NH

MODULE 6

Ventral Roots - Composed of alpha motor neuron axons connecting to the extrafusal muscle fibers. Contains gamma motor neuron axons that innervate the intrafusal muscle fibers of muscle spindles.

Organization of Spinal Nerves

Spinal nerves are categorized by region:

  • Cervical Nerves: C1 - C8

  • Thoracic Nerves: T1 - T12

  • Lumbar Nerves: L1 - L5

  • Sacral Nerves: S1 - S5

  • Coccygeal Nerve

Ventral Horn

  • The location of cell bodies for:

    • Alpha motor neurons.

    • Gamma motor neurons.

    • Interneurons

    • Sensory endings

Spinal Reflexes

Types of Reflexes

  • Superficial Reflexes: Involve skin and mucous membranes, e.g., blinking in response to corneal stimulation.

  • Tendon Reflexes: e.g., patellar reflex.

  • Visceral Reflexes: e.g., pupil dilation, vomiting.

  • Classification by CNS Location:

    • Spinal Reflexes

    • Bulbar Reflexes (located in the medulla, pons, cerebellum)

    • Midbrain Reflexes

Simple Reflex Arc

  • Composed of 5 elements:

    1. Receptor: Detects stimulus.

    2. Afferent (Sensory) Neuron: Carries sensory information to the spinal cord.

    3. Interneuron(s): Optional component for reflex processing (not always present).

    4. Efferent (Motor) Neuron: Conducts impulses away from the spinal cord to the effector.

    5. Effector: The muscle or gland that produces a response.

  • Stimulus-Response Relationship: Defined pathway whereby the stimulus results in a response.

  • Clinical Implications of Reflex Arc Lesions:

    • Receptor/Afferent Neuron Lesion: Leads to absence or diminution of reflex (areflexia/hyporeflexia). Example: sensory neuropathy. Symptoms include sensory loss in a dermatomal or peripheral nerve distribution (e.g., skin, muscles, joints in the affected dermatome/nerve distribution).

    • Efferent Neuron (Ventral Horn/Ventral Root/Spinal Nerve) Lesion: Results in absence or diminution of reflex (areflexia/hyporeflexia) due to disruption of the motor output, accompanied by flaccid paralysis and muscle atrophy (Lower Motor Neuron Syndrome). Example: poliomyelitis, peripheral nerve injury. Symptoms localize to specific myotomes (e.g., specific muscles like biceps, triceps, quadriceps, hand intrinsic muscles).

    • Interneuron Lesion (within CNS): Can affect modulation of complex reflexes, potentially leading to altered reflex activity.

    • Upper Motor Neuron Lesion (above reflex arc): Often results in hyperreflexia and spasticity due to loss of descending inhibitory control from higher centers (affects muscles below the level of the lesion, often in a widespread pattern).

Stretch Reflexes

Anatomy:

  • Involves sensory and motor neurons functioning at the same level in the spinal cord.

  • Provides feedback mechanism for muscle tone regulation.

Muscle Spindles:

  • Classified as intrafusal fibers, act as mechanoreceptors or stretch receptors within muscles.

  • Sensory neurons respond to stretch; firing rate increases with stretch and decreases with contraction.

Motor Neurons:

  • Alpha Motor Neurons: Activate extrafusal muscle fibers to contract upon receiving signals.

  • Gamma Motor Neurons: Adjust the sensitivity of muscle spindle firing based on muscle length changes.

  • Renshaw Cells: An inhibitory interneuron which receives input from alpha motor neurons, inhibiting the same motor neuron for feedback regulation.

Myotomes and Clinical Assessment

Myotomes

  • Define regions of skeletal muscles innervated by the motor components of specific spinal nerves:

    • C3 - C5: Diaphragm (breathing)

    • C5: Biceps (elbow flexion)

    • C6: Deltoids, Extensor Carpii (wrist extension)

    • C7: Triceps (elbow extension)

    • C8: Palmar Interossei (finger flexion)

    • T1: Dorsal Interossei (finger abduction)

    • T1 - T12: Muscles of the chest and abdomen.

    • L2: Iliopsoas (hip flexion)

    • L3: Quadriceps Femoris (knee extension)

    • L4: Anterior Tibialis (ankle dorsiflexion)

    • L5: Extensor Digiti (toe wiggling)

    • S1: Posterior Tibialis (ankle plantar flexion)

    • S3 - S5: Bladder, bowel, and sexual organs.

Clinical Significance

  • Loss of muscle tone associated with damage to alpha motor neurons (Lower Motor Neuron Syndrome - affects specific muscles/myotomes).

  • Impairment of deep tendon reflexes suggests polyneuropathy or damage to the reflex arc (sensory, motor, or interneuron components - affects muscles in the distribution of the damaged reflex arc).

  • Specific reflex impairment indicates injury to the reflex circuit involved. Exaggerated reflexes (hyperreflexia) are indicative of upper motor neuron lesions (affects muscles below the lesion level, often limbs and trunk).

Lesions and Clinical Observations

  • General Lesions external to the spinal cord can cause significant symptoms, including:

    • Spinal cord compression from extramedullary masses such as tumors, abscesses, or herniated disks.

  • Symptoms may include:

    • Weakness or numbness in the legs.

    • Loss or reduction of sensation below specific dermatome levels (e.g., lower extremities, trunk).

    • Pain elicited by percussion over the spinal column.

    • Abnormal reflex responses (can be hypo- or hyper-reflexic depending on exact lesion location).

Ventral Horn and/or Ventral Root Lesions

  • These lesions specifically affect the lower motor neurons (alpha and gamma motor neuron cell bodies in the ventral horn, and their axons in the ventral root).

  • Associated Symptoms (Lower Motor Neuron Syndrome):

    • Flaccid Paralysis: Loss of muscle tone and voluntary movement in the affected muscles (e.g., muscles corresponding to the damaged myotome).

    • Muscle Atrophy: Wasting of muscles due to denervation (e.g., muscles corresponding to the damaged myotome).

    • Fasciculations: Spontaneous, visible twitching of muscle fibers (resulting from denervation hypersensitivity) (e.g., visible in affected muscles).

    • Hyporeflexia or Areflexia: Diminished or absent deep tendon reflexes in the affected myotomes due to disruption of the efferent limb of the reflex arc (e.g., reflexes involving affected muscles).

    • No Sensory Loss: Pure ventral horn/root lesions typically do not cause sensory deficits, as sensory pathways (dorsal horn/root) are spared.

    • Localization: Symptoms localize to specific myotomes corresponding to the damaged ventral horn segment or ventral root. For example, damage to the C5 ventral root would affect biceps function (elbow flexion).

Dorsal Root and/or Dorsal Horn Lesions

  • These lesions affect sensory neurons and their cell bodies (dorsal root ganglia for primary afferents) and synaptic targets in the dorsal horn.

  • Associated Symptoms (Sensory Neuronopathy):

    • Sensory Loss: Numbness, tingling, or loss of specific sensations (e.g., pain, temperature, touch, proprioception) in a dermatomal distribution corresponding to the affected dorsal root (e.g., skin area corresponding to the damaged dermatome).

    • Paresthesias/Dysesthesias: Abnormal or unpleasant sensations (e.g., felt in the affected dermatome).

    • Ataxia: If proprioceptive pathways are affected, leading to impaired coordination and balance (affects limb movements and gait, particularly in the dark).

    • Hyporeflexia: Diminished or absent deep tendon reflexes due to disruption of the afferent limb of the reflex arc. Motor strength remains intact unless the lesion is extensive and also affects motor pathways (e.g., reflexes involving the affected sensory input).

Spinal Nerve Lesions (Mixed Motor and Sensory)

  • A spinal nerve contains both efferent (motor) and afferent (sensory) fibers, so a lesion here combines symptoms of both ventral and dorsal root damage.

  • Associated Symptoms:

    • Flaccid paralysis and muscle atrophy in the muscles innervated by the motor component (e.g., muscles in the affected myotome).

    • Sensory loss (numbness, paresthesias) in the dermatome supplied by the sensory component (e.g., skin in the affected dermatome).

    • Hyporeflexia or Areflexia involving reflexes mediated by that spinal nerve (e.g., reflexes involving both motor and sensory components of the affected nerve).

  • Specific Spinal Nerve Lesion Effects by Region:

    • Cervical Nerves (C1-C8):

    • C1-C4: Weakness in neck muscles (neck flexion, extension, rotation), potentially difficulty breathing if phrenic nerve (C3-C5) is involved (affecting diaphragm); sensory loss in posterior scalp and neck region (back of head, neck).

    • C5: Weakness in shoulder abduction and elbow flexion (deltoid, biceps); sensory loss in lateral arm (outer shoulder to elbow).

    • C6: Weakness in wrist extension (extensor carpii), elbow flexion (biceps); sensory loss in thumb and radial forearm (thumb, index finger, radial forearm).

    • C7: Weakness in elbow extension (triceps), wrist flexion; sensory loss in middle finger (middle finger).

    • C8: Weakness in finger flexion (palmar interossei) and ulnar deviation; sensory loss in little finger and ulnar forearm (little finger, ulnar forearm).

    • Thoracic Nerves (T1-T12):

    • T1: Weakness in finger abduction (dorsal interossei); sensory loss in medial forearm and upper arm (inner forearm to upper arm).

    • T2-T12: Weakness in intercostal and abdominal wall muscles (affecting trunk stability, respiration, coughing); sensory loss in band-like pattern around the trunk corresponding to the specific dermatome level (e.g., chest, abdomen).

    • Lumbar Nerves (L1-L5):

    • L1-L2: Weakness in hip flexion (iliopsoas); sensory loss in upper anterior thigh and groin (upper anterior thigh, groin).

    • L3: Weakness in hip flexion (iliopsoas) and knee extension (quadriceps femoris); sensory loss in anterior and medial thigh (anterior/medial thigh).

    • L4: Weakness in knee extension (quadriceps femoris), ankle dorsiflexion (anterior tibialis); sensory loss in medial leg and foot (medial leg, big toe).

    • L5: Weakness in ankle dorsiflexion (anterior tibialis), toe extension (extensor digiti), hip abduction; sensory loss in lateral leg and dorsum of foot (lateral leg, top of foot, toes 2-5).

    • Sacral Nerves (S1-S5):

    • S1: Weakness in ankle plantar flexion (posterior tibialis), hip extension; sensory loss in posterior leg and lateral foot (back of leg, little toe side of foot).

    • S2-S5: Weakness affecting bladder, bowel, and sexual organ function (e.g., urinary/fecal incontinence, erectile dysfunction); sensory loss in perineal and buttock regions (saddle anesthesia).

    • Coccygeal Nerve: Primarily provides sensory innervation to a small area around the coccyx; lesions typically result in localized sensory loss or pain in this region.

Spinal Anatomy Reminders

External Anatomy

  • The spinal cord extends from the foramen magnum (continuous with the medulla oblongata) to the conus medullaris, typically ending between lumbar vertebrae L1 and L2 in adults.

  • It is protected by the vertebral column, meninges (dura mater, arachnoid mater, pia mater), and cerebrospinal fluid.

  • Spinal Nerves: 31 pairs emerge from the spinal cord, each formed by the union of a dorsal (sensory) root and a ventral (motor) root.

  • Cauda Equina: Below the conus medullaris, the lumbar and sacral nerves descend within the vertebral canal as a bundle, resembling a horse's tail (affects lower extremities, perineum, bladder, and bowel if damaged).

Gray Matter

  • In the spinal cord, significant for processing and relay of information.

    • Ventral Horn: Contains cell bodies of motor neurons (alpha and gamma motor neurons) and some interneurons, controlling voluntary and reflexive skeletal muscle movements. Lesions here (e.g., poliomyelitis) lead to Lower Motor Neuron Syndrome, characterized by flaccid paralysis, muscle atrophy, fasciculations, and hyporeflexia/areflexia in specific myotomes, without sensory loss.

    • Dorsal Horn: Contains cell bodies of sensory neurons (second-order neurons) and interneurons that receive and process sensory information (pain, temperature, touch, proprioception) from the body. Lesions here (e.g., tabes dorsalis if dorsal column is affected, or general sensory neuropathy if primary afferents are damaged) result in sensory loss (numbness, tingling, loss of specific sensations) in a dermatomal distribution, paresthesias, and potentially ataxia if proprioceptive pathways are involved, along with hyporeflexia due to disruption of the afferent limb of the reflex arc.

    • Lateral Horn: Present in thoracic and upper lumbar segments, contains autonomic preganglionic neuron cell bodies.

Ascending Tracts (Sensory Pathways)

  • Responsible for carrying sensory information from the body to the brain:

    • Dorsal Columns: Composed of:

    • Fasciculus Gracilis: Carries fine touch, vibratory sensation, and conscious proprioception from the lower body (legs and lower trunk) to the brain. (Lesion: ipsilateral loss of these sensations below the lesion).

    • Fasciculus Cuneatus: Carries fine touch, vibratory sensation, and conscious proprioception from the upper body (arms and upper trunk) to the brain. (Lesion: ipsilateral loss of these sensations below the lesion).

    • Dorsal Spinocerebellar Tract: Carries unconscious proprioception (muscle and joint position sense) from the lower body and legs to the cerebellum for motor coordination. (Lesion: ipsilateral ataxia in the affected leg/trunk).

    • Ventral Spinocerebellar Tract: Carries unconscious proprioception from the lower body and legs to the cerebellum, particularly information about interneuron activity and motor commands. (Lesion: ipsilateral ataxia in the affected leg/trunk).

    • Lateral Spinothalamic Tract: Transmits crude touch, pain and temperature sensations from the body to the thalamus. These fibers decussate (cross) in the spinal cord. (Lesion: contralateral loss of pain and temperature sensation below the lesion, affecting the opposite side of the body/trunk/limbs).

    • Ventral Spinothalamic Tract: Transmits crude touch and pressure sensations from the body to the thalamus. These fibers also decussate in the spinal cord. (Lesion: contralateral loss of crude touch and pressure sensation below the lesion, affecting the opposite side of the body/trunk/limbs).

Descending Tracts (Motor Pathways)

  • Involved in sending motor information from the brain to the body:

    • Ventral White Commissure: Area where some descending tracts and ascending tract fibers cross the midline.

    • Lateral Corticospinal Tract: The primary pathway for voluntary, skilled movements, especially of the distal limbs (hands and feet). Fibers decussate in the medulla. (Lesion in spinal cord: ipsilateral spastic paralysis, hyperreflexia, Babinski sign, and weakness of voluntary movements below the lesion, affecting the same side limbs/trunk).

    • Ventral Corticospinal Tract: Involved in voluntary movement of axial and proximal limb muscles (trunk, shoulders, hips). Fibers largely remain uncrossed until they reach the spinal level of innervation. (Lesion: bilateral weakness of trunk and proximal muscles, often clinically less pronounced than lateral tract lesions, affecting trunk and proximal limb movements).

    • Rubrospinal Tract: Originates in the red nucleus and influences motor control, particularly for upper limb muscle tone and coordination. (Lesion: less prominent clinical effects in humans compared to corticospinal tracts, but can contribute to flexor posturing of the upper limbs).

    • Medial Vestibulospinal Tract: Plays a role in head and neck movements in response to vestibular (balance) stimuli, aiding in head stabilization. (Lesion: balance issues, difficulty stabilizing the head, especially during movement).

    • Lateral Vestibulospinal Tract: Important for postural control, balance, and maintaining extensor muscle tone against gravity (trunk and limb extensors). (Lesion: balance issues, impaired postural reflexes, difficulty maintaining an upright posture).

    • Tectospinal Tract: Mediates reflexive head and eye movements in response to visual and auditory stimuli, helping to orient the head towards a sensory input. (Lesion: impaired reflex orienting movements of the head and eyes in response to sudden stimuli).

Tract Lesions (General)

  • Dorsal Column Lesion: Ipsilateral loss of discriminative touch, vibratory sense, and proprioception below the level of the lesion (e.g., tabes dorsalis, B12 deficiency; affects limbs and trunk on the same side as the lesion).

  • Spinothalamic Tract Lesion: Contralateral loss of pain and temperature sensation below the level of the lesion (e.g., syringomyelia, spinal cord injury; affects limbs and trunk on the opposite side to the lesion).

  • Corticospinal Tract Lesion (Upper Motor Neuron): Spasticity, hyperreflexia, weakness, positive Babinski sign below the lesion (e.g., stroke, spinal cord compression). Symptoms are ipsilateral if lesion is in spinal cord, contralateral if in brain stem/cortex (affects muscles/limbs below the lesion level, either on the same side or opposite depending on the level of decussation).

  • **Brown-Séquard Syndrome (Hemi-section