Peripheral nervous system Lecture

Cranial nerves

• Apart of the PNS originating from brain

• Numbered with roman numerals according to their position

  • Begin with most anteriorly located nerve

• Name of nerve often related to function

CN I olfactory Nerve

Sense of smell

• Special sensory nerve within temporal lobe that conducts olfactory sensation from nose to the brain

CN II Optic nerve

• Sensory nerve controlling sense of vision

• Crossing over occurs at optic chiasm where some nerves cross to the other side allowing for 3D vision

CN III Oculomotor nerve

• Motor nerve that controls muscles that move eye, lift eyelid, change pupil diameter

• Innervates four of the six extrinsic eye muscles

CN IV Trochlear nerve

Motor nerve that innervates one extrinsic eye muscle: superior oblique

CN V Trigeminal nerve 

• Mixed nerve controlling somatic sensation from face, chewing movements 

• Three branches: ophthalmic, maxillary and mandibular 

• Receives sensory nerve signals from face, oral cavity, nasal cavity, meninges, and anterior scalp, and innervates muscles of mastication 

CN VI Abducens Nerve 

Motor nerve that controls one eye muscle: lateral rectus muscle to abduct eye away from the center

CN VII Facial nerve 

• Controls muscles of facial expression and provides signals for taste from tongue 

• Mixed nerve that conducts taste sensations from anterior two-thirds of the tongue, relays motor output to muscles of facial expression; lacrimal (tear) gland and most salivary glands

CN VIII Vestibulocochlear nerve 

• Sensory nerve controlling sense of hearing and equilibrium

• Sensory nerve with two branches that conducts equilibrium and auditory (hearing) sensations from the inner ear to the brain

• Has three fluid filled canals

CN IX Glossopharyngeal nerve

• Mixed nerve that controls taste and touch from tongue; control of pharynx muscle 

• Receives taste and touch sensations from posterior one-third of the tongue, innervates one pharynx muscle and the parotid salivary gland 

CN X Vagus nerve

Mixed nerve, visceral sensation; parasympathetic nerve to many organs of the body with lots of branches

CN XI Accessory nerve

• Controls muscles of neck, pharynx

• Motor nerve that innervates trapezius and sternocleidomastoid muscles, also assists CN X to innervate pharynx muscles, formerly called the spinal accessory nerve 

CN XII Hypoglossal nerve

Motor nerve that innervates both intrinsic and extrinsic tongue muscles “under tongue”

Spinal nerve characteristics

• 31 pairs

• Each nerve formed from merger of anterior root and posterior root

Anterior root

Many axons of motor neurons whose cell bodies are in anterior and lateral horns (motor output)

Posterior root

Many axons of sensory neurons whose cell bodies are in posterior root ganglion (sensory input)

Naming of spinal nerves

Each is named for part of spinal cord it comes from and a number

Intervertebral foramina

• This is place that cervical nerves exit superior to the vertebra of the same number (C2 nerve exits between C2 and C1 vertebrae)

• Exception is nerve C8 which exits below the C7 vertebra

• Below C8, nerves exit inferior to the vertebra of the same number (T2 nerve exits between T2 and T3 vertebrae)

• Lumbar, sacral and coccygeal spinal nerves have long rootlets that extend inferiorly before exiting vertebrae

  • Rootlets form cauda equina

Somatic nervous system (SNS)

Consciously perceived or controlled sensory and motor processes

Somatic sensory

• Portion detects signals from special senses (vision, hearing, equilibrium, smell taste) and from skin and proprioceptors that we are consciously aware of 

Somatic motor

• Portion sends signals from CNS to skeletal muscles 

• Voluntary movements involve cerebrum

• Reflexive movements involve brainstem and spinal cord 

Sensory nervous system

Detects stimuli and transmits information from receptors to the CNS

• Somatic and visceral sensory

Visceral sensory

Sensory input that is not consciously perceived from receptors of blood vessels and internals organs. It provokes autonomic motor output

Motor nervous system

Initiates and transmits information from CNS to effectors

• Somatic and autonomic motor

Autonomic nervous system (ANS)

• Motor output that is not consciously controlled; transmits signals from CNS to effectors which are heart, cardiac muscle, smooth muscle and glands

• Also called autonomic motor or visceral motor

• Responds to visceral sensory inputs (from blood vessels)

• Functions to maintain homeostasis to keep body conditions within optimal range 

Parasympathetic division (of autonomic motor nervous system)

• Preganglionic neurons located in brainstem nuclei and S2-S4 segments of spinal cord (caniosacral)

• Functions to bring body to homestasis in conditions of rest and digest and conserves energy and replenishes nutrient stores

• Slows down heart rate 

• Airways constrict 

• Caniosacral division

  • Myelinated long preganglionic axon

  • Short unmyelinated postganglionic axon

  • Autonomic ganglion is close to or within effector organ wall

   

Sympathetic division (of autonomic motor nervous system)

• Preganglionic neurons located in lateral horns of T1-L2 segments of spinal cord (thoracolumbar)

• Functions to bring body to homeostasis in conditions of fight or flight 

• Increases alertness and metabolic activity 

• Thoracolumbar division

  • Short branching myelinated preganglionic axon

  • Long unmyelinated postganglionic axon

  • Autonomic ganglion is close to the vertebral column

Sympathetic division function

• Emergency, excitement, exercise

• Thoracolumbar anatomical origin

• Ganglia are close to CNS but anatomical pathways are complex because of branching

Sympathetic trunks and ganglia

• Left and right trunks just lateral to the vertebral column

• Trunk resembles a string of pearls 

  • “string” composed of axons

  • “pearls” composed of sympathetic trunk ganglia housing cell bodies

Adrenal medulla pathway

• Innervates cells in medulla, only one neuron

• For central region of adrenal gland (its medulla)

• Preganglionic sympathetic axons extend through sympathetic trunk and pre vertebral ganglia without synapsing in either

• Preganglionic cells stimulate adrenal medulla cells to release epinephrine and norepinephrine into the blood

Epinephrine and norepinephrine 

Hormones that prolong the “fight or flight” response 

ANS neurotransmitters

• Acetylcholine (ACh) and norepinephrine (NE)

• Either transmission can cause stimulation or inhibition, depending on the postsynaptic receptor (excitatory or inhibitory)

• Cells that release ACh are cholinergic neurons

Cholinergic neurons

• Release ACh

• All ANS preganglionic neurons

• All parasympathetic ganglionic neurons

• Sympathetic ganglionic neurons innervating sweat glands and blood vessels in skeletal muscle

Target cells in ANS that release ACh

Have cholinergic receptors

Adrenergic neurons

Cells that release norepinephrine (NE)

Target cells in ANS that release norepinephrine

Have adrenergic receptors

Types of cholinergic receptors

Nicotinic and muscarinic

Nicotinic receptors

• Type of cholinergic receptor found in all ganglionic neurons and adrenal medulla cells 

• When ACh binds to receptor it opens cation channels to allow (+) cations to move through (stimulatory effect)

  • More Na+ moves into the cell then K+ leaving so it becomes more positive

  • Cell depolarizes: excitatory postsynaptic potential is produced (always excitatory)

Muscarinic receptors 

• Type of cholinergic receptor found in all target organs of parasympathetic division and a few of sympathetic division

  • Sympathetic effectors with these receptors include sweat glands and blood vessels in skeletal muscle

• Different subtypes

  • ACh binds to receptors of smooth muscle in GI it is stimulated to contract more

  • When ACh binds to receptors on cardiac muscle the heart rate decreases

• Have stimulating and inhibitory effects

Types of adrenergic receptors

• Alpha (a) and beta (B) receptors 

  • Can be stimulatory or inhibitory

• Cells with alpha receptors are typically stimulated by NE

• Cells with beta receptors are typically stimulated or inhibited by NE

Dual innervation

• Organ receives input from both the sympathetic and parasympathetic divisions

  •  Two divisions may have antagonistic or cooperative effects

Antagonistic effects

• The parasympathetic and sympathetic divisions oppose each other

• Parasympathetic activity slows heart rate; sympathetic activity increases heart rate

  • Cardiac cells have both cholinergic and adrenergic receptors 

Cooperative effects

• Parasympathetic and sympathetic stimulation have different effects that are part of an overall response

• E.g., male sexual function

  • Penis erection due to parasympathetic activity 

  • Ejaculation due to sympathetic activity

Sensory receptors

• Provide information about external and internal environments

• Respond to stimuli

• Each type of receptor responds best to a type of stimulus 

  • Light energy for eye receptors, sound energy for ear receptors

Modality

Nature of stimulus (light, sound, touch, pressure)

Transducers

• Convert stimulus energy into electrical energy 

  • Receptors have resting membrane potential 

  • Receptor membranes have modality gated channels that respond to their type of stimulus

• Action potentials are conveyed to CNS for interpretation

 Receptive field 

The distribution area of the endings of a sensory neuron

• Smaller receptive fields allow more precise stimulus localization where larger receptive fields are less precise

Sensory receptor classification

Categorized by distribution, stimulus origin, and stimulus modality

Receptor distribution

General vs special

General sense receptors

Simple structures distributed throughout the body

Somatic sensory receptors

Tactile receptors of skin and mucous membranes, proprioceptors of joints, muscles, tendons

Visceral sensory receptors

Found in walls of internal organs, they monitor stretch, chemical environment, temperature, pain

Special sense receptors

• Specialized receptors in complex sense organs of the head

• 5 senses: olfaction, gustation, vision, audition, equilibrium

Stimulus origin categories

• Exteroceptors

• Interoceptors

• Proprioceptors

Exteroceptors

Detect stimulus from external environment

• Skin, mucous membranes, special sense receptors

Interoceptors

Detect stimuli from internal organs

• Visceral sensory receptors monitoring internal environment 

Proprioceptors

• Detect body and limb movements

• Somatosensory receptors of muscles, tendons, joints 

Modality of stimulus categories

chemoreceptors, thermoreceptors, photoreceptors, mechanoreceptors, nociceptors 

Chemoreceptors

• Detect chemicals dissolved in fluid

• Include receptors for external environment (smell of food) or internal environment (oxygen levels in blood)

Thermoreceptors 

• Detect changes in temperature

• Include receptors in skin, hypothalamus 

Photoreceptors

• Detect changes in light intensity, color, movement

• In retina of the eye

Mechanoreceptors

• Detect distortion of cell membrane

• Include touch, pressure, vibration, and stretch receptors

• Function as baroreceptors, proprioceptors, tactile receptors and specialized receptors in inner ear

Nociceptors

• Detect painful stimulus

  • Somatic nociceptors, visceral nociceptors

Somatic nociceptors

Detect chemical, heat or mechanical damage to the body surface or skeletal muscles (Skin, muscle, bone etc.)

Visceral nociceptors

Detect internal organ damage

Classifying a receptor

• Classification based on receptor distribution, stimulus origin, modality

• Eyes

  • Special sense, exteroceptors, photoreceptors

• Receptors for blood vessel stretch

  • General sense, interoceptors, mechanoreceptors (baroreceptors)