Human A&P: Lesson 16 Peripheral Nervous System (PNS)

Main organs of PNS

Peripheral nerves

Peripheral nerves (or nerves)

axons of many neurons bound together in connective tissue

• most are a mix of both sensory and motor neurons

Anterior root

axons of motor neurons exiting anterior and lateral horn

Posterior root

axons of sensory neurons entering porsterior horn

Ganglion

collection of cell bodies

Peripheral nerves anatomy

Very similar organization to muscles

• Epineurium

• Perineurium

• Endonerium

Epineurium

connective tissue surrounding entire nerve, providing protection and support.

Perineurium

small groups= fasicles bound by ___________

Endonerium

each axon within the fascicle is surrounded _________

Sensory transduction

conversion of a stimulus into an electrical signal

1. at rest, gates within sensory neurons are closed

2. stimulus triggers channels that open ——> receptor potential ———> action potential

3. signal propagates to spinal cord

Type of stimuli that causes depolarization of the first order sensory neurons

• Mechanoreceptors

• Thermoreceptor

• Chemoreceptor

• Photoreceptor

• Nociceptor

Mechanoreceptors

mechanically deforms tissue

Thermoreceptor

temperature changes

Chemoreceptor

chemicals in body fluids or air

Photoreceptor

only in eye for changes to light

Nociceptor

noxious stimuli and pain

6 types of mechanoreceptors

• Merkel cell fiber: discriminative touch with fine spatial resolution

• Tactile corpuscle: discriminative touch with less spatial resolution

• Ruffini ending (bulbous corpuscle): strech and movement

• Lamallated corpuscle: vibration and deep pressure

• Hair follicle receptors

• Proprioceptors: in musculoskeletal system to detect movement and position of joints

Structure of 1st order sensory pseudounipolar neuron

• peripheral process

• Central process

Peripheral process

transmits action potential from nerve endings to the central process

Central process

transmit action potential to posterior horn (or brainstem for cranial nerves) synapsing on a 2nd order neurons EITHEIR in the spinal cord or the brain

Reflex classifications

1. Classification of reflexes based upon total # of synapses that occur between all neurons in the arc:

   • Monosynaptic

   • Polysynaptic

2. Classification of reflexes according to type of organ (effector)

   • Visceral reflexes

   • Somatic reflexes

Monosynaptic

1 synapse in spinal cord between sensory and motor neurons

Polysynaptic

multiple synapses

Visceral reflexes

involve ANS neurons innervating internal organs

Somatic reflexes

involve PNS somatic sensory and motor neurons

Lower motor neurons are

housed in the PNS

• reminder lower motor neurons involved are multipolar neurons

  • Cell bodies- within CNS anterior horn of spinal cord OR brainstem

  • Axons- located in the PNS

Stretch reflex involving skeletal muscle as an effector

Motor neuron pools- or groups of lower motor neurons- that are involved:

• Alpha-motor neurons skeletal muscle to contract with excitation-contraction mechanism

• Gamma-motor neurons innervate muscle fibers as part of the stretch reflex (also stimulate muscle)

Skeletal muscle fibers involved

• Extrafusal muscle fibers

• Intrafusal

• Both have the same actin and myosin contractile filaments as we have already seen

Extrafusal muscle fibers

regular contractile muscle fibers outside the muscle spindle

Intrafusal

are within spindle

Sensory neurons involved in skeletal muscle (stretch)

• Primary afferent

• Secondary afferent

• Golgi tendon organ has a side role

  • Proprioceptor

Primary afferent

respond to stretch when it is first initiated

Secondary afferent

static length of muscle and limb position

Proprioceptor

rate at which these neurons fire depends on the amount of muscle tension generated with contractions

Simple stretch reflex

Purpose: prevent damage to muscle from deviating from optimal length and rupturing

• Uses a monosynaptic reflex to shorten the muscle so it returns to the optimal length

• Proprioceptors are stimulated in the triceps brachii due to the contractions

• Sensory afferent neurons involved also synapse with interneurons of the spinal cord » inhibit lower motor neurons that innervate the respective antagonist muscles

Flexion reflex and cross-entension reflex

Purpose: quickly move away from a source of pain (temperature or pressure)

• Recall —→ alpha motor neurons innervate extrafusal muscle fibers

Gag reflex- a cranial nerve reflex

Purpose - to remove foreign objects and prevent chocking

• Involves the glossopharyngeal nerve (IX), Vagus nerve (X)

• Classified as either

  • Somatogenic - requires physical touch to the base of the tongue, tonsilsm and/or pharyngeal well

  • OR psychogenic - mental trigger (i.e., when thinking about something getting close to the back of the throat like during procedure)

Gag reflex involve 2 seperate nerve:

One afferent and one efferent

• Afferent sensory: sensory endings (glossopharyngeal nerve) in posterior throat are stimulated

  !

• Synapse in a medulla oblongata nucleus shared by IX and X

  !

• Efferent motor: motor nerve endings of (Vagus nerve) trigger pharynx contraction

Note that the Vagus nerve and the Glossopharyngeal nerves are mixed nerves (have sensory and motor neurons)