Central Nervous System (CNS) Peripheral Nervous System (PNS)
The Peripheral Nervous System (PNS)
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Transcript of The Peripheral Nervous System (PNS)
The Peripheral Nervous System (PNS)
P A R T A
Peripheral Nervous System (PNS)
PNS – 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
PNS in the Nervous System
Figure 13.1
Sensory Receptors
Structures specialized to respond to stimuli
Activation of sensory receptors results in depolarizations that trigger impulses to the CNS
The realization of these stimuli, sensation and perception, occur in the brain
Receptor Classification by Stimulus Type Mechanoreceptors – respond to
touch, pressure, vibration, stretch, and itch
Thermoreceptors – sensitive to changes in temperature
Photoreceptors – respond to light energy (e.g., retina)
Chemoreceptors – respond to chemicals (e.g., smell, taste, changes in blood chemistry)
Nociceptors – sensitive to pain-causing stimuli
Receptor Class by Location: Exteroceptors
Respond to stimuli arising outside the body
Found near the body surface Sensitive to touch, pressure, pain,
and temperature Include the special sense organs
Receptor Class by Location: Interoceptors
Respond to stimuli arising within the body
Found in internal viscera and blood vessels
Sensitive to chemical changes, stretch, and temperature changes
Receptor Class by Location: Proprioceptors
Respond to degree of stretch of the organs they occupy
Found in skeletal muscles, tendons, joints, ligaments, and connective tissue coverings of bones and muscles
Constantly “advise” the brain of one’s movements
Receptors are structurally classified as either simple or complex
Most receptors are simple and include encapsulated and unencapsulated varieties
Complex receptors are special sense organs
Receptor Classification by Structural Complexity
Simple Receptors: Unencapsulated
Free dendritic nerve endingsRespond chiefly to temperature
and pain Merkel (tactile) discs Hair follicle receptors
Simple Receptors: Encapsulated
Meissner’s corpuscles (tactile corpuscles)
Pacinian corpuscles (lamellated corpuscles)
Muscle spindles, Golgi tendon organs, and Ruffini’s corpuscles
Joint kinesthetic receptors
Unencapsulated Receptors
Table 13.1.1
Simple Receptors:Encapsulated
Table 13.1.2
From Sensation to Perception
Sensation is the awareness of changes in the internal and external environment
Perception is the conscious interpretation of those stimuli
Organization of the Somatosensory System
Input comes from exteroceptors, proprioceptors, and interoceptors
The three main levels of neural integration in the somatosensory system are:Receptor level – the sensor
receptorsCircuit level – ascending pathwaysPerceptual level – neuronal circuits
in the cerebral cortex
Figure 13.2
Processing at the Receptor Lever The receptor must have specificity
for the stimulus energy The receptor’s receptive field
must be stimulated Transduction
Conversion of the energy of a stimulus into the energy of a nerve signal
Processing at the Receptor Lever
Receptor potentialIt is a graded potential happening on
a receptorDepolarization or hyperpolarization
Generator potential It is a receptor potential strong
enough to cause an action potential in an afferent fiber
Adaptation of Sensory Receptors
Adaptation is a reduction in sensitivity in the presence of a stimulusReceptor membranes become
less responsiveReceptor potentials decline in
frequency or stop
Adaptation of Sensory Receptors
Tonic receptors Have little peripheral adaptation
Chemical interoceptors Pain receptorsMacula in the vestibular apparatus
Proprioceptors
Adaptation of Sensory Receptors
Phasic receptors Are fast adapting receptors
PressureTouch Smell
Processing at the Circuit Level
Chains of three neurons that conduct sensory impulses to the cerebral cortex
First-order neurons – soma reside in dorsal root or cranial ganglia, and conduct impulses from the skin to the spinal cord or brain stem
Processing at the Circuit Level
Second-order neurons – soma reside in the dorsal horn of the spinal cord or medullary nuclei and transmit impulses to the thalamus or cerebellum
Third-order neurons – located in the thalamus and conduct impulses to the somatosensory cortex of the cerebrum
Processing at the Perceptual Level
The thalamus projects fibers to: The somatosensory cortexSensory association areas
The exact point in the cortex that is activated will refer to where in the body the stimulus is happening
The result is an internal, conscious image of the stimulus
Main Aspects of Sensory Perception
Perceptual detection – detecting that a stimulus has occurred and requires summation
Magnitude estimation =intensity of the stimulusFrequency of action potentials
Main Aspects of Sensory Perception
Spatial discrimination – identifies the location of the stimulus. It depends on the size of the receptor field. Two-point discrimination test –
smaller fields equals finer two-point discrimination test
Main Aspects of Sensory Perception
Feature abstraction – used to identify a specific feature of the stimulus (texture or shape)
Quality discrimination – the ability to identify submodalities of a sensation (e.g., sweet or sour tastes)
Pattern recognition – ability to recognize patterns in stimuli (e.g., melody, familiar face)
Structure of a Nerve Nerve – peripheral axons enclosed
by connective tissue Connective tissue coverings include:
Endoneurium – loose connective tissue that surrounds axons
Perineurium – coarse connective tissue that bundles fibers into fascicles
Epineurium – tough fibrous sheath around a nerve
Structure of a Nerve
Figure 13.3b
Classification of Nerves
Sensory (afferent) – carry impulse to the CNS
Motor (efferent) – carry impulses from CNS
Mixed nerves – carry somatic and autonomic (visceral) impulsesMost common type
Peripheral Nerves
The four types of mixed nerves are:Somatic
SensoryMotor
Visceral SensoryMotor
Peripheral nerves can be cranial or spinal
Regeneration of Nerve Fibers Mature neurons are amitotic If the soma remains intact, damage
can be repaired Steps
Separated ends seal themselvesWallerian degeneration of the
distal axon by macrophagesFormation of a regeneration tube
by the Schwann cellGuide the axon growth distally
Regeneration of Nerve Fibers
Figure 13.4
Regeneration of Nerve Fibers
Figure 13.4
Cranial Nerves
Twelve pairs of cranial nerves arise from the brain
They have sensory, motor, or both sensory and motor (mixed nerves) functions
Each nerve is identified by a number (I through XII) and a name
Cranial Nerves
Figure 13.5a
Summary of Function of Cranial Nerves
Figure 13.5b
Cranial Nerve I: Olfactory
Arises from the olfactory epithelium Passes through the cribriform plate
of the ethmoid bone Fibers run through the olfactory bulb
and terminate in the primary olfactory cortex
Function is the sense of smell
Cranial Nerve I: Olfactory
Figure I from Table 13.2
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 carry impulses for vision
Cranial Nerve II: Optic
Figure II from Table 13.2
Cranial Nerve III: Oculomotor
Motor for movements of the eyes Parasympathetic fibers innervate
the intrinsic muscles of the eyeConstricting the iris, and
controlling lens shape
Cranial Nerve III: Oculomotor
Figure III from Table 13.2
Cranial Nerve IV: Trochlear
Figure IV from Table 13.2
Cranial Nerve V: Trigeminal
Three divisions: ophthalmic (V1), maxillary (V2), and mandibular (V3)
Conveys sensory impulses from various areas of the face (V1) and (V2), and supplies motor fibers (V3) for mastication
Cranial Nerve V: Trigeminal
Figure V from Table 13.2
Cranial Nerve VI: Abducens• Primarily a somatic motor nerve
Figure VI from Table 13.2
Cranial Nerve VII: Facial
Somatic Motor to the muscles of facial expression, and the transmittal of
Visceral motor to lacrimal and salivary glands
Sensory function is taste from the anterior two-thirds of the tongue
Cranial Nerve VII: Facial
Figure VII from Table 13.2
Cranial Nerve VIII: Vestibulocochlear
Fibers arise from the hearing and equilibrium apparatus of the inner ear,
Two divisions – cochlear (hearing) and vestibular (balance)
A sensory nerve
Cranial Nerve VIII: Vestibulocochlear
Figure VIII from Table 13.2
Cranial Nerve IX: Glossopharyngeal Nerve IX is a mixed nerve with
motor and sensory functions Somatic Motor – innervates part of
the tongue and pharynx, and Visceral Motor fibers to the
parotid salivary gland Visceral Sensory –taste and
general sensory impulses from the tongue and pharynx
Cranial Nerve IX: Glossopharyngeal
Figure IX from Table 13.2
Cranial Nerve X: Vagus
The only cranial nerve that extends beyond the head and neck
The vagus is a mixed nerve Most visceral motor fibers are
parasympathetic fibers to the heart, lungs, and visceral organs
Its visceral sensory function is in taste
Cranial Nerve X: Vagus
Figure X from Table 13.2
Cranial Nerve XI: Accessory
Primarily a somatic motor nerve Supplies fibers to the larynx,
pharynx, and soft palateInnervates the trapezius and
sternocleidomastoid, which move the head and neck
Cranial Nerve XI: Accessory
Figure XI from Table 13.2
Cranial Nerve XII: Hypoglossal
Somatic motor innervates the muscles of the tongue, which contribute to swallowing and speech
Cranial Nerve XII: Hypoglossal
Figure XII from Table 13.2
The Peripheral Nervous System (PNS)
P A R T B
Spinal Nerves Thirty-one pairs of mixed nerves arise
from the spinal cord and supply all parts of the body except the head
They are named according to their point of issue8 cervical (C1-C8)12 thoracic (T1-T12)5 Lumbar (L1-L5)5 Sacral (S1-S5)1 Coccygeal (C0)
Spinal Nerves
Figure 13.6
Spinal Nerves: Roots
Each spinal nerve connects to the spinal cord via two medial roots
Each root forms a series of rootlets that attach to the spinal cord
Ventral roots arise from the anterior horn and contain motor (efferent) fibers
Dorsal roots arise from sensory neurons in the dorsal root ganglion and contain sensory (afferent) fibers
Spinal Nerves: Roots
Figure 13.7a
Spinal Nerves: Rami
The short spinal nerves branch into three or four mixed, distal ramiSmall dorsal ramusLarger ventral ramusRami communicantes at the
base of the ventral rami in the thoracic region visceral nerve fibers
Nerve Plexuses
All ventral rami except T2-T12 form interlacing nerve networks called plexuses
Plexuses are found in the cervical, brachial, lumbar, and sacral regions
Each resulting branch of a plexus contains fibers from several spinal nerves
Nerve Plexuses
Each muscle receives a nerve supply from more than one spinal nerve
Damage to one spinal segment cannot completely paralyze a muscle
The back is innervated by dorsal rami via several branches
The thorax is innervated by ventral rami T1-T12 as intercostal nerves
Intercostal nerves supply muscles of the ribs, anterolateral thorax, and abdominal wall
Spinal Nerve Innervation: Back, Anterolateral Thorax, and Abdominal Wall
Spinal Nerve Innervation: Back, Anterolateral Thorax, and Abdominal Wall
Figure 13.7b
Cervical Plexus Most branches are cutaneous
nerves of the neck, ear, back of head, and shoulders
The most important nerve of this plexus is the phrenic nerveMotor and sensory nerve of the
diaphragm
Cervical Plexus
Figure 13.8
Brachial Plexus
It gives rise to the nerves that innervate the upper limb
There are four major branches of this plexus Roots TrunksDivisions Cords
Brachial Plexus
Figure 13.9a
Brachial Plexus: Nerves Axillary Musculocutaneous Median Ulnar Radial
Brachial Plexus: Distribution of Nerves
Figure 13.9c
Brachial Plexus: Nerves
Figure 13.9b
Lumbar Plexus
Innervates the thigh, abdominal wall, and psoas muscle
The major nerves are the Femoral
For anterior thigh muscles Obturator
Adductors muscles
Lumbar Plexus
Figure 13.10
Sacral Plexus Serves the buttock, lower limb,
pelvic structures, and the perineum (pudendal nerve)
The major nerve is the sciatic, the longest and thickest nerve of the bodyLower limb (except anteromedial
thigh muscles) Branches into two nerves: the tibial
and the common fibular (peroneus)
Sacral Plexus
Figure 13.11
Dermatomes
A dermatome is the area of skin innervated by the cutaneous branches of a single spinal nerve
All spinal nerves except C1 participate in dermatomes
Dermatomes
Figure 13.12
Innervation of Joints
Hilton’s law: any nerve serving a muscle that produces movement at a joint also innervates the joint itself and the skin over the joint
Motor Endings
PNS elements that activate effectors by releasing neurotransmitters at:Skeletal muscles Smooth muscle and glands
Levels of Motor Control
The three levels of motor control areSegmental level
Spinal cord circuitProjection level
Pyramidal and extrapyramidal systems
Precommand levelCerebellum and basal nuclei
Hierarchy of Motor Control
Figure 13.13
Segmental Level
The segmental level 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
Projection Level
Controls the spinal cord Consists of:
Cortical motor areas that produce the direct (pyramidal) system
Brain stem motor areas that oversee the indirect (multineuronal) system
Send information to lower motor neurons and also to higher center
Precommand Level
Cerebellar and basal nuclei systems that:Regulate motor activityPrecisely start or stop movementsCoordinate movements with
postureBlock unwanted movementsMonitor muscle toneControl the output of the cortex
and brain stem motor centers
Reflexes
A reflex is a rapid, predictable motor response to a stimulus
Reflexes may: Be inborn (intrinsic) or learned
(acquired)Involve, peripheral nerves, brain
stem and spinal cord Somatic and visceral reflexes
Reflex Arc
There are five components of a reflex arcReceptorSensory neuron Integration center Motor neuron Effector
Reflex Arc
Figure 13.14
Somatic Reflexes
Spinal: Stretch reflex Golgi tendon reflex Withdrawal reflex Crossed-extensor reflex Superficial: Plantar
Babinski’s Abdominal
Stretch and Deep Tendon Reflexes
For skeletal muscles to perform normally: 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
Stretch reflex - monosynaptic Muscle Spindle Are composed of intrafusal muscle
fibers that lack myofilaments in their central regions, are noncontractile, and serve as receptive surfaces
Afferent fibers Motor fibers:
Extrafusal fibersIntrafusal fibers
Muscle Spindles
Figure 13.15
Operation of the Muscle Spindles
Stretching the muscles activates the muscle spindleThere is an increased rate of action
potential on sensory fibers Contracting the muscle reduces
tension on the muscle spindleThere is a decreased rate of action
potential on sensory fibers
Operation of the Muscle Spindle
Figure 13.17
Stretch Reflex - monosynaptic Stretching the muscle activates the
muscle spindle Excited motor neurons causes the
muscle to contract Afferent impulses from the spindle result
in inhibition of the antagonist Example: patellar reflex
Tapping the patellar tendon stretches the quadriceps and starts the reflex action
The quadriceps contract and the antagonistic hamstrings relax
Stretch Reflex
Figure 13.16
Golgi Tendon Reflex - polysynaptic
The opposite of the stretch reflex Contracting the muscle activates
the Golgi tendon organs Afferent Golgi tendon neurons are
stimulated, neurons inhibit the contracting muscle, and the antagonistic muscle is activated
As a result, the contracting muscle relaxes and the antagonist contracts
It moderates the muscle contraction
Golgi Tendon Reflex
Figure 13.18
Flexor ( Withdrawal) Reflexes
The flexor reflex happens on the limb receiving the painful stimulus Withdrawal reflex by contraction
of the flexor musclesReciprocal inhibition of the
extensorsPolysynaptic reflex
Crossed Extensor Reflex
The crossed extensor reflexHappens on the opposite limbContraction of the extensor
musclesRelaxation of the flexor musclesPolysynaptic
Crossed Extensor Reflex
106
Afferentfiber
Efferentfibers
Extensorinhibited
Flexorstimulated
Right arm(site of stimulus)
Left arm (site ofreciprocal activation)
Arm movements
Interneurons
Key:+ Excitatory synapse– Inhibitory synapse
Efferentfibers
FlexorinhibitedExtensorstimulated
+
–+
–
+
+
Flexes
Extends
Figure 13.19
Superficial Reflexes
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
Superficial Reflexes
Indirectly tests for proper corticospinal tract functioning
Babinski’s sign: abnormal plantar reflex indicating corticospinal damage where the great toe dorsiflexes and the smaller toes fan laterally
The Babinski Reflexes
Figure 13.23
Developmental Aspects of the PNS
Spinal nerves branch from the developing spinal cord and neural crest cellsSupply motor and sensory
function to developing muscles Cranial nerves innervate muscles of
the head
Developmental Aspects of the PNS
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