The Peripheral Nervous System

Peripheral nervous system (PNS)

links CNS to body and to external environment

PNS

 detects sensory stimuli and delivers information to CNS as sensory input

CNS 

processes input and transmits impulse through PNS to muscle cells and glands as motor output

Sensory (Afferent) division 

consists of sensory (afferent) neurons that detect and transmit sensory stimuli to CNS; has 2 anatomical subdivisionss Somatic sensory division, Visceral sensory division

Somatic sensory division

detects both internal and external stimuli; general sense receptors detect stimuli from skin; special sensory receptors detect stimuli from special sense organs

Visceral sensory division

 relays internal information (like blood pressure) from organs of abdominopelvic and thoracic cavities

Motor (Efferent) division

 consists of motor (efferent) neurons; carry out motor functions of nervous system; subdivisions based on organs that neurons contact:Somatic motor division, Visceral motor division (Autonomic Motor Nervous System, ANS)

Somatic motor division

responsible for voluntary motor functions; composed of lower motor neurons (somatic motor neurons) which directly trigger skeletal muscle contractions

Visceral motor division (Autonomic Motor Nervous System, ANS)

 responsible for maintaining many aspects of homeostasis by controlling involuntary motor functions in body; neurons innervate cardiac muscle cells, smooth muscle cells, and secretory cells of glands

sympathetic and parasympathetic

ANS is further divided into … nervous systems

Sympathetic nervous system (fight or flight division)

 involved in homeostasis activities surrounding physical work and visceral responses of emotions

Parasympathetic system (rest and digest division) 

 involved in digestion and maintaining body’s homeostasis at rest

Peripheral nerves

 main organs of PNS; consist of axons of many neurons bound together by connective tissue

Mixed nerves 

contain both sensory and motor neurons

Sensory nerves 

 contain only sensory neurons while motor nerves contain mostly motor neurons (also some sensory neurons involved in muscle stretch and tension)

Spinal nerves

 originate from spinal cord and innervate structures below head and neck; anatomical structures associated with this group of nerves include:

Two collections of axons connect PNS with spinal cord’s gray matter; anterior root (ventral root) consists of motor neurons from anterior horn and posterior root (dorsal root) consists of sensory neurons from posterior horn

posterior root ganglion (or dorsal root ganglion)

Posterior root features a swollen area that houses cell bodies of sensory neurons called …

Epineurium

outermost layer of connective tissue that holds motor and sensory axons together

Fascicles

small groups of bundled axons surrounded by connective tissue called perineurium

Each individual axon within a fascicle is surrounded by its own connective tissue called endoneurium

Cranial nerves 

 attach to brain and mostly innervate structures in head and neck; not formed by fusion of sensory and motor roots (like spinal nerves); allows for purely sensory, mixed, and mostly motor nerves

Functions of PNS are integrated with those of CNS:

Sensory neurons detect stimuli at sensory receptors after which the following events occur:

Detected stimuli are transmitted along sensory neuron (spinal or cranial) to cerebral cortex

In cortex, sensory information is interpreted, integrated, and an appropriate motor response is selected and initiated

Motor response is initiated by commands from motor areas of cerebral cortex, leads to following events:

Impulses travel to spinal cord where neurons synapse with lower motor neurons of PNS

Lower motor neurons carry impulses to appropriate muscles via cranial or spinal nerves where they trigger contractions

Three cranial nerves contain axons of only sensory neurons: 

Olfactory (I)

Optic (II)

Vestibulocochlear (VIII)

The Motor Cranial Nerves 

Five cranial nerves contain primarily axons of motor neurons with their associated sensory axons responsible for proprioception: 

Oculomotor (III) 

Trochlear (IV)

Abducens (VI)

Accessory (XI)

Hypoglossal (XII)

The Mixed Cranial Nerves 

Four cranial nerves contain axons of both sensory and motor neurons: 

Trigeminal (V)

Facial (VII)

Glossopharyngeal (IX)

Vagus (X)

Spinal nerve

short and divides into following 2 mixed nerves; both carry both somatic motor and sensory information: Posterior ramus, Anterior ramus

Posterior ramus

travels to posterior side of body

Anterior ramus

 travels to anterior side of body and/or to an upper or lower limb

31 pairs of spinal nerves:

8 pairs of cervical nerves

12 pairs of thoracic nerves

5 pairs of lumbar and sacral nerves

1 pair of coccygeal nerves 

nerve plexuses

Anterior rami of cervical, lumbar, and sacral spinal nerves each merge to form complicated networks of nerves called …

cervical plexuses

Right and left … are found deep in neck lateral to 1st through 4th cervical vertebrae

Plexus consists of anterior rami of C₁–C₄ (with contributions from C₅)

Phrenic nerve

major motor branch of C₄ with contributions from C₃ and C₅ (3-4-5 to stay alive); innervates diaphragm

Right and left brachial plexuses

lateral to 5th cervical through 1st thoracic vertebrae; provide motor and sensory innervation to upper limbs; includes nerve roots from C_5–T₁

Ulnar nerve

 continuation of medial cord; travels near elbow where it enters forearm to innervate flexor muscles in forearm (not innervated by median nerve), most of intrinsic hand muscles, and skin of 5th digit and medial side of 4th digit 

The contusion of this nerve results in a painful, tingling, electrical sensation that results from hitting your “funny bone”

Thoracic spinal nerves

 do not form plexuses, except T₁

Each posterior ramus innervates deep back muscles

Each anterior ramus travels between two ribs as an intercostal nerve 

Lumbar Plexuses

Left and right lumbar plexuses are derived from anterior rami of L₁–L₄; anterior to vertebrae; embedded deep within psoas muscle; branches innervate pelvic structures and lower extremity after splitting into 2 divisions

Lumbar plexus divisions:

Obturator nerve , Femoral nerve

Obturator nerve 

anterior division’s largest member

Femoral nerve

Posterior division’s largest member; largest branch of lumbar plexus

sacral plexuses

Right and left … are formed from anterior rami of spinal nerves L₄–S₄; nerve branches innervate structures of pelvis, gluteal region, and much of lower extremity; each plexus is divided into anterior and posterior divisions

Sciatic nerve 

 longest and largest nerve in body; contains axons from both anterior and posterior divisions of sacral plexus

Sensory transduction 

 process where stimulus is converted into an electrical signal:

Ion channels in axolemma are closed → stimulus is detected by a sensory receptor → sodium ion channels open → sodium ions flow into axoplasm → temporary depolarization (receptor potential) 

If enough sodium ion enters, membrane potential may reach threshold → voltage-gated sodium ion channels open → action potential is propagated along axon toward CNS

Rapidly adapting receptors 

respond rapidly with high intensity to stimuli but stop sending signals after a certain time period (called adaptation); receptors detect initiation of stimuli but ignore ongoing stimuli

Slowly adapting receptors 

 respond to stimuli with constant action potentials that don’t diminish over time

Sensory receptors exist in many forms: 

Encapsulated nerve endings are surrounded by specialized supportive cells 

Free nerve endings lack supportive cells

Exteroceptors 

 usually close to body’s surface; detect stimuli originating from outside body

Interoceptors

usually found within body’s interior; detect stimuli originating from within body itself

Mechanoreceptors

encapsulated interoceptors or exteroceptors found in musculoskeletal system, skin, and in many other organs; depolarize in response to anything that mechanically deforms tissue where receptors are found; mechanically gated ion channels allow for sensory transduction from vibration, light touch, stretch, and pressure

Thermoreceptors

exteroceptors, most of which are slowly adapting receptors; depolarize in response to temperature changes; separate receptors detect hot and cold

Chemoreceptors 

can be either interoceptors or exteroceptors; depolarize in response to binding to specific chemicals (in body fluids or in air); generate a receptor potential as sodium ion channels open

Photoreceptors

special sensory exteroceptors found only in eye; depolarize in response to light

Nociceptors 

 usually slowly adapting exteroceptors; depolarize in response to noxious stimuli

Merkel cell fibers 

consist of a slowly adapting nerve ending surrounded by a capsule of Merkel cells

Found in epidermal ridges of integumentary system; primarily in skin of hands (especially fingertips)

Receptor potentials are generated by mechanically gated ion channels

Detect discriminative touch stimuli (object form and texture)

Tactile corpuscles (Meissner corpuscles)

in dermal papillae; rapidly adapting tactile exteroceptors; transmit discriminative touch stimuli

Lamellated corpuscles (Pacinian corpuscles) 

 layered onion-shaped appearance; rapidly adapting receptors found deep within dermis; detect high-frequency vibratory and deep pressure stimuli

Hair follicle receptors 

 free nerve endings surrounding base of hair follicles found in thin skin; not on palms and soles; respond to stimuli that cause hair to bend

Proprioceptors 

 in musculoskeletal system; detect movement and position of a joint or body part

Types of thermoreceptors

 usually small knobs at end of free nerve endings in skin

“Cold” receptors

 respond to temperatures between
10 ∞C and 40 ∞C (50–104 ∞F); in superficial dermis

“Hot” receptors

 respond to temperatures between
32 ∞C and 48 ∞C (90–118 ∞F); deep in dermis

Temperatures outside these ranges are detected by nociceptors; reason extremes of temperature are interpreted as pain

Sensory Neurons 

Sensory neurons are classified by two factors that determine speed with which peripheral axons conduct action potentials: diameter of axon and thickness of its myelin sheath

Large-diameter axons with thick myelin sheaths conduct fastest impulses; include axons that:

Conduct proprioceptive information to CNS 

Convey discriminative and nondiscriminative touch information 

Small-diameter axons with little myelin transmit action potentials slowest; include axons that carry pain and temperature stimuli to CNS

Receptive fields 

 areas served by a particular neuron; neuron with more branches innervate larger receptive fields

Body regions whose primary function is sensing environment (fingertips) contain many neurons with smaller receptive fields

Body regions that are not as involved in sensing environment (skin of forearm) have fewer neurons with larger receptive fields

Reflexes 

programmed, automatic responses to stimuli; occur in a three-step sequence of events called a reflex arc; usually protective negative feedback loops

Reflexes begin with a sensory stimulus and finish with a rapid motor response

Neural integration between sensory stimulus and motor response occurs in CNS, at spinal cord or brainstem

Reflexes can be classified by at least two criteria: 

Number of synapses that occur between neurons involved in arc 

Type of organ in which reflex takes place, either visceral or somatic 

(monosynaptic reflexes)

Simplest reflex arcs … involve only a single synapse within spinal cord between a sensory and motor neuron;

(polysynaptic reflexes)

more complicated types of reflex arcs … involve multiple synapses

Simple stretch reflex 

Body’s reflexive response to stretching of muscle to shorten it back to within its “set” optimal length

Patellar (knee-jerk) reflex and jaw-jerk reflex 

are examples of simple stretch reflexes

The jaw-jerk reflex is a stretch reflex used to test a patient's trigeminal nerve (CN V).  The mandible is tapped at a downward angle on the chin while the mouth is held slightly open. The reflex contracts the masseter and temporalis muscles, snapping the mouth shut

Patellar (knee-jerk) reflex

Steps in a simple stretch reflex in a spinal nerve:

External force stretches muscle

Muscle spindles detect stretch; primary and secondary afferents transmit an action potential to spinal cord

In spinal cord, sensory afferents synapse on motor neurons and trigger an action potential

Motor neurons stimulate muscle to contract and it returns to its optimal length

Polysynaptic reflex

A reflex arc with at least one interneuron between the sensory afferent and motor efferent

Has a longer delay than a monosynaptic reflex (more synapses)

Can produce more complex response

Example: flexion reflex, or withdrawal reflex

Reciprocal inhibition 

occurs when one set of motor neurons are stimulated while those controlling antagonistic muscles are inhibited

Crossed-extension reflex

occurs simultaneously on opposite side of body for balance and postural support while other limb is withdrawn from a painful stimulus

Cranial nerve reflexes

 polysynaptic reflex arcs that involve cranial nerves

Gag reflex

 triggered when visceral sensory nerve endings of glossopharyngeal nerve in posterior throat are stimulated

Corneal blink reflex 

triggered when a stimulus reaches somatic sensory receptors of trigeminal nerve in thin outer covering of eye (cornea); something contacts eye leading to a blink response