Session 5: PNS & ANS Notes

Peripheral Nervous System (PNS) & Autonomic Nervous System (ANS)

Learning Objectives: The Peripheral Nervous System

• Name the 12 pairs of cranial nerves; indicate the body region and structures innervated by each.

• Describe the general structure of a spinal nerve.

• Define spinal plexus. Name the four major spinal plexuses and describe the general distribution and function of the peripheral nerves arising from each plexus.

• Name the components of a reflex arc and distinguish between somatic and visceral.

Somatic vs. Autonomic Nervous Systems

• Autonomic nervous system

  • Involuntary branch of the peripheral efferent division

• Somatic nervous system

  • Branch of the efferent division subject to voluntary control

• The AFFERENT division is included in the sensory pathway for both systems.

Peripheral Nervous System (PNS)

• Consists of nerve fibres and cell bodies outside of the central nervous system (CNS)

• Organised into nerves which connect the CNS with the body

  • 12 pairs of cranial nerves

  • 31 pairs of spinal nerves

• Note the use of the term pairs: they occur on the left and right

Nerves and Associated Ganglia

• Classification according to the nerve origin:

  • Spinal nerve from spinal cord

  • Cranial nerve - from brain (brainstem)

• Classification according to the direction nerves transmit impulses

  • Sensory (afferent) nerves: impulses only toward CNS

  • Motor (efferent) nerves: impulses only away from CNS

  • Mixed nerves: contain both sensory and motor fibers

    • Impulses travel both to and from CNS

• Nerve:

  • Cordlike organ of the PNS consisting of axons/dendrites enclosed by connective tissue

Anatomy of a Nerve

• What are the differences between: a neuron, a nerve fiber, a nerve fascicle and a whole peripheral nerve?

• Name the connective tissue membranes that cover the:

  • Axon (Nerve Fiber)

  • Nerve fascicle

  • Entire peripheral nerve

Cranial Nerves

• 12 pairs

• Cranial nerves are in the PNS

• 12 pairs of cranial nerves

• 11 pairs arise from the brain, 1 pair arise from the spinal cord

• Pass through the foramina of the skull

• Named in two ways

  • A descriptive name eg. optic nerve, abducens nerve, accessory nerve

  • Via roman numerals CN II, CN VI, CN XI

Cranial Nerves List

• CN I Olfactory n.

• CN II Optic n.

• CN III Oculomotor n.

• CN IV Trochlear n.

• CN V Trigeminal n.

• CN VI Abducens n.

• CN VII Facial n.

• CN VIII Vestibulocholear n.

• CN IX Glossopharyngeal n.

• CN X Vagus n.

• CN XI Accessory n.

• CN XII Hypoglossal n.

Distribution of Cranial Nerves

• Mnemonic devices to remember the names of the cranial nerves:

  • "On occasion, our trusty truck acts funny—very good vehicle anyhow"

  • "Oh once one takes the anatomy final, very good vacations are heavenly"

Functions of Cranial Nerves

• Carried by afferent fibers.

• Carried by efferent fibers.

Spinal Nerves Numbering

• 31 pairs of spinal nerves exiting the spinal cord

  • 8 cervical (8 pairs)

  • 12 thoracic (12 pairs)

  • 5 lumbar (5 pairs)

  • 5 sacral (5 pairs)

  • 1 coccygeal (1 pair)

Somatic Plexus (refers to a network of intersecting nerves in the somatic nervous system, which controls voluntary movements and sensory perception in the body wall and limbs)

• Ventral rami (larger branches and carry both motor and sensory fibers) from adjacent spinal nerves merge to form networks called a plexus

• Multisegmental peripheral nerves (nerves that receive input from more than one spinal cord segment) arise from a plexus

• Primarily occurs to supply limbs

Innervation of Specific Body Regions

Spinal Nerves

Gray Matter and Spinal Roots

• Gray commissure: bridge of gray matter that connects masses of gray matter on either side

  • Encloses central canal

• Spinal nerves: formed by fusion of dorsal and ventral roots

• Dorsal roots: sensory input to cord

• Dorsal root (spinal) ganglia: cell bodies of sensory neurons

• Ventral roots: bundle of motor neuron axons that exit the spinal cord

Afferent Vs Efferent

• Memory trick to remember the difference between afferent and efferent neurons.

• S – Sensory

• A – Afferent

• M – Motor

• E – Efferent

• D – Dorsal (enter the dorsal aspect of the spinal cord)

• A – Afferent

• V – Ventral (exits the ventral aspect of the spinal cord)

• E - Efferent

Spinal Nerve Formation

• Dorsal Root Ganglion

• Dorsal Horn

• Dorsal Ramus

• Mixed

• Dorsal Roots

• Sensory

• Ventral Spinal Nerve

• Ventral Root

• Ventral Horn

• Ventral Ramus

• Mixed Motor

• Mixed

Spinal Nerves - Neurons

• Blue neurons – sensory

• Red neurons – motor

• Note that the spinal nerve and rami contain both motor and sensory neurons

Structure and Classification (cont.)

• Ganglia: contain neuron cell bodies associated with nerves in PNS

  • Ganglia associated with afferent nerve fibers contain cell bodies of sensory neurons

    • Dorsal root ganglia (sensory, somatic)

  • Ganglia associated with efferent nerve fibers contain autonomic motor neurons

    • Autonomic ganglia (motor, visceral)

Spinal Cord and Innate Reflexes

• A reflex is an involuntary, stereotypical response of the effector tissue from the stimulation of receptors.

• These reflexes are executed by the successive activation (chain reaction) of a certain number of neurons that are mutually connected at the level of spinal cord.

• If there are two neurons with one synapse involved= monosynaptic

• If there are more than 2 neurons and more than one synapse= polysynaptic

• The last neuron generally innervates the effector tissue, which is usually a muscle.

• These reflexes do NOT include brain

• The spinal cord is responsible for the integration of many innate reflexes

Components of a Reflex Arc

• Components of a reflex arc (neural path)

  1. Receptor: site of stimulus action

  2. Sensory neuron: transmits afferent impulses to CNS

  3. Integration center: either monosynaptic or polysynaptic region within CNS

  4. Motor neuron: conducts efferent impulses from integration center to effector organ

  5. Effector: muscle fiber or gland cell that responds to efferent impulses by contracting or secreting

Components of a Reflex Arc (cont.)

• Reflexes are classified functionally as:

  • Somatic reflexes (Spinal reflexes)

    • Activate skeletal muscle

  • Autonomic (visceral) reflexes (homeostatic regulation & feedback loop)

    • Activate visceral effectors (smooth or cardiac muscle or glands)

Spinal Reflexes

• Spinal reflexes occur without direct involvement of higher brain centers

• Brain is still advised of spinal reflex activity and may have an effect on the reflex

• Testing of somatic reflexes important clinically to assess condition of nervous system

• If exaggerated, distorted, or absent, may indicate degeneration or pathology of specific nervous system regions

• Most commonly assessed reflexes are stretch, flexor, and superficial reflexes

Sensory Processing

• Survival depends upon:

  • Sensation: the awareness of changes in the internal and external environment

  • Perception: the conscious interpretation of those stimuli

• Somatosensory system: part of sensory system serving body wall and limbs

  • Receives inputs from: Exteroceptors, proprioceptors, and interoceptors

  • Input is relayed toward head, but processed along the way

Sensory Processing (cont.)

• Processing at the receptor level

  • Generating a signal: For sensation to occur, the stimulus must excite a receptor, and the AP must reach CNS

    • Stimulus energy must match receptor specificity (touch receptors do not respond to light)

    • Stimulus must be applied within receptive field (the nerve that innervates that area)

    • Transduction must occur—energy of stimulus is converted into graded potential called generator potential (in general receptors) or receptor potential (in special sense receptors)

    • Graded potentials must reach threshold → AP

Key Concept: Sensory Receptors

• Sensory receptors: specialized to respond to changes in environment (stimuli)

• Converts mechanical, physical, chemical change into Action potential (opens sodium channels)

• Classification by Stimulus Type:

  • Mechanoreceptors—respond to touch, pressure, vibration and stretch

  • Thermoreceptors—sensitive to changes in temperature

  • Photoreceptors—respond to light energy (example: retina)

  • Chemoreceptors—respond to chemicals (examples: smell, taste, changes in blood chemistry)

  • Nociceptors—sensitive to pain-causing stimuli (examples: extreme heat or cold, excessive pressure, inflammatory chemicals)

Classification by Location

• Exteroceptors

  • Respond to stimuli arising outside body

  • Receptors in skin for touch, pressure, pain, and temperature

• Interoceptors (visceroceptors)

  • Respond to stimuli arising in internal viscera and blood vessels

  • Sensitive to chemical changes, tissue stretch, and temperature changes

  • Sometimes cause discomfort but usually person is unaware of their workings

• Proprioceptors

  • Respond to stretch in skeletal muscles, tendons, joints, ligaments, and connective tissue coverings of bones and muscles

  • Inform brain of one's movements

Peripheral Nervous System Summary

The ANS and CNS

• Many regions of the CNS are involved in the control of autonomic activities

• Some autonomic reflexes are integrated at the spinal- cord level

• Medulla oblongata within the brain stem is the region most directly responsible for autonomic output

• Hypothalamus plays a role in integrating autonomic, somatic, and endocrine responses

Autonomic & Somatic Nervous Systems

• Autonomic nervous system

  • Involuntary branch of the peripheral efferent division

• Somatic nervous system

  • Branch of the efferent division subject to voluntary control

Somatic Nervous System

• Motor neurons supply skeletal muscle

• Bring about movement

• Axons of motor neurons originate in the CNS and end on skeletal muscle. Consist of a one-neuron chain.

• Motor-neuron axon terminals release ACh to stimulate muscle contraction

• Motor neurons are the final common pathway

Pathways and Divisions of the ANS

• An autonomic nerve pathway consists of a two-neuron chain

  • Preganglionic neuron: synapses with the cell body of the postganglionic fiber in a ganglion outside the CNS

  • Postganglionic neuron: sends axons that end on the effector organ

• Autonomic nervous system has two subdivisions

  • Sympathetic and parasympathetic

ANS vs. Somatic Nervous System

Anatomy of ANS

Key Anatomical Differences Between ANS Divisions

Divisions of the ANS

Sympathetic and Parasympathetic Systems

• Parasympathetic postganglionic fibers release acetylcholine

  • Cholinergic fibers

• Sympathetic postganglionic fibers release noradrenaline

  • Adrenergic fibers

• The Adrenal Medulla

  • The adrenal medulla is a modified part of the sympathetic nervous system

  • Adrenal medulla secretes catecholamine hormones on stimulation – epinephrine (80%) & norepinephrine (20%)

Dual Innervation

• Sympathetic and parasympathetic nervous systems dually innervate most visceral organs

  • Dual innervation: innervation of a single organ by both branches of the autonomic nervous system

  • Times of sympathetic dominance: “fight-or-flight” response

  • Times of parasympathetic dominance: “rest-and-digest” response

Illustration of Autonomic Innervation

• Parasympathetic and sympathetic innervation of various organs

• Cranial nerves involved: III, VII, IX, X

• Spinal nerves involved: S2, S3, S4, L1, L2, T1-T12

• Organs include: Nasal mucosa, lacrimal gland, parotid gland, sublingual and submandibular glands, eye, trachea, lung, heart, liver, stomach, spleen, gall bladder, adrenal gland, spinal nerves, kidney, pancreas, colon, small intestine, rectum, urinary bladder, genitalia

Effects of Autonomic Nervous System on Organs

Receptors of the ANS

• Several receptor types are available for each autonomic neurotransmitter:

  • Cholinergic receptors: nicotinic and muscarinic receptors

  • Adrenergic receptors: alpha and beta receptors

  • Autonomic agonists and antagonists: agonist binds to the neurotransmitter’s receptor and an antagonist binds with the receptor