Overview
Lecture covers the Peripheral Nervous System (PNS): sensory receptors, peripheral nerves (cranial and spinal), motor divisions, and reflexes. Focus on classifications, structures, key functions, and clinically relevant examples.
PNS Organization and Flow
- Sensory receptors gather internal and external information; generate graded potentials.
- If threshold is reached, action potentials travel via afferent (sensory) nerves to CNS.
- CNS integrates signals; motor commands exit via efferent (motor) nerves.
- Motor endings activate effectors; sets up next lecture on autonomic nervous system.
Sensory Receptors: Function and Signal Transduction
- Receptors are modified dendritic ends of sensory neurons; widely distributed.
- Respond to diverse stimuli: pressure, temperature, body position, pH, chemicals, light.
- Stimuli produce graded (local) depolarizations; threshold triggers action potentials.
- Some depolarize by mechanical/thermal/chemical gating, not only ligand-gated channels.
Sensory Receptors: Classification by Stimulus
- Mechanoreceptors: respond to touch, pressure, stretch, blood pressure.
- Thermoreceptors: detect temperature changes (hot/cold).
- Photoreceptors: detect light energy and wavelengths.
- Chemoreceptors: detect chemicals in solution; taste, smell, blood chemistry.
- Nociceptors: detect potentially damaging stimuli; pain signaling.
Sensory Receptors: Classification by Location
- Exteroceptors: at/near body surface; touch, pain, pressure, skin temperature, special senses.
- Interoceptors (visceral receptors): in viscera and blood vessels; stretch, pain, temperature, chemical changes.
- Proprioceptors: in skeletal muscles, tendons, ligaments, joints; report body position and movement.
Sensory Receptors: Classification by Structure
- Simple receptors: modified dendritic endings; general senses (touch, temp, stretch, chemical).
- Complex receptors (special sense organs): multiple cell types (hundreds–millions) devoted to special senses.
- Examples: retina photoreceptors (vision), olfactory epithelium (smell), taste buds (taste), inner ear hair cells (hearing, balance).
Peripheral Nerves: Types and Protection
- Functional types: purely sensory (afferent), purely motor (efferent), mixed (both).
- Somatic (voluntary) serve skeletal muscle; visceral (involuntary) serve internal organs.
- Protection strategies:
- Bundling hundreds–thousands of axons into deep-running nerve cables.
- Course alongside bones; names often reflect adjacent bones (e.g., femoral, radial, ulnar).
- Connective tissue wrappings: endoneurium (around axon/myelin), perineurium (around fascicle), epineurium (around nerve).
Cranial Nerves: Names, Locations, Key Functions
- 12 pairs originate from brain/brainstem; primarily serve head and neck (except vagus).
- Know name, Roman numeral, location, and primary function.
| Roman Numeral | Name | Primary Function | Location/Notes |
|---|
| I | Olfactory | Smell (chemoreceptors → olfactory cortex) | Nasal mucosa → cribriform plate → temporal lobe |
| II | Optic | Vision (to visual cortex) | Exits back of eye; occipital lobe target |
| III | Oculomotor | Eye movements; constrict/dilate pupil via iris | Controls 4/6 extrinsic eye muscles; iris smooth muscle |
| IV | Trochlear | Eye movement | Controls one extrinsic eye muscle |
| V | Trigeminal | Facial sensory (skin, nasal/oral mucosa); chewing muscles | Three branches; dental anesthesia target; viral ganglion site |
| VI | Abducens | Eye abduction (lateral rectus) | Abducts eye |
| VII | Facial | Taste (some); facial expression; tear/salivary glands | Viral susceptibility; Bell’s palsy mechanism |
| VIII | Vestibulocochlear | Hearing; balance/equilibrium | Cochlea (hearing), vestibule (balance) |
| IX | Glossopharyngeal | Taste; motor to pharynx; salivary glands | Tongue and pharynx functions |
| X | Vagus | Broad ANS control of visceral organs | Only nerve wandering beyond head/neck; thoracic/abdominal organs |
| XI | Accessory | Motor to sternocleidomastoid and trapezius | Neck and upper back movement |
| XII | Hypoglossal | Motor to tongue; chewing and speech | Below tongue motor control |
- Viral infections and examples:
- HSV in trigeminal (cold sores/fever blisters); can also involve facial and vestibulocochlear nerves.
- Genital herpes with spinal nerve involvement.
- Bell’s palsy: facial nerve inflammation blocks axons; unilateral facial droop; treated with antivirals/steroids; often reversible.
Spinal Nerves: Organization and Plexuses
- 31 pairs; exit via intervertebral foramina; serve body except head and some neck.
- Formation: dorsal root (sensory) + ventral root (motor) fuse to mixed spinal nerve.
- Short spinal nerve splits: dorsal ramus (posterior trunk), ventral ramus (forms plexuses).
- Plexuses (ventral rami recombine) provide redundancy; protect against single-segment paralysis.
| Structure | Components/Origin | Innervation Targets | Notes |
|---|
| Dorsal Ramus | From mixed spinal nerve | Muscles/skin of posterior trunk | Localized distribution |
| Intercostal Nerves | Thoracic ventral rami (no plexus) | Lateral torso; anterior chest and abdomen | Segmental; wrap around torso |
| Cervical Plexus | Upper cervical rami | Cutaneous neck/shoulder; phrenic nerve to diaphragm | Phrenic: respiration; irritation causes hiccups; sever → respiratory arrest |
| Brachial Plexus | Lower cervical/upper thoracic rami | Entire upper limb | Brachial block for surgery; severe damage → upper limb deficits |
| Lumbar Plexus | Lumbar rami | Anterior/medial thigh; knee; medial leg | Femoral nerve: gait; herniation can cause pain/numbness |
| Sacral Plexus | Sacral rami | Posterior lower limb; genital/urinary branches | Sciatic nerve: longest, posterior thigh/leg/foot; sciatica pain; injury → paralysis risk |
- Major terminal nerves:
- Phrenic (cervical): diaphragm motor/sensory.
- Axillary (brachial): armpit region.
- Radial, Ulnar, Median (brachial): forearm/hand; median implicated in carpal tunnel.
- Femoral (lumbar): anterior thigh; medial knee/leg; near tibia.
- Sciatic (sacral): posterior thigh, leg, ankle, foot; avoid in injections.
Motor Division and Neurotransmitters
- Efferent PNS activates effectors by neurotransmitter release.
- Somatic (voluntary): skeletal muscle; neurotransmitter is acetylcholine (ACh); rapid transmission/response.
- Autonomic (involuntary): cardiac muscle, smooth muscle, glands; neurotransmitters ACh or norepinephrine; sympathetic vs parasympathetic.
Autonomic Nervous System (Preview)
- Sympathetic: fight-or-flight; increases HR, respiration, BP; decreases daily living activities.
- Parasympathetic: rest-and-digest; decreases HR, respiration, BP; increases digestive, reproductive, urinary activities.
- Parasympathetic dominates at rest; sympathetic overrides during stress; slightly slower than somatic pathways.
Reflexes: Core Concepts and Circuitry
- Reflex: basic functional unit handling sensory input, integration, motor output.
- Mostly spinal-level, involuntary; serve homeostasis and protection.
- Types:
- Intrinsic (inborn): e.g., arm withdrawal from painful stimulus.
- Learned (acquired): protective postures from practice/repetition.
| Component | Role | Notes |
|---|
| Receptor | Detects stimulus | Initiates graded potentials |
| Sensory Neuron (Afferent) | Carries input to CNS | Often unipolar |
| Integration Center | Processes input; selects response | May include interneuron(s); knee-jerk lacks interneuron |
| Motor Neuron (Efferent) | Sends command to effector | Conducts action potential |
| Effector | Performs response | Skeletal muscle (somatic) or visceral effectors (autonomic) |
-
Somatic reflex examples:
- Stretch (knee-jerk): tap patellar ligament; quadriceps contract; prevents collapse when drowsy.
- Withdrawal (arm): pull away from hot/sharp object; rapid protection.
- Crossed-extensor: one limb flexes while contralateral extends; e.g., stepping on glass.
-
Signal routing nuance:
- Interneurons can branch outputs: immediate motor response and ascending pain perception.
- Perception lags withdrawal due to distance and processing; responses occur within milliseconds.
Key Terms & Definitions
- Afferent (sensory): toward CNS.
- Efferent (motor): away from CNS.
- Exteroceptors: surface stimuli sensors.
- Interoceptors (visceral): internal stimuli sensors.
- Proprioceptors: position/movement sensors in musculoskeletal tissues.
- Endoneurium/Perineurium/Epineurium: nerve connective tissue layers.
- Ganglion: cluster of neuron cell bodies outside CNS.
- Plexus: network of ventral rami recombining to form peripheral nerves.
- Dermatome: skin area innervated by specific spinal nerves (conceptual mapping).
- Bell’s palsy: transient facial paralysis from facial nerve viral inflammation.
- Sciatica: pain from sciatic nerve irritation.
Action Items / Next Steps
- Memorize cranial nerves I–XII: names, numerals, locations, primary functions.
- Learn major plexuses and terminal nerves: phrenic, radial, ulnar, median, femoral, sciatic.
- Practice tracing reflex arc components and examples (knee-jerk, withdrawal, crossed-extensor).
- Prepare for lab: identify plexuses and listed nerves on models; reinforce lecture content.
- Preview autonomic nervous system: sympathetic vs parasympathetic effects and neurotransmitters.