Chapter 13 Peripheral Nervous System (PNS) & Reflex Activity
Objectives
1. Define peripheral nervous system and list its components. 2. Define sensation and perception 3. Classify general sensory receptors by structure, stimulus detected, and body location. 4. Distinguish between receptor and generator potentials and sensory adaptation. 5. Compare and contrast the three main levels of neural integration. 6. Describe the four properties of a stimulus. 7. Distinguish between tonic and phasic receptors and the idea of adaptation. 8. Discuss pain (types, neurotransmitters, fast vs. slow pain, phantom & referred pain). 9. Define ganglion and indicate the general body location of ganglia. 10. Describe the general structure of a nerve. 11. Name the 12 pairs of cranial nerves; indicate the body region and structures innervated by each. 12. Describe the formation of a spinal nerve and the general distribution of its rami. 13. Define plexus. Name the major plexuses and describe the distribution and function of the peripheral nerves arising from each plexus. 14. Outline the three levels of the motor hierarchy. 16. Name the components of a reflex arc and distinguish between visceral and somatic reflexes.
Chapter 12 (CNS) 19. Describe the gross and microscopic structure of the spinal cord. 20. List the major spinal cord tracts, and classify each as a motor or sensory tract.

I Overview (Fig. 13.1) A. General properties of sensory systems 1. They begin with a stimulus, internal or external, that acts on a sensory receptor. 2. The receptor is a biological transducer that converts the stimulus into electrical graded potentials. 3. If the graded potentials are above threshold, action potentials pass from the receptor along an afferent sensory neuron to the CNS. 4. A sensation is the conscious or subconscious awareness of changes in the external or internal environment. Perception is the conscious awareness and interpretation of sensations and is primarily a function of the cerebral cortex. 5. At each synapse along the way, the nervous system can modulate and shape the information.

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II Part 1: Sensory Receptors & Sensation A. Sensory receptors 1. Sensory receptors vary widely in complexity, ranging from branched endings of a single sensory neuron to multicellular sense organs such as the ear (16,000 sensory receptors) or eye (126 million sensory receptors) (Lions and tigers and bears, oh my!) 2. Classification by stimulus type a. Mechanoreceptors (pressure, vibration, gravity, acceleration, sound, stretch) b. Thermoreceptors (varying degrees of heat) c. Photoreceptors (photons of light) d. Chemoreceptors (oxygen, pH, various organic molecules such as glucose) e. Nociceptors (NO-sih-SEP-turs) (tissue damage interpreted as pain) 3. Classification by location a. Exteroceptors b. Interoceptors (visceral pain, nausea, pressure) c. Proprioceptors (position/movements of body) 4. Classification by structural complexity a. Somatosensory receptors (Table 13.1) * A neuron with free or "naked" nerve endings (pain, temperature) * Encapsulated dendritic endings (wrapped in C.T.; mechanoreceptors for touch, pressure) b. Sense organs with highly specialized receptor cells (complex; nerve endings combined with epithelial, connective or muscular tissues that enhance or moderate the response to a stimulus) * The receptors for smell are neurons, but other senses use non-neural receptor cells that synapse onto sensory neurons. B. Sensory pathways carry information to the CNS (general organization of somatosensory system) 1. Sensory stimuli trigger action potentials in first-order (primary) sensory neurons. 2. In the spinal cord, most first-order sensory neurons synapse onto interneurons called second-order (secondary) sensory neurons. 3. Second-order sensory neurons project to other regions of the spinal cord or brain, where they synapse onto third-order (tertiary) sensory neurons. 4. Most sensory information project to the thalamus, which acts as a relay and processing station (note: only olfactory information is not routed through the thalamus). 5. Three main levels of neural integration (Fig. 13.2) a. Processing at the receptor level * Specificity of the receptor: each receptor type has an adequate stimulus, a particular form of energy to which it is most responsive. They also have a minimum stimulus required to activate them or threshold. * The receptor of each first-order neuron picks up information from a single area known as the receptive field of the receptor. In some regions of the skin, such as the arms and legs, many first-order sensory neurons converge on a single second-order sensory neuron, so the secondary receptive area is large. The most sensitive areas of the skin have smaller receptive fields, with perhaps a 1:1 relationship between first- order and second-order neurons (Two-point discrimination test; 2 mm to 40 mm) * Stimulus energy must be converted into a graded potential called a generator potential or receptor potential (see blow) in a process called transduction (conversion of stimulus energy into information that can be processed by the nervous system. What is the "language" of the nervous system, dear ones? The stimulus opens ion channels in the receptor membrane, either directly (ionotropic) or indirectly (metabotropic). In most cases, channel opening results in a net influx of Na+ into the receptor, depolarizing the membrane. In a few cases, the response to the stimulus is hyperpolarization when K+ leaves the cell. * The change in membrane potential is called the generator potential (somatic sense receptors) or receptor potential (special senses) * Four properties of a stimulus: * Sensory modality: which sensory neurons are activated. Each receptor type is most sensitive to a particular type of stimulus (e.g., touch, temperature, pain). The brain then associates a signal coming from a specific group of receptors with a specific modality. This 1:1 association of a receptor to a sensation is called labeled line coding. Stimulation of a cold receptor will always be perceived as cold, whether the actual stimulation was cold or whether an artificial depolarization of the receptor was the source of the stimulus. * Location of the stimulus: remember the somatosensory homunculus in the primary somatosensory cortex? And what about lateral inhibition (a process in which sensory neurons close to a stimulus are inhibited to intensify the perception of the stimulus) * Intensity and duration of the stimulus: number of receptors (population coding) and frequency of action potentials (frequency coding) * Tonic receptors * Phasic receptors: rapidly adapting (pressure, touch, smell) & slowly adapting (pain, body position, chemical composition of blood) b. Processing at the circuit level (ascending pathways): First-order (primary) sensory neurons bring information from somatic receptors to second-order (secondary) sensory neurons in the CNS. The location of the synapses between first-order and second-order neurons varies according to the type of receptor. Those neurons associated with receptors for nociception, temperature, and coarse touch synapse onto their secondary neurons shortly after entering the spinal cord. In contrast, most fine touch and proprioceptive neurons have very long axons that project up the spinal cord all the way to the medulla. All second-order neurons cross the midline of the body, so that sensations from the left side of the body are processed in the right hemisphere of the brain and vice versa. c. Processing at the perceptual level (cerebral cortex) * Perceptual detection: detection of a stimulus * Magnitude estimation: how intense * Spatial discrimination: site or pattern of stimulation * Feature abstraction: complex aspects of a sensation * Quality discrimination: ability to distinguish aspects a sensation * Pattern recognition: familiar pattern

[pic] C. Perception of pain 1. Nociceptors (pain receptors) are especially common in the superficial portions of the skin, in join capsules, in the periostea of bones, and around the walls of blood vessels. Other deep tissues and most visceral organs have few nociceptors. 2. Pain receptors are free nerve endings with large receptive fields. 3. Pain receptors may be sensitive to: * extremes of temperature * mechanical damage * dissolved chemicals, such as chemicals released by injured tissues (histamine, K+, ATP, prostaglandins, & bradykinin, the universal pain stimulus) 4. Two types of axons-type A and type C fibers-carry painful sensations: * Myelinated Type A fibers carry sensations of fast pain or prickling pain * Type C fibers carry sensations of slow pain or burning and aching pain 5. Pain receptors are tonic receptors (no significant peripheral adaptation) 6. Glutamate and/or substance P are the pain neurotransmitters. 7. The level of pain felt can be reduced by the release of the neuromodulators endorphins & enkephalins found in the limbic system, hypothalamus & reticular formation (periaqueductal gray matter of midbrain). These neuromodulators bind to the presynaptic membrane and prevent the release of substance P. 8. The lateral spinothalamic tract carries sensations of pain. Any abnormality along the pathway can result in inappropriate sensations or inaccurate localization of the source. * Phantom limb pain: caused by activity in the sensory neurons or interneurons along the spinothalamic pathway (labeled line) * Referred pain: pain felt in an uninjured part of the body when the pain actually originates at another location. For example, strong visceral pain sensations arriving at a segment of the spinal cord can stimulate interneurons that are part of the spinothalamic pathway. Activity in these interneurons leads to stimulation of the primary somatosensory cortex, so the individual feels pain in a specific part of the body surface.

III Part 2: Transmission Lines: Nerves & Their Structure & Repair A. Nerves & Associated Ganglia 1. Structure & classification [Note: Remember that a nerve fiber is the axon of a single neuron and a nerve is a collection of nerve fibers (a few to more than a million)] a. Structure (Fig. 13.3) * Endoneurium, perineurium & epineurium

[pic] b. Classification * Sensory (afferent) * Motor (efferent) * Mixed (most common) 2. Regeneration of nerve fibers (Fig. 13.4) a. In the PNS, Schwann cells play a part in repairing damaged nerves. In a process known as Wallerian degeneration the axon distal to the injury site degenerates and macrophages migrate into the area and clean up the debris. The Schwann cells proliferate and form a solid cellular cord (regeneration tube) that follows the path of the original axon. As the neuron recovers, its axon grows into the site of injury, and Schwann cells wrap around the axon. b. Limited regeneration occurs in the CNS due to the presence of assorted glial cells. B. Cranial Nerves (Fig. 13.5 & Table 13.2) 1. Mnemonics: Old Opie occasionally tries trigonometry and feels very gloomy, vague, and hypoactive Oh Oh Oh, To Touch And Feel Very Green Vegetables, Ah Heaven! Oh, Once One Takes The Anatomy Final, Very Good Vacations Are Heavenly!

Chapter 12 The Central Nervous System
V The Spinal Cord A. Gross anatomy & protection (Fig. 12.29) 1. General features a. Cervical & lumbar enlargements b. Conus medullaris c. Cauda equina d. Filum terminale e. Spinal nerves (roots) (31) [pic]

f. Anterior median fissure & posterior median sulcus (Fig. 12.31) g. Gray matter * Gray commissure * Anterior, lateral & posterior horns h. White matter * Anterior, lateral & posterior funiculus (columns) 2. Spinal meninges (Fig. 12.31) a. Dural mater: dense C.T. * Epidural space: between dura mater and walls of vertebral canal; contains areolar tissue blood vessels and adipose tissue (epidural block) b. Arachnoid ("spider") mater * Subarachnoid space: filled with CSF, which acts as a shock absorber and a diffusion medium for dissolved gases, nutrients, chemical messengers and waste products * Lumbar tap (Fig. 12.30) c. Pia mater * Denticulate ligaments: extend from the pia through the arachnoid to the dura (prevent lateral movement) 3. Meningitis: inflammation of the meninges due to bacterial or viral infection; can disrupt the normal circulation of CSF

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B. Cross-sectional anatomy 1. Organization of gray matter (Fig. 12.32) a. Sensory & motor nuclei b. Dorsal or posterior horns * Somatic & visceral sensory nuclei c. Ventral or anterior horns * Somatic motor nuclei d. Lateral horns (thoracic & lumbar) * Visceral motor nuclei [pic] 2. Organization of white matter (Fig. 12.33) a. White columns (funiculi): posterior, lateral & anterior * Tracts or fasciculi: bundle of axons relaying sensory or motor info * Ascending (sensory) & descending (motor)

[pic] C. Ascending tracts 1. Neuronal composition (Fig. 12.34) a. First-order neurons (cell bodies in dorsal root ganglia) b. Second-order neurons (cell bodies in dorsal horn) c. Third-order neurons (cell bodies in thalamus) 2. Somatic sensory pathways a. Spinothalamic pathway * Lateral spinothalamic tract: pain and temperature sensations b. Posterior column pathway c. Spinocerebellar pathway: proprioceptive input from Golgi tendon organs, muscle spindles, and joint capsules 3. Somatic motor pathways a. Corticospinal pathway (pyramidal): voluntary control of skeletal muscles; begins at the pyramidal cells of the primary motor cortex (upper motor neurons), descend along the ventral surface of the medulla oblongata (pyramids) with about 85% of the axons crossing over (decussate) to enter the lateral corticospinal tract b. Medial pathway c. Lateral pathway

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C. Spinal Nerves (Back to chapter 13)

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1. General features (Fig. 13.7) a. Proximal branches * Dorsal root * Dorsal root ganglion * Ventral root b. Distal branches * Dorsal ramus (dorsal muscles & joints) * Ventral ramus (ventral & lateral skin & muscles of trunk & limbs) * Meningeal branch (meninges, vertebrae)
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2. Innervation of specific body regions a. Nerve plexuses (Fig. 13.6) * Ventral rami (except thoracic) b. Back (Fig. 13.7) c. Anterolateral thorax & abdominal wall (Fig. 13.7) * Intercostal nerves d. Cervical plexus & neck (Fig. 13.8 & Table 13.3): ventral rami of spinal nerves C1-C5; innervate the muscles of the neck & extend into the thoracic cavity * Phrenic nerve: the major nerve of the cervical plexus; provides nerve supply to diaphragm, a key respiratory muscle. Hiccups are often caused by irritation of the phrenic nerve *Shingles: caused by herpes virus that also causes chickenpox. This virus attacks neurons within the dorsal root of spinal nerves and sensory ganglia of cranial nerves. e. Brachial plexus of pectoral girdle & upper limb (Table 13.4 & Fig. 13.9) * Axillary nerve * Musculocutaneous nerve * Median nerve * Ulnar nerve * Radial nerve * "Saturday night paralysis" f. Lumbosacral plexus of lower limb * Lumbar plexus (Table 13.5 & Fig. 13.10) * Femoral & obturator nerves * Sacral plexus (Table 13.6 & Fig. 13.11) * Sciatic nerve g. Innervation of skin: Dermatomes (derma = skin + tom = segment) (Fig. 13.12) h. Innervation of joints
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IV Part 3: Motor Endings & Motor Activity A. Peripheral Motor Endings 1. Innervation of skeletal muscle a. Neuromuscular junctions (Fig. 9.7) b. Innervation of visceral muscle & glands (Fig. 9.25) B. Overview of Motor Integration: From Intention to Effect 1. Levels of motor control (Fig. 13.13) a. The segmental level: neural circuits at various segments of spinal cord; control locomotion and specific, oft-repeated motor activity * Central pattern generators (CPGs): a network of neurons that can produce a repetitive and properly sequenced pattern of motor signals entirely by virtue of the connections that exist among the neurons in it, and without the necessity for external timing cues. CPGs are important in the generation of most, if not all, rhythmic behaviors, such as the running after a ball. b. The projection level (upper motor neurons of motor cortex & brain stem) c. The precommand level (cerebellum & basal nuclei) * Regulate motor activity * Precisely start and stop movements * Coordinate movements with posture * Block unwanted movements * Monitors muscle tone

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V Part 4: Reflex Activity A. The Reflex Arc: the "wiring" of a single reflex (rapid, automatic responses to specific stimuli) 1. Components of a reflex arc (Fig. 13.14) a. Arrival of a stimulus and activation of a receptor b. Activation of a sensory receptor c. Information processing d. Activation of a motor neuron e. Response of peripheral effector

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B. Classification of reflexes 1. Innnate (e.g., removal of hand from hot plate, blinking) vs. acquired (e.g., braking a car) 2. Somatic vs. visceral 3. Monosynaptic vs. polysynaptic 4. Spinal vs. cranial reflexes B. Spinal Reflexes 1. Stretch reflex (e.g., patellar reflex): muscle spindles