Lect 13 PNS 2024 Bio101 Marieb 10th edition

Peripheral Nervous System (PNS)

Definition

The Peripheral Nervous System (PNS) includes all neural structures outside the brain and spinal cord, integrating sensory receptors, peripheral nerves, associated ganglia, and motor endings. This system is crucial for connecting the central nervous system (CNS) to the limbs and organs, facilitating communication across the body.

Purpose

The PNS plays a vital role in providing links to and from the external environment, ensuring sensory information is relayed to the CNS and motor commands are transmitted from the CNS to the peripheral organs, thus enabling appropriate responses.

Learning Objectives

  • Spinal Nerve Nomenclature and Anatomical Relationships: Understand the classification of spinal nerves based on their regions of origin and how they relate to body function.

  • Dermatomes and Their Spinal Nerve Basis: Describe the relationship between dermatomes and specific spinal nerves, exemplifying how sensory information is distributed across the skin.

  • Reflex Arc Components: Explain the critical components of reflex arcs, including:

    • Monosynaptic Stretch Reflex: Involves sensory neurons that directly activate motor neurons, leading to muscle contraction, while simultaneously inhibiting antagonist muscles to facilitate movement.

    • Cutaneous Flexion Reflex: Engages a di-synaptic mechanism that interacts bilaterally to elicit a protective withdrawal response in reaction to harmful stimuli.

  • Reflex Modification and Spinal Cord Circuit Functions: Explore how reflexes can adapt under different conditions and how spinal cord circuits contribute to rhythmic locomotion patterns.

Spinal Nerves

  • Quantity: There are 31 pairs of mixed nerves that arise from the spinal cord, each combining motor and sensory fibers.

  • Classification: Named based on their origin from the spinal column:

    • Cervical: 8 pairs (C1-C8) - Supply the neck, shoulders, arms, and hands.

    • Thoracic: 12 pairs (T1-T12) - Innervate the thoracic region and upper abdomen.

    • Lumbar: 5 pairs (L1-L5) - Serve the lower back and parts of the legs.

    • Sacral: 5 pairs (S1-S5) - Innervate the pelvic region and parts of the legs.

    • Coccygeal: 1 pair (C0) - Located at the tailbone.

Dermatomes

  • Definition: A dermatome is an area of skin that is innervated by sensory fibers from a single spinal nerve root, providing a precise mapping of spinal afferent functions.

  • Exclusion: The only spinal nerve not involved in dermatomes is C1, which typically does not have a corresponding skin area. Variations in dermatomes can help pinpoint neurological damage.

Reflexes

  • Definition: Reflexes are rapid, predictable motor responses to specific stimuli that are essential for survival, allowing for quick reactions without the need for conscious thought.

  • Types:

    • Inborn (Intrinsic) Reflexes: These reflexes are natural responses (like withdrawing a hand from heat) that occur without prior experience.

    • Learned (Acquired) Reflexes: These are responses developed through experience and practice, such as riding a bicycle.

  • Components and Functions: Reflexes involve peripheral nerves and can engage the spinal cord directly or involve higher brain centers for modulation.

Reflex Arc Components

  • Receptor: The specific site that senses the stimulus.

  • Sensory Neuron: Transmits afferent impulses towards the CNS for processing.

  • Integration Center: This can be a simple monosynaptic connection or a more complex polysynaptic center where different signals are processed in the CNS.

  • Motor Neuron: Conducts efferent impulses from the CNS to an effector organ (muscle or gland).

  • Effector: The muscle or gland that responds to the motor impulse, resulting in movement or secretion.

Muscle Spindles

  • Function: Muscle spindles are specialized sensory organs located within muscles that measure changes in muscle length and rate of length change, triggering reflex actions that help maintain posture and muscle tone.

  • Activation: When muscles are stretched, the muscle spindles increase their rate of action potentials, informing the CNS about the change in muscle length, crucial for the stretch reflex mechanism.

Stretch Reflex

  • Mechanism: Triggered by the stretching of muscles, activating muscle spindles, which facilitate a monosynaptic response leading to muscle contraction on the same side while inhibiting the antagonist muscle. The strength of this response can be modified, although the direction remains consistent.

  • Example: The patellar reflex exemplifies this wherein the quadriceps muscle contracts while the hamstrings relax, demonstrating a coordinated response.

Flexor and Crossed Extensor Reflexes

  • Flexor Reflex: An automatic withdrawal response from a painful stimulus, allowing quick, protective movements to be executed.

  • Crossed Extensor Reflex: This reflex involves both sides of the body, allowing coordinated actions like withdrawal of a limb while extending the opposite limb to maintain balance. Characteristics include being polysynaptic and having variable magnitude and direction of response.

Locomotion and Over-Riding Reflexes

  • Case Study: Experiments with cats on a treadmill demonstrated how spinal reflexes could generate locomotion independent of brain input, indicating a sophisticated level of spinal cord circuit function.

  • Key Insight: Reflexes such as flexion, stretch, and crossed extensor reflexes are integral to generating complex movement patterns, including walking and trotting.

Review and Questions

  • Reflect on Improved Understanding: Engage with exercises provided in Marieb's 10th edition, specifically targeting reflex activity to reinforce learning.

  • Short Answer Questions: Respond to recommended questions for deeper comprehension of the material covered.