PNS and Sensory Receptors

PNS Organization

• The peripheral nervous system consists of cranial and spinal nerves
• The PNS functions as communication lines between the CNS and the rest of the body
• The sensory division carries impulses from receptors to the CNS
• The motor division conducts impulses from the CNS to effectors like muscles and glands
• Cranial nerves originate from the brain
• Spinal nerves originate from the spinal cord

Sensory Receptors

• Sensory receptors are specialized structures responding to stimuli and changes in the local environment
• Chemoreceptors are activated by changes in chemical levels such as O2, CO2, and hormones
• Photoreceptors respond to light
• Thermoreceptors are activated by changes in temperature
• Mechanoreceptors respond to movement like stretch, vibration, and touch
• Nociceptors respond to stimuli that causes damage, resulting in triggering pain
• Interoceptors respond to internal stimuli
• Exteroceptors respond to external stimuli
• Proprioceptors are scattered and monitor body position and movement
• Simple receptors are modified dendrite endings of sensory neurons
• Complex receptors include whole sense organs such as the eye and ear

Nerve Structure

• A nerve constitutes a bundle of neuronal axons enclosed by connective tissues
• Endoneurium is a loose connective tissue that surrounds a single axon
• Perineurium is a connective tissue that wraps fascicles of axons
• Epineurium is a connective tissue that encapsulates all fascicles and blood vessels

Nerve Types

• Sensory nerves carry afferent impulses towards the CNS
• Motor nerves carry efferent impulses away from the CNS
• Mixed nerves carry both afferent and efferent impulses to and from the CNS

Cranial Nerves

• There are twelve cranial nerves
• I. Olfactory (sensory)
• II. Optic (sensory)
• III. Oculomotor (motor)
• IV. Trochlear (motor)
• V. Trigeminal (both/mixed)
• VI. Abducens (motor)
• VII. Facial (both)
• VIII. Vestibulocochlear (sensory)
• IX. Glossopharyngeal (both)
• X. Vagus (both)
• XI. Accessory (motor)
• XII. Hypoglossal (motor)
• Mnemonic for remembering the cranial nerves: "On Occasion Our Trusty Truck Acts Funny Very Good Vehicle AnyHow"
• Mnemonic for remembering cranial nerve functions: "Some Say Marry Money But Mary's Brother Says Bad Business My Man"

Olfactory Nerve (I)

• The olfactory nerve provides sensory input regarding the sense of smell from the nasal cavity

Optic Nerve (II)

• The optic nerve provides sensory input regarding vision from the eyes' retina
• Nerves unite at the optic chiasm

Oculomotor Nerve (III)

• The oculomotor nerve provides motor commands to four of the eye muscles

Trochlear Nerve (IV)

• The trochlear nerve provides motor commands to one of the eye muscles

Trigeminal Nerve (V)

• The trigeminal nerve provides sensory input from the tongue, teeth, and skin of the face
• Provides motor commands to muscles involved in mastication

Abducens Nerve (VI)

• The abducens nerve provides motor commands to one of the eye muscles

Facial Nerve (VII)

• The facial nerve provides sensory input from taste buds
• Provides motor commands to muscles of facial expression

Vestibulocochlear Nerve (VIII)

• The vestibulocochlear nerve provides sensory input from the inner ear related to hearing and balance (equilibrium)

Glossopharyngeal Nerve (IX)

• The glossopharyngeal nerve provides sensory input from taste buds, tongue, and throat
• Provides motor commands to throat muscles and salivary glands

Vagus Nerve (X)

• The vagus nerve provides sensory input from taste buds, throat, and multiple organs
• Provides motor commands to throat muscles and various organs

Accessory Nerve (XI)

• The accessory nerve provides motor commands to muscles of the neck

Hypoglossal Nerve (XII)

• The hypoglossal nerve provides motor commands to the muscles of the tongue

Spinal Nerves

• There are 31 pairs of spinal nerves that emerge from the spinal cord
• 8 cervical pairs
• 12 thoracic pairs
• 5 lumbar pairs
• 5 sacral pairs
• 1 coccygeal pair
• Spinal nerves are mixed, carrying both sensory and motor impulses

Spinal Nerves

• A spinal nerve is formed by the fusion of a dorsal root (sensory fibers) and a ventral root (motor fibers)
• They quickly branch into rami

Spinal Nerve Branches

• Dorsal/Posterior Ramus: Carries sensory impulses from receptors in the skin and muscles of the body's posterior surface (back). Carries motor impulses to back muscles.
• Ventral/Anterior Ramus: Carries sensory impulses from receptors in the skin and muscles of the body's anterior and lateral surfaces. Carries motor impulses to anterior and lateral muscles.

Skin Innervation

• A dermatome is an area of skin innervated by a single spinal nerve
• Numbness along a dermatome indicates damage to a specific spinal nerve

Nerve Plexus

• A nerve plexus is an interlacing network of the ventral rami of spinal nerves, primarily innervating structures within the limbs
• Benefit of nerve plexus: damage to a single spinal nerve does not completely paralyze any limb muscle

Reflex Activity

• Reflexes are innate (inborn) or learned responses to specific stimuli
• Intrinsic: you are born with them, reflexes are fast, predictable, and involuntary
• Learned reflexes: reflexes that develop with life experience
• Somatic reflexes: activate skeletal muscles
• Autonomic reflexes: activate organs or glands

Reflex Arc

• The reflex arc is a sequence of neural structures involved in executing reflexes
• Receptor: structure that recognizes the stimulus
• Sensory neuron: carries afferent impulse to CNS
• Integration center: synapse within CNS
• Motor neuron: carries efferent impulse to effector organs
• Effector: muscle or gland that responds to the efferent impulse