The Peripheral Nervous System and Reflex Activity

1
Chapter 13

• The Peripheral Nervous System and Reflex Activity
• J.F. Thompson, Ph.D. J.R. Schiller, Ph.D. G.
  Pitts, Ph.D.

2
Overview of PNS

• Modality
• each input is a specific type of sensation
  temperature, pain, pressure, touch, body
  position, equilibrium, hearing, vision, smell,
  taste
• individual sensory neurons generally carry only
  one modality
• Selectivity of Receptors
• sensory neurons respond strongly to one type of
  stimulus and weakly or not at all to other types
• some respond accidentally to other types of
  stimuli rubbing ones eyes mechanically
  stimulates the eyes light receptors in the retina

3
Components of Sensation

• stimulation - stimulus or change in the
  environment
• transduction
• requires a sensory receptor cell or organ which
• responds to specific stimuli and converts them
  into receptor/generator potentials
• impulse generation and conduction
• if a receptor/generator potential (graded
  potential) reaches a threshold, then
• the neurons action potential will be sent to the
  CNS
• integration
• some region of the CNS must receive and translate
  nerve impulses into sensations and perceptions
• this generally occurs in the cerebral cortex

4
Sensory Receptor Classification

• Classified by the type of stimulus
• Mechanoreceptors
• mechanical pressure or stretching
• generate action potentials when deformed
• Thermoreceptors - changes in temperature
• Nociceptors - pain due to physical or chemical
  damage to nearby tissue
• Photoreceptors
• light strikes retinal receptor cells
• generates action potentials in response to light
  energy
• Chemoreceptors certain specific chemical
  molecules are detected in the mucous fluids of
  the GI respiratory tracts, or in the blood or
  other body fluids

5
Sensory Receptor Classification

• Classified by location
• Exteroceptors
• located at or near the bodys surface
• provide information about the external
  environment
• Interoceptors (visceroceptors)
• found in blood vessels, connective tissues, and
  organs
• provide information about the internal
  environment
• Proprioceptors
• located in muscles, tendons, joints and the
  internal ear
• provide information about gravity, body and limb
  positions and skeletal muscle movements

6
Adaptation by Sensory Receptors

• a change in sensitivity to a long-lasting
  stimulus
• primarily by rapidly-adapting phasic receptors
• pressure, touch, hearing, smell
• adapt very quickly, i.e., respond less if the
  stimulus remains constant
• allows us to shut out background noise
• little adaptation by slowly-adapting tonic
  receptors
• pain, body position, chemicals in the blood or
  CSF
• adapt slowly, continue to respond even when the
  stimulus remains constant
• continuous input is useful for some modalities
  because the body needs to make continuous
  responses to that kind of information

7
Anesthesia

• a partial or complete loss of sensation
• General anesthesia gas agents act in the CNS
  through poorly understood mechanisms
• Local anesthesia drugs injected near peripheral
  nerves inhibit the opening of gated sodium
  channels, preventing local transmission of action
  potentials
• Analgesia reduced perception of pain without
  loss of other sensory information or loss of
  consciousness biochemical interference with
  local stimulus (NSAIDs) or mimic endogenous
  endorphins in CNS (opiates)
• Paresthesias abnormal sensations (burning,
  tingling, numbness) not related to normal
  stimulation, e.g., mechanical pressure on nerves
  in your leg puts your foot to sleep

8
Functional Types of Nerves

• mixed nerves contain both sensory and motor
  fibers
• motor (efferent) nerves
• Note so-called pure motor nerves do also
  carry proprioceptive sensory signals back to the
  CNS from the skeletal muscles, joints, and
  tendons being served by that nerve.
• sensory (afferent) nerves

9
Anatomy of Nerves

• bundles/fascicles of axons dendrites
• endoneurium around individual processes
• perineurium
• around fascicles
• individual nerve fibers with their endoneurium
• epineurium - outermost covering around entire
  peripheral nerve

10
Cranial Nerves

• Twelve pairs of nerves which originate from the
  brain and exit through foramina of the skull
• First 2 pairs originate from the forebrain
  (olfactory, optic)
• Remaining 10 pairs originate from the brain stem

11
What You Should KnowAbout the Cranial Nerves

• name and number
• general region(s) served
• main functional roles
• modality (sensory, motor, mixed (m/s))
• ( motor nerves carry proprioception sensory
  information back to the CNS)
• See Table 13.2 pp. 501- 507 and slides after end
  slide in this PPT for details

12
Spinal Nerves

• 31 pairs of spinal nerves originate from the
  spinal cord
• All are mixed (m/s) nerves
• Thousands of fibers per spinal nerve
• Each pair serves a particular region of the body
• Each pair also provides some service to the
  region supplied by the spinal nerve above it and
  the spinal nerve below it (redundancy)

13
Spinal Nerve Anatomy

• Spinal nerves are very short, they divide almost
  immediately
• dorsal ramus - supplies posterior body trunk
• ventral ramus - supplies the rest of body trunk
  and the limbs
• meningeal branch - supplies the meninges and
  blood vessels within meninges

14
Dorsal and Ventral Rami of a Typical Spinal Nerve
15
Dermatomes

• Areas of skin innervated by the cutaneous
  branches of each pair of spinal nerves
• Each pair also provides some service to the
  region of the spinal nerve above and the spinal
  nerve below (redundancy)

16
Reflex Activity

• a reflex is a rapid, predictable, automatic
  response to a stimulus
• a reflex is unlearned, unpremeditated, and
  involuntary
• one is conscious of somatic reflexes only after
  they occur
• reflexes are involved in homeostasis
• two fundamental types of reflexes
• somatic reflexes - produce contraction of
  skeletal muscle
• autonomic (visceral) reflexes
• generally, they are not perceived consciously
• produce responses by smooth muscle, cardiac
  muscle, glands to adjust conditions of the
  internal environment

17
Components of a Reflex Arc

• 5 Functional Components
• receptor - dendrites or other sensory structures
  respond to changes in the environment
• sensory neuron - conducts an impulse from a
  receptor to its axon terminals
• integrating center (some region within the CNS)
• simple - monosynaptic (2 cells only sensory and
  motor neurons)
• complex polysynaptic (gt 2 cells interneurons
  involved)
• motor neuron - impulses from integrating center
  to an effector
• effector - body part (muscle or gland) which
  responds to the motor nerve impulse

18
Stretch Reflexes

• receptors - muscle spindles and Golgi tendon
  organs
• sensory mechanoreceptors which respond to
  stretching
• increased tension (stretching) stimulates the
  receptors
• sends proprioceptive inputs to spinal cord
• contraction of the skeletal muscle reduces
  tension on the muscle spindle
• lowers the rate of action potential generation
• decreases input to the spinal cord and higher
  centers cortex and cerebellum

19
Stretch Reflexes

• Remember that if a muscle is being stretched, the
  stretch is caused by the contraction of its
  antagonist.
• This sensory proprioception information
  contributes to maintaining proper muscle tone.

20
Patellar Reflex

• monosynaptic
• ipsilateral (same side)
• segmental (at one level of the spinal cord)
• polysynaptic component for reciprocal
  inhibition of the antagonist

21
Golgi (Deep) Tendon Reflex

• an increase in muscle tension activates receptors
  (Golgi tendon organ) in the tendon
• the muscle relaxes and lengthens in response to
  its antagonists contraction
• D-T reflex inhibits the agonist
• D-T reflex excites the antagonist
• helps to regulate a smooth start and stop for a
  contraction
• input from the Golgi tendon organs are sent to
  the cerebellum and the cortex
• polysynaptic, ipsilateral, and segmental

22
Flexor Reflex

• a pull on the limb, extending it, will trigger
  the reflex
• also a painful stimulus a burn, pin prick, toe
  stub, etc.
• F-R causes an automatic withdrawal from the
  (dangerous) stimulus
• polysynaptic, ipsilateral, and segmental

23
Crossed Extensor Reflex

• flexion of a body part is often balanced by
  extension of the same body part on the opposite
  side of the body
• polysynaptic
• contralateral
• segmental

24
End Chapter 13Note Summary slides for the
cranial nerves appear after this slide for your
exam 4 review.
25
Cranial Nerve I Olfactory

• Passes through the cribriform plate of the
  ethmoid bone
• Fibers run through the olfactory bulb and
  terminate in the primary olfactory cortex
• Functions solely by carrying
• afferent impulses for the
• sense of smell

26
Cranial Nerve II Optic

• Arises from the retina of the eye
• Optic nerves pass through the optic canals and
  converge at the optic chiasm
• They continue to the thalamus where they synapse
• From there, the optic radiation fibers run to the
  visual cortex
• Functions solely by carrying afferent impulses
  for vision

27
Cranial Nerve III Oculomotor

• Fibers extend from the ventral midbrain, pass
  through the superior orbital fissure, and go to
  the extrinsic eye muscles
• Functions in raising the eyelid, directing the
  eyeball, constricting the iris, and controlling
  lens shape
• Parasympathetic cell bodies are in the ciliary
  ganglia

• Proprioceptor afferents from extrinsic eye muscles

28
Cranial Nerve IV Trochlear

• Fibers emerge from the dorsal midbrain and enter
  the orbits via the superior orbital fissures
  innervate the superior oblique muscle
• Primarily a motor nerve that directs the eyeball

• Proprioceptor afferents from extrinsic eye muscles

29
Cranial Nerve V Trigeminal

• Composed of three divisions ophthalmic (V1),
  maxillary (V2), and mandibular (V3)
• Fibers run from the face to the pons via the
  superior orbital fissure (V1), the foramen
  rotundum (V2), and the foramen ovale (V3)
• Conveys sensory impulses from various areas of
  the face (V1) and (V2), and supplies motor fibers
  (V3) for mastication

30
Cranial Nerve VI Abdcuens

• Fibers leave the inferior pons and enter the
  orbit via the superior orbital fissure
• Primarily a motor nerve innervating the lateral
  rectus muscle

31
Cranial Nerve VII Facial

• Fibers leave the pons, travel through the
  internal acoustic meatus, and emerge through the
  stylomastoid foramen to the lateral aspect of the
  face
• Mixed nerve with five major branches
• Motor functions include facial expression, and
  the transmittal of autonomic impulses to lacrimal
  and salivary glands
• Sensory function is taste from the anterior
  two-thirds of the tongue

32
Cranial Nerve VIII Vestibulocochlear

• Fibers arise from the hearing and equilibrium
  apparatus of the inner ear, pass through the
  internal acoustic meatus, and enter the brainstem
  at the pons-medulla border
• Two divisions cochlear (hearing) and vestibular
  (balance)
• Functions are solely sensory equilibrium and
  hearing

33
Cranial Nerve IX Glossopharyngeal

• Fibers emerge from the medulla, leave the skull
  via the jugular foramen, and run to the throat
• Nerve IX is a mixed nerve with motor and sensory
  functions
• Motor innervates part of the tongue and
  pharynx, and provides motor fibers to the parotid
  salivary gland
• Sensory fibers conduct taste and general
  sensory impulses from the tongue and pharynx

34
Cranial Nerve X Vagus

• The only cranial nerve that extends beyond the
  head and neck
• Fibers emerge from the medulla via the jugular
  foramen
• The Vagus is a mixed nerve
• Most motor fibers are parasympathetic fibers to
  the heart, lungs, and visceral organs
• Its sensory function is in taste

35
Cranial Nerve XI Accessory

• Formed from a cranial root emerging from the
  medulla and a spinal root arising from the
  superior region of the spinal cord
• The spinal root passes upward into the cranium
  via the foramen magnum
• The accessory nerve leaves the cranium via the
  jugular foramen
• Primarily a motor nerve
• Supplies fibers to the larynx, pharynx, and soft
  palate
• Innervates the trapezius and sternocleidomastoid,
  which move the head and neck

36
Cranial Nerve XII Hypoglossal

• Fibers arise from the medulla and exit the skull
  via the hypoglossal canal
• Innervates both extrinsic and intrinsic muscles
  of the tongue, which contribute to swallowing and
  speech

37
End Chapter 13Cranial Nerve Slides