Chapter 13: Peripheral Nervous System

Cervical Plexus

• Buried deep in the neck; Supply the muscles of the shoulder and neck. • It's single most important nerve is the phrenic nerve, which supplies both motor and sensory innervation to the diaphragm. • The primary danger of a broken neck is that the phrenic nerve may be severed. Severing of the phrenic nerve and/or C3-C5 results in paralysis of the diaphragm.

General Sensory Receptors

• Involved in tactile sensations (touch, pressure, stretch), temperature monitoring, pain, and muscle sensation. • These receptors are either free dendritic endings or encapsulated dendritic endings.

Patellar Reflex

Knee jerk reflex. Stretch reflex that helps keep knees from buckling when standing upright.

Nerve Classifications (3):

Nerves are classified according to the direction in which they transmit impulses. Most nerves are mixed. • Sensory (Afferent): Carry impulses towards the CNS. • Motor (Efferent): Carry impulses away from the CNS. • Mixed Nerves: Contain both sensory and motor fibers and transmit impulses in both directions.

Perineurium

A coarse connective tissue wrapping which binds together a group of fibers forming fascicles.

Endoneurium

A delicate, connective tissue that surrounds each nerve fiber. It also encloses the fiber's associated myelin and/or neurilemma sheath.

Epineurium

A dense, fiberous sheath of connective tissue that surrounds an entire nerve including fascicles and lymphatics.

Fiber Classifications (4):

Fibers can be classified further according to the region they innervate: • Somatic Afferent: Sensory impulses from the body surface. • Somatic Efferent: Motor impulses to the skeletal muscles. • Visceral Afferent: Sensory impulses from the internal organs. • Visceral Efferent: Motor impulses to the internal organs.

Superficial Reflex

Plantar reflex. Elicited by gentle cutaneous stimulation.

Joint Kinesthetic Receptors

Proprioceptors that monitor stretch in the articular capsules of synovial joints.

Crossed Extensor Reflex

Reflex combined with withdrawal reflex with a contralateral extensor reflex. Reflex when someone grabs your arm unexpectedly or you step on a piece of glass barefooted.

Pacinian Corpuscles

Scattered deep in the dermis and in subcutaneous tissue. Sense deep pressure.

Muscle Spindles

Sense muscle stretch.

Golgi Tendon Organs

Sense tendon stretch and tension.

Meissner's Corpuscles

Small, egg-shaped receptors found just beneath skin epidermis of the dermal papillae (discriminative touch).

Dermatomes

The area of skin innervated by the branches of a single nerve.

Sensory or Afferent Neurons (PNS)

The input component of the PNS consists of nerve cells called sensory of afferent neurons. They conduct nerve impulses from sensory receptors in various parts of the body to the CNS.

Motor or Efferent Neurons (PNS)

The output component of the PNS consists of nerve cells called motor or efferent neurons. They originate within the CNS and conduct nerve impulses from the CNS to muscles and glands.

Spinal Nerves

Thirty-one pairs of nerves arising from the fusion of the ventral and dorsal roots of the spinal cord. Supply all parts of the body, except the head and some areas of the neck. All spinal nerves are mixed nerves. Spinal nerves are named according to their point of issue from the spinal cord: • 8 pairs of cervical nerves (C1-C8) • 12 pairs of thoracic nerves (T1-T12) • 5 pairs of lumbar nerves (L1-L5) • 5 pairs of sacral nerves (S1-S5) • 1 pair of tiny coccygeal nerves

Location (Sensory Receptor Classification):

Three Classes Recognized: • Exteroceptors: Sensitive to stimuli arising outside the body. Receptors located at or near the body surface. Include touch, pressure, pain, temperature, and most special sense organs. • Interoceptors: Respond to stimuli arising from within the body. Excited by a variety of stimuli, including chemical, stretching, and temperature. Activation may cause the feeling of pain, discomfort, hunger, or thirst. • Proprioceptors: Special category that responds to internal stimuli, however, their location is restricted to the musculoskeletal system. Occur in skeletal muscles, tendons, joints, and ligaments. Constantly advise the brain on body movements.

Flexor Reflex

Withdrawal reflex. Initiated by a painful stimulus and causes automatic withdrawal of the threatened part from the stimulus.

Principal Divisions of the Nervous System (2):

• CNS consists of the brain and spinal cord. Within the CNS, various sorts of incoming sensory information are integrated and correlated. The CNS is connected to sensory receptors, muscles and glands in peripheral parts of the body by the PNS. • PNS consists of cranial nerves that arise from the brain and spinal nerves that emerge from the spinal cord. Portions of these nerves carry nerve impulses into the CNS, and other portions carry them out.

Encapsulated General Sensory Receptors

• Exhibit one or more terminal fibers enclosed in a C.T. capsule. • Virtually all are mechanoreceptors.

Cranial Nerves (12 Listed Nerves and Functions):

• I: Olfactory: Sensory for smell. • II:Optic: Sensory for vision. • III:Oculomotor: Directs eyeball, moves eyelid, focusing of lens. • IV: Trochlear: Motor fibers to superior oblique muscle of the eye. • V: Trigeminal: Major sensory nerve of face; Motor fibers to chewing muscles. • VI: Abducens: Abducts eye. • VII: Facial: Motor impulses to skeletal muscles of face; Sensory impulses from taste buds on anterior portion of tongue. • VIII: Vestibulocochlear: Sensory for equilibrium and hearing. • IX: Glossopharyngeal: Motor impulses for swallowing and gag reflexes; Sensory impulses from posterior tongue and pharynx. • X: Vagus: Motor and sensory to pharynx and larynx; Parasympathetic innervation of heart and abdominal organs. • XI: Accessory: Motor fibers to muscles of neck. • XII: Hypoglossal: Motor impulses for tongue functions of food manipulation, swallowing, and speech.

Deep Tendon Reflex

• Initiated by the Golgi tendon organ ( a special sensory receptor in tendons). • When muscle tension increases, the Golgi tendon organ is stimulated and causes the contracting muscle to relax and also reciprocal activation (the contraction of antagonistic muscles).

Stretch Reflex

• Initiated by the muscle spindle (a special sensory receptor in muscle) when the muscle is stretched. Used when carrying heavy weight. • The muscle spindle senses the stretch and caused the stretched muscle to contract in order to support the weight. • Also causes reciprocal inhibition (the relaxation of the antagonistic muscles).

Free General Sensory Receptors

• Invade virtually all body tissues; Particularly abundant in epithelia and connective tissues. • Sensory fibers have a small diameter and their distal dendrites usually have small, knob-like swellings. Respond to pain and temperature, but some respond to tissue movements. • Some are associated with disc-shaped epidermal cells (merkel cells) that attach to the deeper layers of skin epidermis and function as light touch receptors. • Root hair plexuses are light touch receptors that detect bending of hairs.

Brachial Plexuses (5):

• Located partially in the neck and partially in the axilla. Provides virtually all the nerves that innervate the upper limb. • Five nerves are especially important: 1. Axillary: Supplies the deltoid and teres minor and the skin and joint capsule of the shoulder. 2. Musculocutaneous: Supplies motor fibers to the arm muscles that flex the forearm. It provides cutaneous sensation to the lateral forearm. 3. Median: Gives off branches in the forearm to the skin and most of the flexor muscles. It activates muscles to probate the forearm, flex the wrist and fingers, and to oppose the thumb. 4. Ulnar: Produces wrist and finger flexion and adduction, and abduction of the medial fingers. 5. Radial: The largest nerve of the brachial plexus. It innervates all the extensor muscles of the upper limb.

Classifications of Sensory Receptors (3):

• Location in the Body • Type of Stimulus Detected • Relative Complexity of Their Structure

Stimulus Detected (Sensory Receptor Classification):

• Mechanoreceptors: Touch, Pressure, Vibrations, and Stretches. • Thermoreceptors: Sensitive to temperature changes. • Photoreceptors: Retina of the eye. • Chemoreceptors: Respond to chemicals in solution, molecule smells or taste, and changes in blood chemistry. • Nociceptors: Respond to potentially damaging stimuli that result in pain. Virtually all receptors function as nociceptors at one time or another.

Plexus

• Network of interlacing blood vessels or nerves. • A feature of all spinal nerves except T1-T12; acts as an advantage in that damage to me spinal segment or root cannot completely paralyze any limb muscle.

Peripheral Nervous System (PNS)

• Provides links to and from the external environment. It's nerve fibers connect virtually every part of the body, enabling the CNS to receive information and to carry out its decisions. • Includes: Neural structures outside of the brain and spinal cord. Includes sensory receptors, peripheral nerves and their associated ganglia, and efferent motor endings.

Components of a Reflex Arc

• Receptor: Site of the stimulus action. • Sensory Neurons: Transmit the afferent impulses to the CNS. • Integration Center: - Simple: May be a single synapse between the sensory neuron and a motor neuron. - Complex: Involves multiple synapses with chains of interneurons. The center is always within the CNS. • Motor Neuron: Conducts efferent impulses from the center to an effector organ. • Effector: Muscle fiber or gland cell that responds in a characteristic way.

Structural Complexity (Sensory Receptor Classification):

• Simple: Majority of receptors. Equivalent structurally to modified dendritic endings of sensory neurons. Found in the skin mucus membranes, muscles, and connective tissue. Monitor most types of general sensory information. • Complex: Actually sense organs - Localized collections of cells working together to accomplish a specific receptive process. Associated with special senses - Vision, smell, hearing, smell, and taste.

PNS Subdivisions (2):

• Somatic: Body Consists of sensory neurons that convey information from cutaneous and special sense receptors (primarily in the head, body wall, and extremities) to the CNS and motor neurons from the CNS that conduct impulses to skeletal muscles only. • Autonomic: Smooth/Cardiac Muscle, Glands Consists of sensory neurons that convey information from receptors primarily in the viscera (internal organs) to the CNS and motor neurons from the CNS that conduct impulses to smooth muscle, cardiac muscle, and glands.

Lumbosacral Plexus

• The sacral and lumbar plexuses overlap substantially. They mainly serve the pelvic region of the trunk and the lower limbs. • Lumbar plexuses arises from the first four lumbar spinal nerves. The major branches descend to innervate the anterior and medial thigh. The femoral nerve innervates the quadriceps (thigh flexors, knee extenders). The obturator nerve innervates the adductor muscles. • The sacral plexus arises from the spinal nerves L4-S4. About half the nerves serve the buttocks and the lower limbs, the rest innervate pelvic structures. The major branch of the sacral plexus is the sciatic nerve, the thickest and longest nerve in the entire body (actually made up of two nerves wrapped in a common sheath).

Cranial Nerves (Defining Factors):

• There are 12 pairs of nerves that are associated with the brain and pass through various foramina of the skull. • The first two pairs originate from the forebrain; the rest originate from the brain stem. • Cranial nerves serve only head and neck structures, except the Vagus Nerve, which serves abdominal and thoracic structures.