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 (V 1 ), maxillary (V 2 ), and mandibular (V 3 ) Fibers run from the face to the pons via the superior orbital fissure (V 1 ), the foramen rotundum (V 2 ), and the foramen ovale (V 3 ) Conveys sensory impulses from various areas of the face (V 1 ) and (V 2 ), and supplies motor fibers (V 3 ) 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

Sympathetic

Origin of Fibers

Thoracolumbar region of the spinal cord

Length of Fibers

Short preganglionic and long postganglionic

Location of Ganglia

Close to the spinal cord in the sympathetic trunk (chain) Parasympathetic Brain and sacral spinal cord Long preganglionic and short postganglionic 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  heart is stimulatory receptors of the

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