: Spinal cord supplied by anterior/posterior spinal arteries along with additional radicular arteries; damage leads to various deficits.

10.9 Vascularization of the Spinal Cord

The spinal cord has a rich vascular supply critical for its function, primarily supplied by the following vessels:

• Anterior Spinal Artery: A single artery that runs along the anterior surface of the spinal cord, supplying the anterior two-thirds of the spinal cord through penetrating branches.

• Posterior Spinal Arteries: Two arteries (left and right) that run along the posterior side of the spinal cord, supplying the posterior one-third of the spinal cord. These arteries arise from the vertebral arteries.

• Radicular Arteries: Feed into the spinal cord from the segmental arteries (like intercostal and lumbar arteries) to enhance the blood supply, particularly to the lower thoracic and lumbar regions.

• Vascular Supply Variation: The vascularization can vary between individuals, and insufficient blood supply due to injury or anatomical variations can lead to ischemia and dysfunction of spinal cord functions.