Download Nervous System - Alamo Colleges

Part II
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
Sensory Receptors
 Mechanoreceptors –sense mechanical force (touch,
pressure)
 Thermoreceptors –sense temperature changes
 Photoreceptors –sense light
 Chemoreceptors –respond to chemicals (taste, smell)
 Nociceptors –respond to pain
Classes of Receptors Based on Location
 Exteroceptors –respond to stimuli from outside the
body
 Interoceptors –respond to stimuli from within the
body
 Proprioceptors –found at skeletal muscles, joints,
tendons, and ligaments; keep the body aware of
movements and position
Nerves
 Nerve = bundle of axons
 Found only in PNS
 Endoneurium –surrounds each axon
 Perineurium –surrounds groups of axons called
fascicles
 Epineurium –surrounds outside of entire nerve
 Most nerves are both sensory and motor
 Ganglia = cell bodies found in PNS
Structure of a Nerve
Figure 13.3b
Cranial Nerves
 Associated with brain
 All are paired
 Serve only head and neck structures, except for
vagus (X)
 Olfactory, optic, and vestibulocochlear are purely
sensory
 Oculomotor, trochlear, abducens, accessory, and
hypoglossal are purely motor
 The rest are mixed
Olfactory Nerve (I)
 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
 Functions solely by carrying afferent impulses for the sense of
smell
Optic Nerve (II)
 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
Oculomotor (III)
 Fibers extend from the
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
Trochlear (IV)
 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
Trigeminal (V)
 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
Abducens (VI)
 Fibers leave the inferior pons and enter the orbit via the superior orbital
fissure
 Primarily a motor nerve innervating the lateral rectus muscle
Facial (VII)
 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 (temporal,
zygomatic, buccal,
mandibular, and cervical)
 Motor functions include
facial expression, and
stimulation of lacrimal and
salivary glands
 Sensory function is taste from
the anterior two-thirds of the
tongue
Vestibulocochlear (VIII)
 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
Glossopharyngeal (IX)
 Fibers emerge from the medulla, leave the skull via the jugular
foramen, and run to the throat
 Motor – innervates part of the tongue and pharynx
 Sensory – fibers conduct taste and general sensory impulses from the
tongue and pharynx
Vagus (X)
 The only cranial nerve that
extends beyond the head and
neck
 Fibers emerge from the
medulla via the jugular
foramen
 Most motor fibers are to the
heart, lungs, and visceral
organs
 Its sensory function is in
taste
Accessory (XI)
 Formed from a cranial root
(medulla) and a spinal root
(superior 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
Hypoglossal (XII)
 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
Spinal Nerves
 31 pairs (8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1
coccygeal)
 Dorsal roots = sensory fibers from dorsal root ganglia,
send receptive impulses to spinal cord
 Ventral roots = motor fibers from spinal cord,
innervate skeletal muscles, send impulses from spinal
cord
Spinal Nerves
Figure 13.6
Cervical Nerves
 Sensory branches – innervate skin of neck and
shoulders
 Motor branches – innervate neck muscles
 Phrenic nerve – part of cervical plexus; innervates
diaphragm
 Cervical nerves form part of brachial plexus, which
innervates upper limbs
 Median nerve – innervates most of forearm, responsible for
carpal tunnel
 Radial and ulnar nerves – also innervate arm and forearm
Cervical Plexus
Figure 13.8
Brachial Plexus: Distribution of Nerves
Figure 13.9c
Thoracic Nerves
 Mostly are responsible for innervation at each rib
 Each one provides a nerve supply to muscles, skin, and
abdominal wall
Lumbar Nerves
 Lumbar plexus – supplies thigh and abdominal wall
 Femoral nerve – nerve supply to thigh as well as part of
leg
 Obturator nerve – nerve supply to medial thigh and
adductor muscles
Lumbar Plexus
Sacral Nerves
 Sacral plexus – supplies buttocks, lower limbs and
pelvis
 Sciatic nerve – longest nerve in body, supplies whole
lower limb
 Tibial and fibular nerves – supply knee and leg
Sacral Plexus
Figure 13.11
Spinal Reflexes
 Not all nerve impulses travel to brain
 Reflex arc:
 Receptor – site of stimulus
 Sensory neuron –carries signal to spinal cord
 Integration center – sensory and motor neurons
connected by interneurons
 Motor neuron – conducts impulses to effector
 Effector – muscle fiber or gland cell that responds to
impulses
Reflex Examples
1.
2.
Stretch reflex – patellar reflex keeps knees straight
when standing up, if pressure is applied to tendon or
muscle is stretched, reflex contraction will occur
Withdrawal reflex – painful stimulus causes you to
withdraw from the stimulus; several muscle groups
are activated
Autonomic Nervous System
 The ANS consists of motor neurons that:
 Innervate smooth and cardiac muscle and glands
 Make adjustments to ensure optimal support for body
activities
 Operate via subconscious control
 Have viscera as most of their effectors
Autonomic Nervous System
 Somatic nervous system has motor neurons which
go from CNS all the way to effectors
 Autonomic nervous system has a chain of 2
neurons, a preganglionic neuron and a
postganglionic neuron
 Cell body of preganglionic neuron is in CNS
 Cell body of postganglionic neuron is outside of CNS
 Postganglionic axon extends to effector
Autonomic Nervous System
 All somatic motor neurons release acetylcholine
(ACh), which has an excitatory effect
 In the ANS:
 Preganglionic fibers release ACh
 Postganglionic fibers release norepinephrine or ACh and
the effect is either stimulatory or inhibitory
 ANS effect on the target organ is dependent upon the
neurotransmitter released and the receptor type of the
effector
Divisions of the ANS
 ANS divisions: sympathetic and parasympathetic
 The sympathetic mobilizes the body during extreme
situations
 The parasympathetic performs maintenance activities
and conserves body energy
 The two divisions counterbalance each other
Parasympathetic
 Concerned with keeping body energy use low
 Involves the D activities – digestion, defecation,
and diuresis
 Its activity is illustrated in a person who relaxes
after a meal
 Blood pressure, heart rate, and respiratory rates are low
 Gastrointestinal tract activity is high
 The skin is warm and the pupils are constricted
Sympathetic
 The sympathetic division is the “fight-or-flight”
system
 Involves E activities – exercise, excitement,
emergency, and embarrassment
 Promotes adjustments during exercise – blood flow
to organs is reduced, flow to muscles is increased
 Its activity is illustrated by a person who is
threatened
 Heart rate increases, and breathing is rapid and deep
 The skin is cold and sweaty, and the pupils dilate
Anatomy of ANS
Division
Origin of Fibers
Length of Fibers
Sympathetic
Thoracolumbar
region of the spinal
cord
Short preganglionic
and long
postganglionic
Parasympathetic
Brain and sacral spinal Long preganglionic
cord
and short
postganglionic
Location of
Ganglia
Close to the
spinal cord in the
sympathetic trunk
(chain)
In the visceral
effector organs
Neurotransmitters and Receptors
 Acetylcholine (ACh) and norepinephrine (NE) are
the two major neurotransmitters of the ANS
 ACh is released by all preganglionic axons and all
parasympathetic postganglionic axons
 Cholinergic fibers – ACh-releasing fibers
 Adrenergic fibers – sympathetic postganglionic
axons that release NE
 Neurotransmitter effects can be excitatory or
inhibitory depending upon the receptor type
Cholinergic Receptors
 The two types of receptors that bind ACh are nicotinic
and muscarinic
 These are named after drugs that bind to them and
mimic ACh effects
Nicotinic Receptors
 Nicotinic receptors are found on:
 Motor end plates (somatic targets)
 All ganglionic neurons of both sympathetic and
parasympathetic divisions
 The hormone-producing cells of the adrenal medulla
 The effect of ACh binding to nicotinic receptors is
always stimulatory
Muscarinic Receptors
 Muscarinic receptors occur on all effectors innervated
by parasympathetic system
 The effect of ACh binding:
 Can be either inhibitory or excitatory
 Depends on the receptor type of the target organ (for
example, inhibits heart activity, but stimulates digestive
tract activity)
Adrenergic Receptors
 The two types of adrenergic receptors are alpha and
beta
 Each type has two or three subclasses
(1, 2, 1, 2 , 3)
 Effects of NE binding to:
  receptors is generally stimulatory
  receptors is generally inhibitory
 A notable exception – NE binding to  receptors of the
heart is stimulatory
Sympathetic and Parasympathetic
Interaction
 Most visceral organs are innervated by both
sympathetic and parasympathetic fibers
 This results in dynamic antagonisms that precisely
control visceral activity
 Sympathetic fibers increase heart and respiratory
rates, and inhibit digestion and elimination
 Parasympathetic fibers decrease heart and
respiratory rates, and allow for digestion and the
discarding of wastes
CNS Control of the ANS
 The hypothalamus is the main integration center of
ANS activity, it controls:
 Heart activity and blood pressure
 Body temperature, water balance, and endocrine activity
 Emotional stages (rage, pleasure) and biological drives
(hunger, thirst, sex)
 Reactions to fear and the “fight-or-flight” system