chap 13

Overview of the Peripheral and Central Nervous Systems

  • Central Nervous System (CNS)

    • Comprises the brain and spinal cord.

    • Integrates sensory information and initiates responses.

  • Peripheral Nervous System (PNS)

    • Consists of cranial and spinal nerves that link the CNS to the rest of the body.

    • Facilitates motor and sensory functions.

Functional Divisions of the PNS

  • Sensory (Afferent) Division

    • Somatic Sensory Division

    • Carries general sensory stimuli from:

      • Muscles

      • Bones

      • Joints

      • Skin

    • Includes special sensory stimuli such as vision and taste.

  • Motor (Efferent) Division

    • Visceral Motor Division

    • Carries stimuli from the internal organs to the central nervous system.

    • Somatic Motor Division

    • Carries stimuli to skeletal muscles.

    • Autonomic Nervous System (ANS)

    • Carries stimuli to smooth muscle, cardiac muscle, and glands.

Anatomy and Classification of Nerves

  • Anatomy of a Nerve

    • Composed of bundles of axons from many neurons bound together by connective tissue.

    • Hierarchical organization similar to skeletal muscle:

    • Axon - wrapped in endoneurium.

    • Fascicles - wrapped in perineurium.

    • Nerve - wrapped in epineurium.

  • Classification of Nerves Based on Function

    • Sensory Nerves

    • Contain only axons of sensory neurons.

    • Function: Afferent pathways (carry signals to CNS).

    • Motor Nerves

    • Contain primarily axons of motor neurons with a few sensory neuron axons related to muscle stretch and tension.

    • Function: Efferent pathways (carry signals from CNS).

    • Mixed Nerves

    • Contain axons of both sensory and motor neurons.

Cranial and Spinal Nerves

  • Cranial Nerves

    • Total of 12 pairs connecting to the brain, mainly innervating structures in the head and neck.

    • Can be classified as:

    • Sensory Nerves:

      • Olfactory (I)

      • Optic (II)

      • Vestibulochochlear (VIII)

    • Primarily Motor Cranial Nerves (although they contain some sensory):

      • Oculomotor (III)

      • Trochlear (IV)

      • Abducens (VI)

      • Accessory (XI)

      • Hypoglossal (XII)

    • Mixed Nerves:

      • Trigeminal (V)

      • Facial (VII)

      • Glossopharyngeal (IX)

      • Vagus (X)

  • Spinal Nerves

    • Total of 31 pairs originating from the spinal cord, supplying all parts of the body except the head.

    • Named according to their point of issue:

    • 8 Cervical (C1-C8)

    • 12 Thoracic (T1-T12)

    • 5 Lumbar (L1-L5)

    • 5 Sacral (S1-S5)

    • 1 Coccygeal (C0)

    • All spinal nerves are mixed nerves due to the fusion of anterior and posterior roots.

Roots and Rami of Spinal Nerves

  • Roots

    • Anterior (ventral) roots contain axons of motor neurons, exiting the spinal cord.

    • Posterior (dorsal) roots contain axons of sensory neurons entering the spinal cord.

  • Rami

    • Spinal nerves are short and branch into posterior and anterior rami after exiting the vertebral column.

    • Posterior (Dorsal) Ramus:

    • Innervates deep muscles and skin of the back, carrying both somatic motor and sensory signals.

    • Anterior (Ventral) Ramus:

    • Innervates skin and skeletal muscles of the anterior trunk, lateral trunk, and extremities.

    • Contains both sensory and motor signals.

    • Rami Communicantes:

    • Branches off the anterior ramus, contains visceral motor axons from the sympathetic nervous system.

  • Intercostal Nerves

    • Formed from the anterior rami of thoracic spinal nerves T1 to T11, serving the intercostal muscles and overlying skin.

Plexuses of the PNS

  • Nerve Plexuses

    • All anterior rami except T2-T12 form interlacing nerve networks called plexuses.

    • Found in the cervical, brachial, lumbar, and sacral regions.

    • Each branch of a plexus contains fibers from several spinal nerves; beneficial as injury to one spinal nerve does not completely disrupt motor or sensory function in that region.

Cervical Plexus

  • Composed of nerve roots C1 through C5.

  • Innervates:

    • Skin of the scalp and posterior ear.

    • Diaphragm (via the phrenic nerve, C3-C5).

    • Geniohyoid and infrahyoid muscles.

Brachial Plexus

  • Comprises nerve roots C5-T1.

  • Innervates upper limbs.

  • Carpal tunnel syndrome arises from median nerve pressure due to inflammation/anatomical changes.

Lumbar Plexus

  • Composes of nerve roots L1-L4.

  • Innervates:

    • Muscles of the anterior and medial thigh.

    • Femoral Nerve:

    • Responsible for sensations of the anterior thigh and motor innervation of the quadriceps.

Sacral Plexus

  • Contains nerve roots L4-S4.

  • Sciatic Nerve:

    • The largest and longest nerve in the body; branches into tibial and common fibular nerves.

Summary of Nerve Function and Innervation

  • Spinal Nerve Branches from the Plexuses

    • Each plexus has distinct nerves that provide sensory and motor functions.

    • Cervical Plexus:

    • Includes lesser occipital, greater auricular, ansa cervicalis, transverse cervical, supraclavicular, and phrenic nerves.

    • Brachial Plexus:

    • Includes radial, median, ulnar, musculocutaneous, and axillary nerves.

    • Lumbar Plexus:

    • Includes femoral, obturator, lateral femoral cutaneous nerves.

    • Sacral Plexus:

    • Includes sciatic, tibial, common fibular nerves, as well as inferior and superior gluteal nerves.

Ethical, Philosophical, and Practical Implications

  • Understanding the structure and function of the PNS is crucial for diagnosing and treating neurological conditions.

  • Awareness of nerve injuries and their complex nature emphasizes the importance of appropriate medical intervention.

1. Basic Pathway of Sensation and Sensory Transduction

  • Basic Pathway of Information Flow during Sensation

    • Sensory receptor detects a stimulus and converts it into an electrical signal (receptor potential).

    • Generates action potentials in a sensory neuron (first-order neuron).

    • Signal travels along the afferent pathway to the central nervous system (CNS).

    • In the spinal cord or brainstem, it synapses with a second-order neuron, which typically decussates (crosses to the opposite side) and ascends to the thalamus.

    • From the thalamus, a third-order neuron projects to the primary somatosensory cortex for conscious perception.

  • Sensory Transduction

    • The process by which a sensory receptor converts a physical or chemical stimulus into an electrical signal (graded potential like receptor potential or generator potential), which can be interpreted by the nervous system.

2. Receptor Potential and Adaptation

  • Receptor Potential

    • A graded, local potential that develops in a sensory receptor in response to a stimulus.

    • If it reaches threshold, it can trigger action potentials in the afferent neuron.

  • Adaptation

    • The decrease in the frequency of action potentials generated by a sensory receptor even when the stimulus remains constant, leading to a diminished perception of the stimulus.

  • Rapidly vs. Slowly Adapting Receptors

    • Rapidly Adapting (Phasic) Receptors: Respond strongly at the onset and offset of a stimulus but stop responding quickly if the stimulus is maintained (e.g., touch receptors).

    • Slowly Adapting (Tonic) Receptors: Continue to produce action potentials throughout the duration of the stimulus, providing sustained information (e.g., pain receptors, Merkel's discs).

3. Types of Receptors

  • Interoreceptors: Detect internal stimuli from within the body (e.g., blood pressure, pH).

  • Exteroreceptors: Detect external stimuli from the environment (e.g., touch, temperature, sight).

  • Mechanoreceptors: Respond to mechanical forces such as pressure, stretch, vibration, and touch.

  • Chemoreceptors: Respond to chemical stimuli (e.g., changes in pH, O<em>2O<em>2 and CO</em>2CO</em>2 levels, taste, smell).

  • Thermoreceptors: Respond to changes in temperature (hot or cold).

  • Photoreceptors: Respond to light stimuli (e.g., rods and cones in the retina).

  • Nociceptors: Respond to noxious (harmful or damaging) stimuli perceived as pain.

4. Receptive Fields

  • Receptive Fields: The specific physical area or region from which a sensory neuron can be activated by a stimulus.

  • Neurons innervating more sensitive areas (e.g., fingertips) have smaller receptive fields, allowing for greater spatial discrimination.

  • Neurons innervating less sensitive areas (e.g., back) generally have larger receptive fields, resulting in less precise localization.

5. Dermatomes

  • Dermatomes: Specific areas of skin predominantly innervated by sensory nerve fibers from a single spinal nerve root.

  • Clinical Usefulness: Mapping sensory deficits to specific dermatomes helps pinpoint the level of spinal cord injury or nerve root compression.

6. Referred Pain

  • Underlying Cause: Occurs because visceral sensory neurons (from internal organs) and somatic sensory neurons (from skin/muscles) often converge on the same spinal cord neurons. The brain misinterprets visceral pain as originating from the more common somatic location.

7. Reflexes

  • Reflex: A rapid, involuntary, predictable motor response to a stimulus that does not require conscious thought.

  • Elements of a Reflex Pathway (Reflex Arc):

    1. Sensory receptor: Detects the stimulus.

    2. Sensory neuron (afferent neuron): Transmits the signal from the receptor to the CNS.

    3. Integration center: One or more synapses within the CNS where sensory and motor neurons communicate (often via interneurons).

    4. Motor neuron (efferent neuron): Carries the motor command from the CNS to the effector.

    5. Effector: The muscle or gland that carries out the response.

8. Monosynaptic vs. Polysynaptic Reflex Pathways

  • Monosynaptic Reflex Pathways: Involve only one synapse in the integration center between the sensory neuron and the motor neuron, with no interneuron (e.g., stretch reflex). They are the fastest.

  • Polysynaptic Reflex Pathways: Involve one or more interneurons between the sensory neuron and the motor neuron, resulting in two or more synapses in the integration center (e.g., withdrawal reflex). They are typically more complex and slower.

9. Muscle Spindles and Stretch Reflex

  • Muscle Spindles: Encapsulated sensory receptors within skeletal muscles that detect changes in muscle length and rate of change of length.

  • Involvement in Stretch Reflex: When a muscle is stretched, muscle spindles are activated. Their sensory neurons directly synapse with alpha motor neurons in the spinal cord (monosynaptic reflex) causing the same muscle to contract and resist the stretch.

10. Golgi Tendon Organs

  • Golgi Tendon Organs (GTOs): Encapsulated sensory receptors located in tendons, monitoring muscle tension and force of contraction.

  • Activation by forceful contraction inhibits alpha motor neurons of the contracting muscle and excites antagonistic muscles, protecting the muscle and tendon from excessive force.

11. Flexion Reflex and Crossed Extension Reflex Pathways

  • Flexion (Withdrawal) Reflex: A polysynaptic reflex protecting the body from harmful stimuli.

    • Noxious stimulus activates nociceptors; sensory neurons transmit signals to the spinal cord.

    • Interneurons excite alpha motor neurons causing ipsilateral (same side) flexor muscles to contract, withdrawing the limb.

    • Simultaneously, other interneurons inhibit ipsilateral extensor muscles.

  • Crossed Extension Reflex: Often accompanies the flexion reflex in weight-bearing limbs.

    • As the injured limb withdraws, interneurons in the spinal cord send branches to the contralateral side.

    • These excite alpha motor neurons of contralateral extensor muscles, causing that limb to extend and support body weight.

    • Simultaneously, they inhibit contralateral flexor muscles.

    • Both are polysynaptic reflexes.