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| Peripheral Nervous System (PNS)– |
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– all neural structures outside the brain and spinal cord
Includes sensory receptors, peripheral nerves, associated ganglia, and motor endings
Provides links to and from the external environment |
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are structurally classified as either simple or complex
most are are simple and include encapsulated and unencapsulated varieties
Complex ones are special sense organs |
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| is the awareness of changes in the internal and external environment |
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| is the conscious interpretation of those stimuli |
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| receptor level, circuit level and perceptual level |
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| The three main levels of neural integration in the somatosensory system are |
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| neuronal circuits in the cerebral cortex |
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| from exteroceptors, proprioceptors, and interoceptors |
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| Processing at the Receptor Lever |
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The receptor must have specificity for the stimulus energy
The receptor’s receptive field must be stimulated |
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| Processing at the Receptor Lever |
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Stimulus energy must be converted into a graded potential
A generator potential in the associated sensory neuron must reach threshold |
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| Adaptation of Sensory Receptors |
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| occurs when sensory receptors are subjected to an unchanging stimulus |
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| Adaptation of Sensory Receptors |
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Receptor membranes become less responsive
Receptor potentials decline in frequency or stop |
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| Receptors responding to pressure, touch, and smell |
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| Receptors responding slowly include |
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| Merkel’s discs, Ruffini’s corpuscles, and interoceptors that respond to chemical levels in the blood |
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| Pain receptors and proprioceptors do not exhibit adaptation. Why is this statement true? |
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| Chains of three neurons conduct sensory impulses |
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| Processing at the Perceptual Level |
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| First one modality is sent, then those considering more than one |
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| Processing at the Perceptual Level |
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| The result is an internal, conscious image of the stimulus |
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| – detecting that a stimulus has occurred and requires summation |
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| – how much of a stimulus is acting |
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| – identifying the site or pattern of the stimulus |
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| – used to identify a substance that has specific texture or shape |
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| the ability to identify submodalities of a sensation (e.g., sweet or sour tastes) |
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| ability to recognize patterns in stimuli (e.g., melody, familiar face) |
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| cordlike organ of the PNS consisting of peripheral axons enclosed by connective tissue |
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| Connective tissue coverings include: |
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Endoneurium – loose connective tissue that surrounds axons
Perineurium – coarse connective tissue that bundles fibers into fascicles
Epineurium – tough fibrous sheath around a nerve
How do these relate to muscle coverings? |
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Sensory and motor divisions
Sensory (afferent) – carry impulse to the CNS
Motor (efferent) – carry impulses from CNS
Mixed – sensory and motor fibers carry impulses to and from CNS; most common type of nerve |
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| carry somatic and autonomic (visceral) impulses |
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| The four types of mixed nerves are: |
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Somatic afferent and somatic efferent
Visceral afferent and visceral efferent |
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| If the soma of a damaged nerve remains intact, damage can be |
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| nerves arise from the brain |
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| They have sensory, motor, or both sensory and motor functions |
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| carry parasympathetic fibers that serve muscles and glands |
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| Cranial Nerve III: Oculomotor |
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Definition
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 |
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| Cranial Nerve IV: Trochlear |
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Definition
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
The superior oblique muscle travels through a loop called the trochlea |
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| Cranial Nerve V: Trigeminal |
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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 |
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| Cranial Nerve VI: Abducens |
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Fibers leave the inferior pons and enter the orbit via the superior orbital fissure
Primarily a motor nerve innervating the lateral rectus muscle…helps the eye to “abduct”
If the nerve is damaged, the eye will rotate medially and lead to strabismus (crossed-eyes) |
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| Cranial Nerve VII: Facial |
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Definition
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. Damage leads to Bell’s palsy
Sensory function is taste from the anterior two-thirds of the tongue |
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| Cranial Nerve VIII: Vestibulocochlear |
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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 (equilibrium)
Functions are solely sensory: equilibrium and hearing |
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| Cranial Nerve IX: Glossopharyngeal |
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Definition
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 |
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| Cranial Nerve IX: Glossopharyngeal |
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functions
Motor – innervates part of the tongue and pharynx, and provides motor fibers to the parotid salivary gland |
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| Cranial Nerve IX: Glossopharyngeal |
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| Sensory – fibers conduct taste (posterior one-third) and general sensory impulses from the tongue and pharynx |
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The only cranial nerve that extends beyond the head and neck
Fibers emerge from the medulla via the jugular foramen
is a mixed nerve |
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| Motor – most motor fibers are parasympathetic fibers to the heart, lungs, and visceral organs |
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| Sensory – transmitting impulses (1) from thoracic and abdominal vicera, (2) from aortic baroreceptors and chemoreceptors, (3) from taste buds |
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| Cranial Nerve XI: Accessory |
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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
nerve leaves the cranium via the jugular foramen |
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| Cranial Nerve XI: Accessory |
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Primarily a motor nerve
Supplies fibers to the larynx, pharynx, and soft palate
Innervates the trapezius and sternocleidomastoid, which move the head and neck |
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| Cranial Nerve XII: Hypoglossal |
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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 |
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| Thirty-one pairs of mixed nerves arise from the spinal cord and supply all parts of the body except the head |
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| Each spinal nerve connects to the spinal cord via two |
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| forms a series of rootlets that attach to the spinal cord |
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| arise from the anterior horn and contain motor (efferent) fibers |
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| arise from sensory neurons in the dorsal root ganglion and contain sensory (afferent) fibers |
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Small dorsal ramus
Larger ventral ramus
Tiny meningeal branch
Rami communicantes at the base of the ventral rami in the thoracic region |
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| The short spinal nerves branch into three or four mixed, distal rami |
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| except T2-T12 form interlacing nerve networks called plexuses |
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| are found in the cervical, brachial, lumbar, and sacral regions |
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| Each resulting branch of a plexus contains fibers from several |
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Fibers travel to the periphery via several different routes
Each muscle receives a nerve supply from more than one spinal nerve
Damage to one spinal segment cannot completely paralyze a muscle |
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| The back is innervated by dorsal rami via |
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| supply muscles of the ribs, anterolateral thorax, and abdominal wall |
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plexus is formed by ventral rami of C1-C4
Most branches are cutaneous nerves of the neck, ear, back of head, and shoulders
The most important nerve of this plexus is the phrenic nerve |
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| is the major motor and sensory nerve of the diaphragm |
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Formed by C5-C8 and T1 (C4 and T2 may also contribute to this plexus)
It gives rise to the nerves that innervate the upper limb |
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There are four major branches of this plexus
Roots – five ventral rami (C5-T1)
Trunks – upper, middle, and lower, which form divisions
Divisions – anterior and posterior serve the front and back of the limb
Cords – lateral, medial, and posterior fiber bundles |
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| – innervates the deltoid and teres minor |
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| sends fibers to the biceps brachii and brachialis |
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| – branches to most of the flexor muscles of arm |
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| – supplies the flexor carpi ulnaris and part of the flexor digitorum profundus |
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| – innervates essentially all extensor muscles |
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Arises from L1-L4 and innervates the thigh, abdominal wall, and psoas muscle
The major nerves are the femoral and the obturator and saphenous |
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Arises from L4-S4 and serves the buttock, lower limb, pelvic structures, and the perineum
The major nerve is the sciatic, the longest and thickest nerve of the body |
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| is actually composed of two nerves: the tibial and the common fibular (peroneal) nerves |
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is the area of skin innervated by the cutaneous branches of a single spinal nerve
All spinal nerves except C1 participate in |
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PNS elements that activate effectors by releasing neurotransmitters at:
Neuromuscular junctions
Varicosities at smooth muscle and glands |
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| Innervation of Visceral Muscle and Glands |
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Acetylcholine and norepinephrine are used as neurotransmitters
Visceral responses are slower than somatic responses |
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| The three levels of motor control are |
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Segmental level
Projection level
Precommand level |
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is the lowest level of motor hierarchy
It consists of segmental circuits of the spinal cord
Its circuits control locomotion and specific, oft-repeated motor activity
These circuits are called central pattern generators (CPGs) |
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| The projection level consists of: |
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Cortical motor areas that produce the direct (pyramidal) system
Brain stem motor areas that oversee the indirect (multineuronal) system |
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| Helps control reflex and fixed-pattern activity and houses command neurons that modify the segmental apparatus |
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Cerebellar and basal nuclei systems that:
Regulate motor activity
Precisely start or stop movements
Coordinate movements with posture
Block unwanted movements
Monitor muscle tone |
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| There are five components of a reflex arc |
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Receptor
Sensory neuron
Integration center
Motor neuron
Effector |
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reflex arc
– site of stimulus |
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reflex arc
transmit the afferent impulse to the CNS |
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reflex arc
either monosynaptic or polysynaptic region within the CNS |
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reflex arc
conducts efferent impulses from the integration center to an effector |
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reflex arc
muscle fiber or gland that responds to the efferent impulse |
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| For skeletal muscles to perform normally: |
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The Golgi tendon organs (proprioceptors) must constantly inform the brain as to the state of the muscle
Stretch reflexes initiated by muscle spindles must maintain healthy muscle tone |
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| Are composed of 3-10 intrafusal muscle fibers that lack myofilaments in their central regions, are noncontractile, and serve as receptive surfaces |
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| are wrapped with two types of afferent endings: primary sensory endings of type Ia fibers and secondary sensory endings of type II fibers |
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These regions are innervated by gamma () efferent fibers
Note: contractile muscle fibers are extrafusal fibers and are innervated by alpha () efferent fibers |
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| Stretching the muscles activates the muscle spindle |
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| There is an increased rate of action potential in Ia fibers |
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| Contracting the muscle reduces tension on the muscle spindle |
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| There is a decreased rate of action potential on Ia fibers |
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Stretching the muscle activates the muscle spindle
Excited motor neurons of the spindle cause the stretched muscle to contract
Afferent impulses from the spindle result in inhibition of the antagonist |
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Example: patellar reflex
Tapping the patellar tendon stretches the quadriceps and starts the reflex action
The quadriceps contract and the antagonistic hamstrings relax |
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The opposite of the stretch reflex
Contracting the muscle activates the Golgi tendon organs |
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| Afferent Golgi tendon neurons are stimulated, neurons inhibit the contracting muscle, and the antagonistic muscle is activated |
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| As a result, the contracting muscle relaxes and the antagonist |
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| is initiated by a painful stimulus (actual or perceived) that causes automatic withdrawal of the threatened body part |
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| The crossed extensor reflex has two parts |
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The stimulated side is withdrawn
The contralateral side is extended |
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Initiated by gentle cutaneous stimulation
Example:
Plantar reflex is initiated by stimulating the lateral aspect of the sole of the foot
The response is downward flexion of the toes
Indirectly tests for proper corticospinal tract functioning |
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| abnormal plantar reflex indicating corticospinal damage where the great toe dorsiflexes and the smaller toes fan laterally |
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| Cranial nerves innervate muscles of the |
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| Developmental Aspects of the PNS |
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Distribution and growth of spinal nerves correlate with the segmented body plan
Sensory receptors atrophy with age and muscle tone lessens
Peripheral nerves remain viable throughout life unless subjected to trauma |
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