Lecture notes on reflex, tracts of spinal cord, receptors, thalamus-sensory motor cortex

PIO 203 Overview

  • Focus: Simple reflex and spinal reflexes

  • Key pathways: Spinal cord ascending, descending pathways

  • Key components: Receptors, thalamus-sensory motor cortex

  • Instructor: Dr. Archibong EfioK

Introduction to Reflexes

  • Definition: A rapid, involuntary, preprogrammed response activating muscles or glands to avoid tissue damage.

  • Protective Mechanism:

    • Immediate response to stimuli (e.g., withdrawing from a hot object).

    • Responses to bright light protect the retina by constricting pupils.

Reflex Arc

  • Anatomical Pathway for reflex action comprising five components:

    1. Receptor: Sense organ responding to stimulus; generates impulses in the afferent nerve.

    2. Afferent Nerve: Sensory nerve transmitting impulses to the integrating center through the dorsal root.

    3. Central Integrating Station: Located in the brain/spinal cord; generates motor impulses from sensory inputs.

    4. Efferent Nerve: Motor nerve transmitting commands from the center via the ventral root to the effector organ.

    5. Effector Organ: Muscle or gland where responses occur.

    • Interneurons may connect afferent and efferent fibers, enhancing complexity.

Classification of Reflexes

  • Based on various criteria:

    1. Development:

      • Inborn Reflexes: Natural reflexes present at birth (e.g., saliva secretion upon tasting honey).

      • Acquired Reflexes: Learned responses (e.g., salivation at sight/smell of food).

    2. Situation (Anatomical): Spinal, cerebellar, and cortical reflexes.

    3. Purpose (Physiological): Protective (withdrawal reflexes) and antigravity (extensor reflexes).

    4. Number of Synapses:

      • Monosynaptic Reflexes: One synapse in the reflex arc (e.g., stretch reflex).

      • Polysynaptic Reflexes: More than one synapse (e.g., withdrawal reflex).

    5. Clinical Basis:

      • Superficial (e.g., sneezing), deep (e.g., knee jerk), visceral (e.g., pupillary light), and pathological reflexes (e.g., Babinski sign).

Spinal Reflexes

  • Central integrating center located in the spinal cord.

  • Examples: Stretch reflex, Golgi tendon reflex, crossed extensor reflex, and withdrawal reflex.

Stretch Reflex

  • Type: Monosynaptic reflex (e.g., knee jerk).

  • Function: Maintains muscle length and prevents strain due to muscle stretch.

  • Receptor: Muscle spindle activated by stretch, sending impulses to the spinal cord resulting in muscle contraction.

  • This reflex protects the muscle from being overstretched.

Golgi Tendon Reflex

  • Type: Bisynaptic reflex also known as the tendon reflex.

  • Function: Regulates muscle tension to prevent tendon damage by inducing relaxation.

  • Receptor: Golgi tendon organ; afferent nerve is type Ib sensory nerve.

  • Mechanism: Tension applied activates the Golgi tendon organ, sending impulses that inhibit alpha motor neurons, causing muscle relaxation.

Withdrawal Reflex

  • Type: Polysynaptic reflex or flexor withdrawal reflex.

  • Purpose: Protects against damaging stimuli by activating flexor muscles while inhibiting extensors.

  • Example: Withdrawal from a hot object through a complex pathway of sensory, interneuron, and motor neuron activation.

Crossed Extensor Reflex

  • Function: Compensates for weight shifts during withdrawal from a painful stimulus.

  • Mechanism: Afferent fibers synapse across to the opposite side of the spinal cord, coordinating limb movements to maintain balance.

Spinal Cord Pathways

Anatomy

  • Structure: Spinal cord extends from medulla to the first lumbar vertebra with 31 nerve segments.

  • Types of Nerves:

    • Cervical (8), thoracic (12), lumbar (5), sacral (5), coccygeal (1).

Ascending Pathways

  • Carry sensory impulses from spinal cord to brain via three neurons:

    1. 1st Order Neuron: Receives impulses from receptors; transmits to dorsal gray horn.

    2. 2nd Order Neuron: Located in the dorsal gray horn; ascends to subcortical areas (thalamus).

    3. 3rd Order Neuron: Located in the thalamus; transmits to the cerebral cortex.

Specific Sensory Pathways

  1. Dorsal Column-Medial Lemniscal Pathway: Carries touch, pressure, vibration, and proprioceptive sensations to the somatosensory cortex through specific fasciculus pathways.

  2. Spinothalamic Pathway: Transmits pain, temperature, and crude sensations, synapsing and ascending through the anterior and lateral columns.

  3. Spinocerebellar Pathway: Provides unconscious proprioceptive information to the cerebellum, crucial for balance and coordination.

Descending Pathways

Overview

  • Formed by motor nerve fibers descending from the brain, controlling voluntary and reflexive movements.

Pyramidal Tracts

  • Function: Responsible for voluntary motor activities with two main tracts (anterior and lateral corticospinal tracts).

  • These tracts decussate in the medulla before descending into the spinal cord.

Extrapyramidal Tracts

  • Involved in gross motor control and postural reflexes (e.g., reticulospinal, vestibulospinal, rubrospinal tracts).

  • Integrate motor responses from various stimuli and maintain reflexive actions.

Receptors

Overview

  • Sensory nerve endings that respond to stimuli; exist in various forms.

Types of Receptors

  1. Mechanoreceptors: Detect mechanical deformation (touch, pressure).

  2. Thermoreceptors: Sense temperature changes.

  3. Nociceptors: Respond to damage or pain.

  4. Photoreceptors: Detect light (vision).

  5. Chemoreceptors: Respond to chemical changes (taste, smell).

Properties of Receptors

  • Transduction: The conversion of stimuli into action potentials.

  • Adaptation: Decrease in action potential frequency with constant stimulus strength.

  • Fatigue: Occurs when receptors can no longer respond, despite stimulus presence.

  • Law of Specific Energies: Each sensory fiber produces one sensation type regardless of the stimulus type.

Thalamus and Sensory-Motor Cortex

Thalamus Role

  • Acts as a relay for sensory information (except olfactory), connecting subcortical structures with the cerebral cortex.

Functions of Thalamus

  1. Processing Sensory Information: Integrates all incoming sensory impulses before directing them to the cortex.

  2. Arousal and Alertness: Regulates sleep-wake states; damage can result in coma.

  3. Motor Activity Integration: Coordinates with cerebellum for motor function.

  4. Quality of Sensation: Determines the nature and affective quality of sensory experiences.