The Peripheral Nervous System Flashcards
Autonomic Nervous System
-Involuntary
-Regulates cardiac muscle, smooth muscle, glandular tissue
-Autonomic motor neurons regulate visceral activities by exciting or inhibiting activities in effector tissues
Somatic Nervous System
-Voluntary
-Sensory neurons (afferent) convey input from receptors for somatic senses and special senses
-Motor neurons (efferent) innervate skeletal muscles
Afferent Division of PNS
Somatic sensory: carries signals from receptors in skin, muscles, bones and joint (ex. step on something)
Visceral sensory: carries signals mainly from organs in the thoracic and abdominal cavities (ex. eat and feel bloated/stomach hurts)
Sensory Receptors of PNS- 3 Types
- Free Nerve Endings
- Encapsulated
- Proprioceptors
Sensory Receptors: Free Nerve Endings (3)
- Free nerve endings of sensory neurons
- Modified free nerve endings (Merkel discs)
- Hair follicle receptors
- Free Nerve Endings of Sensory Neurons
Functional class: nociceptors (pain), thermoreceptors (heat and cold), mechanoreceptors (pressure), chemoreceptors
Body location: most body tissues; most dense in connective tissues
- Modified Free Nerve Endings (Merkel discs)
Functional class: mechanoreceptors (light pressure)
Body location: deepest layer of epidermis
- Hair Follicle Receptors
Functional class: mechanoreceptors (hair deflection)
Body location: in and surrounding hair follicles
*free nerve endings
Sensory Receptors: Encapsulated (3)
- Tactile (Meissner’s) corpuscles
- Lamellar (Pacinian) corpuscles
- Ruffini corpuscles
- Tactile (Meissner’s) corpuscles
Functional class: mechanoreceptors (light pressure, discriminative touch, vibration of low frequency)
Body location: hairless skin, particularly nipples, external genitalia, fingertips, eyelids
*encapsulated
- Lamellar (Pacinian) corpuscles
Functional class: mechanoreceptors (deep pressure, stretch, vibration of high frequency)
Body location: dermis and hypodermis; most abundant on fingers, soles of feet, external genitalia, nipples
*encapsulated
- Ruffini corpuscles
Functional class: mechanoreceptors (deep pressure and stretch)
Body location: deep in dermis, hypodermis and joint capsules
*encapsulated
Sensory Receptors: Proprioceptors (2)
- Muscle Spindles
- Tendon Organs
- Muscle spindles
Functional class: mechanoreceptors (muscle stretch)
Body location: skeletal muscles, particularly those of the extremities
*proprioceptors
- Tendon Organs
Functional class: mechanoreceptors (tendon stretch)
Body location: tendons
*proprioceptors
Purpose of Spinal Nerves (4)
1) Communication system between brain and various body parts
2) Essential for voluntary and involuntary actions
3) Highway for sensory and motor integrations
4) Important for rapid automatic responses through reflexes
Spinal Column Vertebrae
7 cervical
12 thoracic
5 lumbar
5 sacral fused
3-5 coccygeal fused
Pairs of Spinal Nerves
31 pairs of spinal nerves
8 pairs cervical
12 pairs thoracic
5 pairs lumbar
5 pairs sacral
1 pair coccygeal
*pairs 1-7 leave the spinal cord above their corresponding vertebrae level
Layers of Spinal Nerves (3)
Epineurium
Perineurium
Endoneurium
*superficial to deep layers
Provide structural support, protection and organization to nerves
Dorsal and Ventral Roots
Each spinal nerve connects to the spinal cord through the Dorsal Root and Ventral Root
Dorsal root: contains axonal processes of SENSORY neurons arising from POSTERIOR horn
Ventral root: contains axonal processes of MOTOR neurons whose cell bodies are located in ANTERIOR horn
Dorsal Root and Dorsal Root Ganglion
Dorsal root contains the dorsal root ganglion:
-Enlargement of the dorsal root
-Cluster of cell bodies of sensory neurons
-Functions as sensory relay stations by processing and modulating sensory signals before reaching CNS
*spinal nerve and dorsal root ganglion lie within intervertebral foramina
Dorsal and Ventral Rami
Dorsal and ventral roots branch into dorsal and ventral rami
*both contain sensory and motor fibers, and exit vertebral column through intervertebral foramina and supply the entire somatic region of the body
Dorsal rami: supplies dorsum of neck and back
Ventral rami: supplies anterior and lateral neck and trunk, all regions of limbs (and is much thicker than dorsal rami)
Spinal Nerve Pathway Overview
Posterior horn -> dorsal rootlets -> dorsal roots -> dorsal root ganglion -> dorsal rami
Anterior horn -> ventral rootlets -> ventral roots -> ventral rami
Ventral Rami and Plexuses
Ventral rami do not travel directly to their target tissues but instead branch out extensively to form complex networks known as nerve plexuses
There are 4 primary nerve plexuses from ventral rami, of which individual nerves travel to specific target tissues (muscles, skin, organs) to facilitate motor control and sensory perception
4 Nerve Plexuses
- Cervical plexus
- Brachial plexus
- Lumbar plexus
- Sacral plexus
Cervical Plexus
-Spinal nerves C1-C4
-Supply sensory and motor innervation to the neck and portions of the head
Brachial Plexus
-Spinal nerves C5-T1
-Supply sensory and motor innervation to shoulder and upper limbs
Lumbar Plexus
-Spinal nerves L1-L4
-Supplies nerves to lower back, abdominal wall and part of lower limb
Sacral Plexus
-Spinal nerves L4-S5
-Innervates buttocks, pelvic structures and remainder of lower limb
Summary: Path From Spinal Cord
Nerves originate from spinal cord through dorsal and ventral roots (dorsal= carries sensory info into spinal cord, ventral= carries motor info away from spinal cord)
Dorsal and ventral roots unite outside spinal cord to form a spinal nerve (each nerve has both sensory and motor fibers)
Spinal nerve branches into dorsal and ventral rami (dorsal= branches that supply dorsal parts of the body, including skin and muscles, ventral= larger branches that supply ventral parts of the body and form complex nerve plexuses)
Dermatomes
-Sections of the skin innervated by a single spinal nerve
-Motor and sensory innervation for those specific areas
-Important dermatomes:
T4: Nipples
T10: Umbillicus
Clinical Case: Shingles
Cause: viral infection caused by reactivation of varicella-zoster virus (VZV)
-VZV dormant in nerve cells after person recovers from chickenpox
-Rash of scaly, painful blisters usually to one or several adjacent dermatomes
-Aging and weakened immune system triggers reactivation
-After chickenpox, VZV transported from skin lesions through peripheral processes of sensory neurons to cell bodies in a sensory ganglion, and it multiplies and travels back through sensory axons to the skin when immune system is weakened
Motor Reflex Arc
-Neural pathway that mediates rapid involuntary actions
-Integrates sensory inputs and motor outputs
-Physiological and neural response against potential harmful stimuli
*Spinal reflex= monosynaptic (only one synapse)
Anatomy of Motor Reflex Arc (9)
- Sensory receptor
- Sensory neuron (afferent)
- Dorsal root ganglion
- Dorsal root
- Posterior horn
- Interneurons
- Anterior horn
- Motor neuron (efferent)
- Effector muscle
