Peripheral Nervous System: Sensory Receptors, Nerves, and Reflexes

Peripheral Nervous System (PNS)

Overview of the PNS

The Peripheral Nervous System (PNS) consists of cranial and spinal nerves that branch out from the brain and spinal cord to all body parts. It is divided into the somatic system (cranial and spinal nerves to skin and skeletal muscle) and the autonomic nervous system (cranial and spinal nerves to cardiac and smooth muscle, and glands). The PNS includes sensory receptors, ganglia, and efferent motor endings.

• Somatic system: Voluntary control of skeletal muscles.

• Autonomic system: Involuntary control of cardiac muscle, smooth muscle, and glands.

Sensory Receptors and Sensation

Definition and Function

Sensory receptors are specialized structures that detect changes in the environment (stimuli) and convert them into nerve impulses, which are interpreted by the brain as sensations. Sensation is the state of awareness or feeling that occurs when sensory impulses are interpreted by the brain. Perception is the conscious recognition of the stimulus.

Classification of Sensory Receptors

Sensory receptors can be classified by complexity, location, and the nature of the stimulus.

By Complexity

• General senses: Receptors widely distributed throughout the body (touch, pressure, pain, temperature, proprioception).

• Special senses: Receptors localized in specific organs (vision, hearing, equilibrium, taste, smell).

By Location

• Exteroceptors: Located near body surface; respond to external stimuli (e.g., touch, pressure, pain, temperature).

• Interoceptors (visceroceptors): Detect stimuli originating from within the body (e.g., chemical changes, tissue stretch).

• Proprioceptors: Detect changes in movement, equilibrium, or body position; found in muscles, tendons, joints.

By Nature of Stimulus

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

• Thermoreceptors: Respond to temperature changes (heat, cold).

• Photoreceptors: Respond to light (found in the retina).

• Chemoreceptors: Respond to chemical changes (smell, taste, blood chemistry).

• Nociceptors: Respond to pain (can be mechanical, chemical, or thermal).

General Sensory Receptors: Structure and Function

Structure	Location	Stimulus	Body Location
Free Nerve Endings	E, I, P	M, T	Most body tissues
Merkel Discs	E	M	Basal epidermal layer
Hair Follicle Receptors	E	M	Hair follicles
Meissner's Corpuscles	E, P	M	Dermal papillae of hairless skin
Pacini (Pacinian) Corpuscles	E, I, P	M	Skin, mesentery, tendons, joint capsule
Ruffini's Corpuscles	E, P	M	Dermis, subcutaneous tissue
Muscle Spindles	P	M	Skeletal muscles
Golgi Tendon Organs	P	M	Tendons
Joint Kinesthetic Receptors	P	M, N	Joint capsules

Key: E = Exteroceptor, I = Interoceptor, P = Proprioceptor, M = Mechanoreceptor, T = Thermoreceptor, N = Nociceptor

Receptor Adaptation and Processing

• Adaptation: Decrease in sensitivity to a constant stimulus over time.

• No adaptation: Pain and proprioceptors do not adapt.

• Perceptual detection: Awareness that a stimulus has occurred.

• Magnitude estimation: Ability to detect how much stimulus is present.

• Spatial discrimination: Ability to identify the site or pattern of stimulation.

• Feature abstraction: Ability to identify more complex aspects of a sensation.

• Quality discrimination: Ability to distinguish submodalities (e.g., sweet vs. bitter taste).

• Pattern recognition: Ability to recognize familiar or significant patterns in stimuli.

Nerves and Nerve Structure

Definition and Function

Nerves are bundles of axons (nerve fibers) that link the body to the CNS. They transmit sensory and motor signals. Each nerve contains connective tissue coverings:

• Endoneurium: Surrounds each axon.

• Perineurium: Surrounds bundles of axons (fascicles).

• Epineurium: Surrounds the entire nerve.

Cranial Nerves

There are twelve pairs of cranial nerves, each with specific sensory, motor, or mixed functions. They connect the brain to the head, neck, and trunk.

Nerve #	Type	Role
I Olfactory	Sensory	Smell
II Optic	Sensory	Vision
III Oculomotor	Motor	Eye movement, pupil constriction
IV Trochlear	Motor	Eye movement (superior oblique muscle)
V Trigeminal	Mixed	Sensory from face, motor to muscles of mastication
VI Abducens	Mixed (primarily motor)	Motor to lateral rectus muscle of the eye
VII Facial	Mixed	Motor to facial muscles, sensory for taste (anterior 2/3 of tongue)
VIII Vestibulocochlear	Sensory	Hearing and equilibrium
IX Glossopharyngeal	Mixed	Motor to pharynx, sensory for taste (posterior 1/3 of tongue)
X Vagus	Mixed	Motor and sensory to thoracic and abdominal organs
XI Accessory	Mixed (primarily motor)	Motor to sternocleidomastoid and trapezius muscles
XII Hypoglossal	Mixed (primarily motor)	Motor to tongue muscles

Mnemonic: "Some say marry money, but my brother says (it's) bad business (to) marry money" helps remember sensory, motor, or both for each cranial nerve.

Spinal Nerves and Plexuses

There are 31 pairs of spinal nerves, each emerging from the spinal cord by a dorsal and ventral root. Spinal nerves are grouped by the region they arise from and are mixed nerves (sensory and motor fibers).

• Cervical plexus: Supplies muscles and skin of neck (notably the phrenic nerve to the diaphragm).

• Brachial plexus: Supplies upper limb (major nerves: musculocutaneous, ulnar, median, radial, axillary).

• Lumbar plexus: Supplies lower abdomen, anterior and medial thigh (major nerves: femoral, obturator).

• Sacral plexus: Supplies buttocks, lower limb, pelvic structures (major nerve: sciatic).

Plexus	Comments and/or Nerves To Know
Cervical	Phrenic nerve (diaphragm)
Brachial	Musculocutaneous, ulnar, median, radial, axillary nerves
Lumbar	Femoral, obturator nerves
Sacral	Sciatic, tibial, peroneal, gluteal nerves

Dermatomes and Innervation

• Dermatome: Area of skin supplied by cutaneous branches of a single spinal nerve.

• Hilton's Law: Joints are innervated by the same nerves that supply the muscles moving the joint.

Reflexes

Definition and Pathway

Reflexes are automatic, unconscious responses to changes inside or outside the body. The simplest pathway is a reflex arc, which includes:

1. Receptor

2. Sensory neuron

3. Integration center

4. Motor neuron

5. Effector

Functional Classes of Reflexes

• Visceral reflexes: Involve smooth or cardiac muscle or glands (e.g., regulation of heart rate, digestion).

• Somatic reflexes: Involve contraction of skeletal muscle (e.g., withdrawal reflex).

Clinical Relevance

• Reflexes are used to assess the condition of the nervous system.

• Damage to the brain or spinal cord can affect reflex activity.

Example: Patellar Reflex

• Striking the patellar tendon causes contraction of the quadriceps muscle, resulting in knee extension.

Additional info: The notes also mention referred pain, which is pain perceived at a location other than the site of the painful stimulus, due to convergence of sensory pathways in the CNS.