Peripheral Nervous System and Reflexes: Study Guide Notes

Peripheral Nervous System (PNS)

Composition of the PNS

The Peripheral Nervous System (PNS) consists of all neural structures outside the brain and spinal cord. It includes sensory receptors, peripheral nerves, associated ganglia, and motor endings.

• Sensory (afferent) division: Transmits impulses from receptors to the CNS.

• Motor (efferent) division: Transmits impulses from the CNS to effector organs.

Classification of Sensory Receptors

Sensory receptors are specialized to respond to changes in the environment (stimuli). They can be classified by:

• Type of stimulus detected:

  • Mechanoreceptors (touch, pressure, vibration)

  • Thermoreceptors (temperature)

  • Photoreceptors (light)

  • Chemoreceptors (chemicals)

  • Nociceptors (pain)

• Location:

  • Exteroceptors (external environment)

  • Interoceptors (internal environment)

  • Proprioceptors (body position)

• Receptor structure:

  • Encapsulated (surrounded by connective tissue)

  • Non-encapsulated (free nerve endings)

Peripheral Nerves and Connective Tissue Layers

Structure of a Nerve

Peripheral nerves are composed of bundles of axons surrounded by connective tissue layers:

• Endoneurium: Surrounds individual axons.

• Perineurium: Surrounds bundles of axons (fascicles).

• Epineurium: Surrounds the entire nerve.

Additional info: A labeled diagram is useful for visualizing these layers.

Classification of Nerves

• Mixed nerves: Contain both sensory and motor fibers.

• Sensory nerves: Carry impulses toward the CNS.

• Motor nerves: Carry impulses away from the CNS.

Regeneration of Nerve Fibers

Regeneration in the PNS

Peripheral nerve fibers can regenerate if the cell body is intact and Schwann cells are present. The process involves:

• Macrophages remove debris.

• Schwann cells form a regeneration tube.

• Axon sprouts grow through the tube to reconnect.

Additional info: CNS axons have limited regeneration due to inhibitory factors and lack of supportive cells.

Processing at Different Levels

Levels of Neural Processing

• Receptor level: Sensory receptors detect stimuli.

• Circuit level: Processing in ascending pathways to the brain.

• Perceptual level: Interpretation in the cerebral cortex.

Pain Perception

Referred Pain

Referred pain occurs when pain from one body region is perceived as coming from another region. This is due to convergence of nerve fibers in the spinal cord.

• Example: Pain from a heart attack may be felt in the left arm.

Spinal Nerves and Plexuses

Spinal Nerve Structure

Spinal nerves are formed by the union of dorsal and ventral roots. Each nerve divides into:

• Dorsal ramus: Supplies muscles and skin of the back.

• Ventral ramus: Forms plexuses and supplies limbs and anterior trunk.

• Meningeal branch: Supplies meninges and blood vessels.

Nerve Plexuses

Plexuses are networks of nerves formed by ventral rami. Major plexuses include:

• Cervical plexus: Serves the neck and diaphragm.

• Brachial plexus: Serves the upper limb.

• Lumbar plexus: Serves the anterior thigh.

• Sacral plexus: Serves the posterior thigh, leg, and foot.

Major Nerves and Their Functions

Nerve	Origin Plexus	Main Function
Axillary	Brachial	Deltoid and teres minor muscles; shoulder sensation
Musculocutaneous	Brachial	Flexor muscles in anterior arm; lateral forearm sensation
Radial	Brachial	Extensor muscles in posterior arm and forearm; posterior limb sensation
Median	Brachial	Flexor muscles in forearm; lateral palm sensation
Ulnar	Brachial	Flexor muscles in forearm and hand; medial hand sensation
Femoral	Lumbar	Anterior thigh muscles; anterior thigh and medial leg sensation
Obturator	Lumbar	Medial thigh muscles; medial thigh sensation
Sciatic	Sacral	Posterior thigh, leg, and foot muscles; leg and foot sensation

Additional info: The sciatic nerve is the largest nerve in the body.

Reflexes

Types of Reflexes

Reflexes are rapid, involuntary responses to stimuli. They can be classified as:

• Somatic reflexes: Involve skeletal muscles.

• Autonomic reflexes: Involve smooth muscle, cardiac muscle, or glands.

Spinal Reflexes

• Stretch reflex: Maintains muscle tone and posture (e.g., knee-jerk reflex).

• Tendon reflex: Prevents muscle and tendon damage.

• Flexor (withdrawal) reflex: Protects from injury by withdrawing from painful stimuli.

• Plantar reflex: Tests integrity of spinal cord and brain (Babinski sign).

• Abdominal reflex: Tests integrity of spinal cord segments T8-T12.

Reflex Arc Components

• Receptor

• Sensory neuron

• Integration center

• Motor neuron

• Effector

Additional info: Reflex testing is used clinically to assess nervous system function.