Overview of the Peripheral Nervous System and Sensory Pathways

Facial Nerve

Has five major branches (and two minor ones) that innervate muscles of facial expression and other structures.

Temporal Branch

Innervates forehead muscles (e.g., raises eyebrows).

Zygomatic Branch

Innervates orbital muscles (e.g., closes eyelids).

Buccal Branch

Innervates cheek muscles (e.g., smiling).

Mandibular Branch

Innervates lower lip and chin (e.g., pouting).

Cervical Branch

Innervates platysma (neck muscle).

PNS

Provides links from and to world outside our body.

Sensory Receptors

Specialized to respond to changes in environment (stimuli).

Graded Potentials

Activation of sensory receptors results in graded potentials that trigger nerve impulses.

Sensation

Awareness of stimulus occurs in the brain.

Perception

Interpretation of meaning of stimulus occurs in the brain.

Classification of Receptors

Three ways to classify receptors: by type of stimulus, body location, and structural complexity.

Mechanoreceptors

Respond to touch, pressure, vibration, and stretch.

Thermoreceptors

Sensitive to changes in temperature.

Photoreceptors

Respond to light energy (example: retina).

Chemoreceptors

Respond to chemicals (examples: smell, taste, changes in blood chemistry).

Nociceptors

Sensitive to pain-causing stimuli (examples: extreme heat or cold, excessive pressure, inflammatory chemicals).

Exteroceptors

Respond to stimuli arising outside body, including receptors in skin for touch, pressure, pain, and temperature.

Interoceptors

Respond to stimuli arising in internal viscera and blood vessels, sensitive to chemical changes, tissue stretch, and temperature changes.

Proprioceptors

Respond to stretch in skeletal muscles, tendons, joints, ligaments, and connective tissue coverings of bones and muscles.

Simple Receptors

Modified dendritic endings of sensory neurons found throughout the body that monitor various sensations.

Special Sense Receptors

Housed in complex organs (e.g., retina for vision, cochlea for hearing) and highly specialized for one type of stimulus.

Nonencapsulated Nerve Endings

Bare dendritic endings (no connective tissue wrapping) abundant in epithelia and connective tissues.; Mostly unmyelinated C fibers (slow, dull pain/temperature) or lightly myelinated Aδ fibers (sharp, fast pain).

Transmission Lines

Nerves are bundles of axons (myelinated/unmyelinated) in connective tissue that can regenerate if damaged.

Somatic Motor Neurons

Innervate skeletal muscles (voluntary).

Autonomic Motor Neurons

Innervate smooth/cardiac muscles and glands (involuntary).

Reflex Activity

Rapid, involuntary responses (e.g., pulling hand from heat).

Cold receptors

Active at 10-40°C (superficial dermis).

Heat receptors

Active at 32-48°C (deeper dermis).

Vanilloid receptor (TRPV1)

Ion channel activated by heat (>43°C), acid (low pH), or capsaicin (why chili peppers feel 'hot').

Itch receptors

Stimulated by chemicals like histamine (allergic reactions).

Tactile (Merkel) discs

Light touch receptors in epidermis (detect steady pressure/texture).

Hair follicle receptors

Detect hair movement (e.g., mosquito landing).

Encapsulated Nerve Endings

Structure: Dendrites wrapped in connective tissue (enhances sensitivity/specificity).

Meissner's corpuscles

Light touch/vibration in hairless skin (e.g., fingertips).

Pacinian corpuscles

Deep pressure/vibration (e.g., subcutaneous tissue).

Ruffini endings

Stretch/persistent pressure (e.g., joints, ligaments).

Somatosensory system

Part of sensory system serving body wall and limbs; Receives inputs from: Exteroceptors, proprioceptors, and interoceptors.

Receptor level

Sensory receptors.

Circuit level

Processing in ascending pathways.

Perceptual level

Processing in cortical sensory areas.

Adaptation

Change in sensitivity in presence of constant stimulus.

Phasic receptors

(fast-adapting) send signals at beginning or end of stimulus.

Tonic receptors

Adapt slowly or not at all.

Spinal Cord/Brainstem

Initiates reflex arcs (e.g., withdrawing a hand from heat before feeling pain).

Thalamus

Acts as the 'gateway' to the cortex, filtering irrelevant inputs (e.g., ignoring constant clothing pressure).

Spinal Cord

Enclosed in vertebral column, begins at the foramen magnum and ends at L1 or L2 vertebra; provides two-way communication to and from brain and body.

Major Reflex Center

Reflexes are initiated and completed at spinal cord.

Spinal Dura Mater

Is one layer thick; does not attach to vertebrae.

Epidural Space

Cushion of fat and network of veins in space between vertebrae and spinal dura mater.

CSF

Fills subarachnoid space between arachnoid and pia maters.

Meninges and CSF

Create a protected physical environment for sensory pathways to transmit signals uninterrupted.

Spinal Nerves

Part of PNS; attach to spinal cord by 31 paired roots.

Cervical and Lumbosacral Enlargements

Nerves serving upper and lower limbs emerge here.

Cauda Equina

Bundle of nerve roots below L1/L2; the cord itself ends here.

Ventral Median Fissure

Deep groove dividing the spinal cord.

Dorsal Median Sulcus

Shallow groove dividing the spinal cord posteriorly.

Gray Matter

H-shaped core of spinal cord containing neuron cell bodies, synapses, and interneurons.

Dorsal Horns

Receive sensory input (somatic + visceral).

Ventral Horns

Send motor output (somatic muscles).

Lateral Horns

House sympathetic neurons (visceral motor) and are present only in T1-L2.

Gray Commissure

Connects left/right sides; surrounds the central canal (filled with CSF).

White Matter

Outer region of spinal cord containing myelinated axons for rapid signal transmission.

Dorsal Funiculus

Ascending sensory tracts (e.g., touch).

Lateral Funiculus

Contains both ascending (e.g., pain) and descending (e.g., motor) tracts.

Ventral Funiculus

Mostly descending motor tracts.

Dorsal Roots

Carry sensory input into the cord.

Dorsal Root Ganglia

Swellings containing sensory neuron cell bodies.

Ventral Roots

Carry motor output away from the cord.

Decussation

Most pathways cross from one side of CNS to other at some point.

Relay

Consist of chain of two or three neurons.

Somatotopy

Precise spatial relationship in CNS correspond to spatial relationship in body.

Symmetry

Pathways are paired symmetrically (right and left).

Dorsal column-medial lemniscal pathways

Transmit input to somatosensory cortex for discriminative touch and vibrations.

Fasciculus cuneatus

Paired structure in spinal cord part of dorsal column-medial lemniscal pathways.

Fasciculus gracilis

Paired structure in spinal cord part of dorsal column-medial lemniscal pathways.

Medial lemniscus

Pathway in brain (medulla to thalamus) for dorsal column-medial lemniscal pathways.

Spinothalamic pathways

Transmit pain, temperature, coarse touch, and pressure impulses.

Lateral spinothalamic tract

Part of spinothalamic pathways that transmits pain and temperature.

Ventral spinothalamic tract

Part of spinothalamic pathways that transmits coarse touch and pressure impulses.

Spinocerebellar tracts

Convey information about muscle or tendon stretch to cerebellum.

First-order neuron

Conducts impulses from cutaneous receptors and proprioceptors.

Second-order neuron

Interneuron with cell body in dorsal horn of spinal cord or medullary nuclei.

Third-order neuron

Interneuron with cell bodies in thalamus, extending to somatosensory cortex.

Descending pathways

Deliver efferent impulses from brain to spinal cord.

Direct pathways

Pyramidal tracts in primary motor cortex for precise, voluntary movements.

Indirect pathways

All other pathways originating in brainstem nuclei for automatic movements.

Upper motor neurons

Pyramidal cells in primary motor cortex that initiate voluntary movement.

Lower motor neurons

Ventral horn motor neurons that innervate skeletal muscles.

Reticulospinal tracts

Maintain balance by varying tone of postural muscles.

Rubrospinal tracts

Control flexor muscles.

Tectospinal tracts

Originate from superior colliculi and mediate head movements in response to visual stimuli.

Perceptual detection

Ability to detect a stimulus (requires summation of impulses).

Magnitude estimation

Intensity coded in frequency of impulses.

Spatial discrimination

Identifying site or pattern of stimulus (studied by two-point discrimination test).

Feature abstraction

Identification of more complex aspects and several stimulus properties.

Quality discrimination

Ability to identify submodalities of a sensation (e.g., sweet or sour tastes).

Pattern recognition

Recognition of familiar or significant patterns in stimuli (e.g., melody in piece of music).