PNS lecture

Nervous System Overview

• The nervous system is primarily divided into two main components:

  • Central Nervous System (CNS)

  • Peripheral Nervous System (PNS)

Peripheral Nervous System (PNS)

• Focus of the lecture is on the PNS organization.

  • Includes sensory receptors and cranial/spinal nerves.

  • Discussed in detail, including disorders and development.

Organization of the PNS

• Functionally divided into:

  • Sensory Division: Receives information from sensory receptors.

  • Motor Division: Initiates actions directed by the CNS.

  • Subdivided into:

    • Somatic: Relating to external stimuli.

    • Visceral: Relating to internal organs.

• Additional classifications: General vs. Special Senses.

• Acts as a communication link between the body and the CNS, transmitting information from the environment to the CNS and executing actions directed by the CNS.

Sensory Receptors

• Sensory receptors can be classified by:

  1. Location:

     • Exteroceptors: Detect stimuli from outside the body (e.g., touch, pain).

     • Interoceptors: Detect stimuli from internal organs (e.g., chemical changes, stretch).

     • Proprioceptors: Monitor body position and movement (located in muscles, tendons, joints).

  2. Type of Stimulus Detected:

     • Mechanoreceptors: Sensitive to mechanical forces (touch, pressure, vibration).

     • Thermoreceptors: Sensitive to temperature changes.

     • Chemoreceptors: Sensitive to chemical changes in the body (e.g., blood chemical concentrations).

     • Photoreceptors: Sensitive to light (only in the eye).

     • Nociceptors: Sensitive to pain (detect painful stimuli).

• Receptors can also be categorized by their structure:

  • Free Nerve Endings: Unencapsulated nerve endings that respond to various stimuli.

  • Specialized Free Nerve Endings:

    • Merkel Discs: Light touch sensors under the epidermis.

    • Hair Follicle Receptors: Detect hair movement.

  • Encapsulated Nerve Endings: Surrounded by connective tissue.

  • Examples include tactile corpuscles (Meissner corpuscles) and lamellar corpuscles (Pacinian corpuscles), which detect pressure and vibration respectively.

Cranial Nerves

• There are 12 Cranial Nerves; the lecture emphasizes their functions and innervations.

  • Cranial Nerve I (Olfactory Nerve): Sensory for smell; does not pass through the thalamus.

  • Cranial Nerve II (Optic Nerve): Sensory for vision; passes through the optic canal.

  • Cranial Nerve III (Oculomotor Nerve): Somatic motor; controls most eye movements and pupil constriction.

  • Cranial Nerve IV (Trochlear Nerve): Somatic motor; innervates the superior oblique muscle for eye movement.

  • Cranial Nerve V (Trigeminal Nerve): Largest cranial nerve; responsible for facial sensation and muscles of mastication.

  • Divides into three branches: ophthalmic, maxillary, and mandibular.

  • Cranial Nerve VI (Abducens Nerve): Somatic motor; lateral eye movement.

  • Cranial Nerve VII (Facial Nerve): Motor function for facial expressions, taste sensations, and certain glands.

  • Cranial Nerve VIII (Vestibulocochlear Nerve): Special somatic sense; responsible for hearing and balance.

  • Cranial Nerve IX (Glossopharyngeal Nerve): Sensory for taste; motor function for swallowing.

  • Cranial Nerve X (Vagus Nerve): Longest cranial nerve; autonomic functions in thoracic and abdominal organs.

  • Cranial Nerve XI (Accessory Nerve): Motor function for neck muscles; enters through foramen magnum.

  • Cranial Nerve XII (Hypoglossal Nerve): Motor function for tongue movement.

Spinal Nerves

• 31 Pairs of Spinal Nerves: Named for where they exit the vertebral column, not where they originate from the spinal cord.

  • Comprised of dorsal (sensory) and ventral (motor) roots that merge to form mixed spinal nerves.

Dorsal and Ventral Rami

• After forming a spinal nerve, it splits into:

  • Dorsal Ramus: Innervates muscles and skin of the back.

  • Ventral Ramus: Innervates anterior thoracic and abdominal wall muscles and forms nerve plexuses (except in the thoracic region).

Nerve Plexuses

1. Cervical Plexus: Innervates neck; includes the phrenic nerve which innervates the diaphragm – crucial for breathing.

2. Brachial Plexus: Innervates the upper limb; includes major nerves:

   • Musculocutaneous Nerve: Innervates muscles flexing the forearm.

   • Median Nerve: Innervates forearm flexors.

   • Ulnar Nerve: Innervates flexors on the medial side.

   • Axillary Nerve: Innervates shoulder muscles and abducts the arm.

   • Radial Nerve: Innervates muscles extending the arm.

3. Lumbar Plexus: Innervates anterior and medial thigh; includes

   • Femoral Nerve: Innervates quadriceps for leg extension.

   • Obturator Nerve: Innervates adductor muscles.

4. Sacral Plexus: Innervates the lower limb; includes:

   • Sciatic Nerve: The largest nerve in the body, splits into tibial and common fibular nerves, responsible for innervating the posterior and lateral side of the leg, respectively.

Disorders and Development of the PNS

• Shingles: Viral infection linked to prior chickenpox; affects skin innervated by affected sensory nerves.

• Peripheral Neuropathy: Can involve sensory or motor nerves causing sensations such as numbness or burning pain.

• Development of the PNS: Begins in week 4 of embryonic development; spinal nerves connect to specific muscles and skin areas, retaining original nerve support throughout life.