Neuro Lecture Review

Peripheral Nervous System (PNS)

  • PNS is outside the brain and spinal cord.
  • Includes:
    • Sensory receptors
    • Peripheral nerves
    • Associated ganglia
    • Motor endings
  • Provides links to and from the external environment.

PNS in the Big Picture

  • The nervous system is divided into:
    • Central nervous system (CNS)
    • Peripheral nervous system (PNS)
  • PNS divisions:
    • Sensory (afferent) division
    • Motor (efferent) division
  • Motor division is further divided into:
    • Somatic nervous system
    • Autonomic nervous system (ANS)
  • ANS divisions:
    • Sympathetic division
    • Parasympathetic division

General Sensory Receptors Classified by Structure and Function

  • Nonencapsulated (free) nerve endings:
    • Free nerve endings of sensory neurons:
      • Location: Exteroceptors, interoceptors, and proprioceptors
      • Stimulus: Thermoreceptors (warm and cool), chemoreceptors (itch, pH, etc.), mechanoreceptors (pressure), nociceptors (pain, hot, cold, pinch, and chemicals)
      • Body location: Most body tissues; most dense in connective tissues (ligaments, tendons, dermis, joint capsules, periostea) and epithelia (epidermis, cornea, mucosae, and glands)
    • Modified free nerve endings: Tactile (Merkel) discs:
      • Tactile cell and tactile disc
      • Location: Exteroceptors
      • Stimulus: Mechanoreceptors (light pressure); slowly adapting
      • Body location: Basal layer of epidermis
    • Hair follicle receptors:
      • Location: Exteroceptors
      • Stimulus: Mechanoreceptors (hair deflection); rapidly adapting
      • Body location: In and surrounding hair follicles
  • Encapsulated nerve endings:
    • Tactile (Meissner's) corpuscles:
      • Location: Exteroceptors
      • Stimulus: Mechanoreceptors (light pressure, discriminative touch, vibration of low frequency); rapidly adapting
      • Body location: Dermal papillae of hairless skin, particularly nipples, external genitalia, fingertips, soles of feet, eyelids
    • Lamellar (Pacinian) corpuscles:
      • Location: Exteroceptors, interoceptors, and some proprioceptors
      • Stimulus: Mechanoreceptors (deep pressure, stretch, vibration of high frequency); rapidly adapting
      • Body location: Dermis and hypodermis; periostea, mesentery, tendons, ligaments, joint capsules; most abundant on fingers, soles of feet, external genitalia, nipples
    • Bulbous corpuscles (Ruffini endings):
      • Location: Exteroceptors and proprioceptors
      • Stimulus: Mechanoreceptors (deep pressure and stretch); slowly or nonadapting
      • Body location: Deep in dermis, hypodermis, and joint capsules
    • Muscle spindles:
      • Intrafusal fibers
      • Location: Proprioceptors
      • Stimulus: Mechanoreceptors (muscle stretch, length)
      • Body location: Skeletal muscles, particularly in the extremities
    • Tendon organs:
      • Location: Proprioceptors
      • Stimulus: Mechanoreceptors (tendon stretch, tension)
      • Body location: Tendons
    • Joint kinesthetic receptors:
      • Location: Proprioceptors
      • Stimulus: Mechanoreceptors and nociceptors
      • Body location: Joint capsules of synovial joints

Classification of Sensory Receptors

  • By location:
    • Exteroceptors: Near or at the body surface; touch, pain, pressure, temperature receptors in the skin.
    • Interoceptors: Visceral receptors, monitor chemical, tissue, and temperature stimuli.
    • Proprioceptors: Constantly advise the brain and cerebellum of movements.
  • By stimulus type:
    • Thermoreceptors, chemoreceptors, mechanoreceptors, nociceptors, photoreceptors.
  • By structural complexity:
    • Simple (unencapsulated or encapsulated).
    • Complex (special senses).

Sensory Receptors

  • Monitor the environment.
  • Structures are specialized to respond to stimuli.
  • Activation of receptors = Depolarizations that trigger impulses to the CNS.
  • Sensory Integration: from Sensation to Perception.
    • Sensation (awareness of stimuli in the Thalamus).
    • Perception (Interpretation/understanding in the Cortex).

Sensation to Perception

  • Survival depends upon Sensation/Perception.
  • Sensation = awareness of changes in the internal and external environment.
    • The receptor has flagged a change, but awareness occurs a split second later (Thalamus).
  • Perception = conscious interpretation of those stimuli occurring in the Cortex.
    • Occurs the moment of understanding.

Organization of the Somatosensory System

  • Input comes from exteroceptors, proprioceptors, and interoceptors.
  • Three main levels of neural integration in the somatosensory system:
    • Receptor level: Sensory receptors.
    • Circuit level: Ascending pathways (“wiring” pathway).
    • Perceptual level: Neuronal circuits in the cerebral cortex.

Processing at the Receptor Level

  • Receptor = Specificity for that particular stimulus energy.
  • Receptor’s Receptive field: must be stimulated (the smaller the field = more accurate localization).
  • Stimulus energy converted into graded potential (Transduction).
  • A generator potential in the associated sensory neuron must reach threshold (sums up to elicit an AP).

Adaptation of Sensory Receptors

  • Tonic Receptors (Equilibrium): always on, alterations in frequency.
  • Phasic Receptors: normally “off” (Touch) / Report a change.
  • Adaptation: occurs when sensory receptors are subjected to an unchanging stimulus.
    • Receptor membranes become less responsive.
    • Receptor potentials decline in frequency/stop.
    • Pain receptors DO NOT adapt.
    • Slowly adapt/Not at all: Merkel’s discs, Ruffini’s corpuscles, and interoceptors that respond to chemical levels in the blood.
    • Adapt quickly: Pressure/Touch/Smell.

Processing at the Perceptual Level

  • Thalamus projects fibers to:
    • Somatosensory cortex.
    • Sensory association areas.
  • The brain interprets the activity of a specific receptor ALWAYS the same based on where it went (occipital lobe interprets visual signals).
  • The result is an internal, conscious image of the stimulus.
  • Projection: the brain refers sensations to their usual point of stimulation.

Main Aspects of Sensory Perception

  • Perceptual detection: detecting that a stimulus has occurred.
  • Magnitude estimation: how intense a stimulus is (frequency of APs).
  • Spatial discrimination: identifying the site or pattern of the stimulus.
    • 2-Point discrimination test.
    • Varies depending on the site (Tongue - 5mm5mm / Back - 50mm50mm).
  • Feature abstraction: used to identify a substance that has a specific texture or shape.
  • Quality discrimination: the ability to identify submodalities of a sensation (e.g., sweet or sour tastes).
    • Taste is the Modality / Type of taste is the Submodality (bitter, salty).
  • Pattern recognition: ability to recognize patterns in stimuli (e.g., melody, familiar face).

Nerve Structure

  • Epineurium: surrounds the entire nerve
  • Perineurium: surrounds fascicles of nerve fibers
  • Endoneurium: surrounds individual nerve fibers (axons)
  • Contains:
    • Axons
    • Nerve fibers
    • Blood Vessels
    • Myelin Sheath

Regeneration of Nerve Fibers

  • If the damage is not severe, axons can regenerate.
    • 1. The axon becomes fragmented at the injury site.
    • 2. Macrophages clean out the dead axon distal to the injury.
    • 3. Axon sprouts, or filaments grow through a regeneration tube formed by Schwann cells.
    • 4. The axon regenerates and a new myelin sheath forms.

Cranial Nerves

  • I - Olfactory:
    • Sensory - Smell
    • Not Motor; Not Parasympathetic
  • II - Optic:
    • Sensory - Vision
    • Not Motor; Not Parasympathetic
  • III - Oculomotor:
    • Motor; Parasympathetic
    • Not Sensory
  • IV - Trochlear:
    • Motor
    • Not Sensory; Not Parasympathetic
  • V - Trigeminal:
    • Sensory - General Sensation
    • Motor
    • Not Parasympathetic
  • VI - Abducens:
    • Motor
    • Not Sensory; Not Parasympathetic
  • VII - Facial:
    • Sensory - Taste
    • Motor; Parasympathetic
  • VIII - Vestibulocochlear:
    • Sensory - Hearing and Balance
    • Some Motor
    • Not Parasympathetic
  • IX - Glossopharyngeal:
    • Sensory - Taste
    • Motor; Parasympathetic
  • X - Vagus:
    • Sensorytaste
    • Motor; Parasympathetic
  • XI - Accessory:
    • Motor
    • Not Sensory; Not Parasympathetic
  • XII - Hypoglossal:
    • Motor
    • Not Sensory; Not Parasympathetic

Cranial Nerves - Detailed

  • I - Olfactory:
    • Origin: Olfactory receptor cells in the olfactory epithelium of the nasal cavity, passing through the cribriform plate of the ethmoid bone to synapse in the olfactory bulb.
    • Function: Purely sensory; carries afferent impulses for the sense of smell.
    • Clinical Testing: Identify aromatic substances.
    • Homeostatic Imbalance: Anosmia (partial or total loss of smell) due to fracture of the ethmoid bone or lesions of olfactory fibers.
  • II - Optic:
    • Origin: Retina of the eye, passing through the optic canal of the orbit, converging to form the optic chiasma where fibers partially cross over, continuing on as optic tracts to the thalamus.
    • Function: Purely sensory; carries afferent impulses for vision.
    • Clinical Testing: Eye chart, testing visual fields, ophthalmoscope to detect papilledema.
    • Homeostatic Imbalance: Blindness due to optic nerve damage; anopsias (visual defects) due to damage to the visual pathway beyond the optic chiasma.
  • III - Oculomotor:
    • Origin: Ventral midbrain, passing through the superior orbital fissure to the eye.
    • Function: Chiefly motor; contains proprioceptive afferents.
      • Somatic motor fibers to four of the six extrinsic eye muscles (inferior oblique and superior, inferior, and medial rectus muscles) and to the levator palpebrae superioris muscle.
      • Parasympathetic motor fibers to sphincter pupillae and ciliary muscle.
      • Sensory (proprioceptor) afferents from the same four extrinsic eye muscles to the midbrain.
    • Clinical Testing: Examination of pupils for size, shape, and equality; pupillary reflex with penlight; convergence for near vision; ability to follow objects with the eyes.
    • Homeostatic Imbalance: Oculomotor nerve paralysis, eye cannot be moved up, down, or inward, rotates laterally (external strabismus), upper eyelid droops (ptosis), double vision, and trouble focusing on close objects.
  • IV - Trochlear:
    • Origin: Dorsal midbrain, coursing ventrally around the midbrain to enter the orbit through the superior orbital fissure along with the oculomotor nerves.
    • Function: Primarily motor; supplies somatic motor fibers to the superior oblique muscle (one of the extrinsic eye muscles) and carries proprioceptor fibers from it.
    • Clinical Testing: Tested in common with cranial nerve III.
    • Homeostatic Imbalance: Double vision and reduced ability to rotate the eye inferolaterally due to trauma or paralysis of the trochlear nerve.
  • V - Trigeminal:
    • Homeostatic Imbalance: Trigeminal neuralgia (tic douloureux) caused by inflammation of the trigeminal nerve, produces excruciating pain. Analgesics and anticonvulsants are only partially effective. Surgery may be required.
    • Divisions:
      • Ophthalmic division (V1)
      • Maxillary division (V2)
      • Mandibular division (V3)
  • VI - Abducens:
    • Origin: Inferior pons entering the orbit via the superior orbital fissure to run to the eye.
    • Function: Primarily motor; supplies somatic motor fibers to the lateral rectus muscle (an extrinsic muscle of the eye); conveys proprioceptor impulses from the same muscle to the brain.
    • Clinical Testing: Tested in common with cranial nerve III.
    • Homeostatic Imbalance: Eye cannot be moved laterally due to abducens nerve paralysis; at rest, the affected eyeball rotates medially (internal strabismus).
  • VII - Facial:
    • Origin: Pons, entering the temporal bone via the internal acoustic meatus and running within the bone before emerging through the stylomastoid foramen, then coursing to the lateral aspect of the face.
    • Function: Mixed nerves; chief motor nerves of the face with five major branches (temporal, zygomatic, buccal, mandibular, and cervical).
      • Conveys motor impulses to skeletal muscles of the face (muscles of facial expression) and transmits proprioceptor impulses from the same muscles to the pons.
      • Transmits parasympathetic motor impulses to lacrimal glands, nasal and palatine glands, and submandibular and sublingual salivary glands.
      • Conveys sensory impulses from taste buds of the anterior two-thirds of the tongue.
    • Clinical Testing: Testing the anterior two-thirds of the tongue for the ability to taste sweet, salty, sour, and bitter substances. Checking for symmetry of the face; asking the subject to close eyes, smile, whistle, etc. Assessing tearing with ammonia fumes.
    • Homeostatic Imbalance: Bell's palsy, characterized by paralysis of facial muscles on the affected side and partial loss of taste sensation. Often caused by herpes simplex 1 viral infection.
  • VIII - Vestibulocochlear:
    • Origin: Hearing and equilibrium apparatus located within the inner ear of the temporal bone and passing through the internal acoustic meatus to enter the brain stem at the pons-medulla border. The cochlear division arises from hearing receptors in the cochlea, and the vestibular division arises from equilibrium receptors in the semicircular canals and vestibule.
    • Function: Mostly sensory; vestibular branch transmits afferent impulses for the sense of equilibrium, and the cochlear branch transmits afferent impulses for the sense of hearing. A small motor component adjusts the sensitivity of sensory receptors.
    • Clinical Testing: Hearing checked by air and bone conduction using a tuning fork.
    • Homeostatic Imbalance: Lesions of the cochlear nerve or cochlear receptors result in central or nerve deafness, whereas damage to the vestibular division produces dizziness, rapid involuntary eye movements, loss of balance, nausea, and vomiting.
  • IX - Glossopharyngeal:
    • Origin: Medulla, leaving the skull via the jugular foramen to run to the throat.
    • Function: Mixed nerves that innervate part of the tongue and pharynx.
      • Provides somatic motor fibers to the stylopharyngeus muscle and carries proprioceptor fibers from it.
      • Provides parasympathetic motor fibers to the parotid salivary glands.
      • Sensory fibers conduct taste and general sensory impulses from the pharynx and posterior tongue, from chemoreceptors in the carotid body, and from baroreceptors of the carotid sinus.
    • Clinical Testing: Checking the position of the uvula; gag and swallowing reflexes; asking the subject to speak and cough; testing the posterior third of the tongue for taste.
    • Homeostatic Imbalance: Injury or inflammation of glossopharyngeal nerves impairs swallowing and taste.
  • X - Vagus:
    • Origin: Medulla, passing through the skull via the jugular foramen and descending through the neck region into the thorax and abdomen.
    • Function: Mixed nerves; nearly all motor fibers are parasympathetic efferents, except those serving skeletal muscles of the pharynx and larynx.
      • Parasympathetic motor fibers supply the heart, lungs, and abdominal viscera and are involved in the regulation of heart rate, breathing, and digestive system activity.
      • Transmits sensory impulses from thoracic and abdominal viscera, from the aortic arch baroreceptors, and the carotid and aortic bodies, and taste buds of the posterior tongue and pharynx.
      • Carries proprioceptor fibers from muscles of the larynx and pharynx.
    • Clinical Testing: As for cranial nerve IX (IX and X are tested in common).
    • Homeostatic Imbalance: Vagal nerve paralysis can lead to hoarseness or loss of voice, difficulty swallowing, and impaired digestive system motility. Total destruction of both vagus nerves is incompatible with life.
  • XI - Accessory:
    • Origin: Unique in that they are formed from ventral rootlets that emerge from the spinal cord (C1-C5). They pass upward along the spinal cord and enter the skull as the accessory nerves via the foramen magnum. They exit from the skull through the jugular foramen together with the vagus nerves and supply two large neck muscles.
    • Function: Mixed nerves, but primarily motor in function. Supply motor fibers to the trapezius and sternocleidomastoid muscles, which together move the head and neck, and convey proprioceptor impulses from the same muscles.
    • Clinical Testing: Sternocleidomastoid and trapezius muscles are checked for strength by asking the person to rotate their head and shrug their shoulders against resistance.
    • Homeostatic Imbalance: Injury to the spinal root of one accessory nerve causes the head to turn toward the injury side due to sternocleidomastoid muscle paralysis; shrugging of that shoulder becomes difficult.
  • XII - Hypoglossal:
    • Origin: Medulla, exiting from the skull via the hypoglossal canal to travel to the tongue.
    • Function: Mixed nerves, but primarily motor in function. Carry somatic motor fibers to intrinsic and extrinsic muscles of the tongue and proprioceptor fibers from the same muscles to the brain stem.
    • Clinical Testing: The person is asked to protrude and retract their tongue. Any deviations in position are noted.
    • Homeostatic Imbalance: Damage to hypoglossal nerves causes difficulties in speech and swallowing. If both nerves are impaired, the person cannot protrude their tongue. If only one side is affected, the tongue deviates (points) toward the affected side; eventually, the paralyzed side begins to atrophy.

Spinal Nerves

  • Cervical: C1-C8
  • Thoracic: T1-T12
  • Lumbar : L1-L5
  • Sacral : S1-S5
  • Coccygeal: Co1

Spinal Cord Structure

  • The dorsal and ventral roots arise medially as rootlets and join laterally to form the spinal nerve.
  • Dorsal root ganglion
  • Dorsal ramus of spinal nerve
  • Ventral ramus of spinal nerve
  • Spinal nerve
  • Gray matter
  • White matter
  • Ventral root
  • Rami communicantes
  • Sympathetic trunk ganglion
  • Dorsal and ventral rootlets of spinal nerve

Spinal Nerve Branches

  • Dorsal Ramus: Supplies the posterior body trunk
  • Ventral Ramus: Supplies the anterior body trunk and limbs
  • Meningeal Branch: Reenters the vertebral canal to innervate the meninges and blood vessels
  • Rami Communicantes: Autonomic nerve fibers that attach to the ventral rami; lead to the sympathetic trunk ganglia

Cervical Plexus

  • Formed by ventral rami of C1-C5
  • Segmental Branches
  • Innervates:
    • Neck
    • Ear Area
    • Back of Head
    • Shoulder
  • Major Nerves:
    • Lesser occipital
    • Greater auricular
    • Transverse cervical
    • Ansa cervicalis
    • Supraclavicular
    • Phrenic
    • Hypoglossal Nerve (XII)
    • Accessory Nerve (XI)

Brachial Plexus

  • Located in the neck and axilla
  • Formed by ventral rami of C5-T1
  • Major nerves:
    • Axillary
    • Musculocutaneous
    • Median
    • Ulnar
    • Radial

Lumbar Plexus

  • Arises from L1-L4
  • Innervates:
    • Thigh
    • Abdominal wall
    • Psoas Muscle
  • Major nerves:
    • Iliohypogastric
    • Ilioinguinal
    • Femoral
    • Lateral femoral cutaneous
    • Obturator
    • Anterior femoral cutaneous
    • Saphenous

Sacral Plexus

  • Arises from L4-S4
  • Innervates the buttock, lower limb, pelvic structures, and perineum
  • Major nerves:
    • Superior gluteal
    • Inferior gluteal
    • Pudendal
    • Sciatic
    • Posterior femoral cutaneous
    • Common fibular
    • Tibial
    • Sural
    • Deep fibular
    • Superficial fibular
    • Plantar branches

Dermatomes

  • Area of skin innervated by the cutaneous branches of a single spinal nerve
  • All spinal nerves except C1 are involved

Hierarchy of Motor Control

  • Precommand Level (highest):
    • Cerebellum
    • Basal nuclei
    • Programs and Instructions (modified by feedback)
  • Projection Level (middle):
    • Motor cortex (pyramidal system) and brain stem nuclei
    • Convey instructions to spinal cord motor neurons and send copy of that information to higher levels.
  • Segmental Level (lowest):
    • Spinal cord
    • Contains central pattern generators (CPGs)
    • Reflex Activity

Segmental Level

  • The lowest level of motor hierarchy
  • Segmental circuits of the spinal cord
  • Circuits control locomotion and specific, oft-repeated motor activity
  • Circuits are called central pattern generators (CPGs)

Projection Level

  • Consists of:
    • Cortical motor areas that produce the direct (pyramidal) system
    • Brain stem motor areas that oversee the indirect (multineuronal) system
  • Helps control reflex and fixed-pattern activity and houses command neurons that modify the segmental apparatus

Precommand Level

  • Cerebellar and basal nuclei systems that:
    • Regulate motor activity
    • Precisely start or stop movements
    • Coordinate movements with posture
    • Block unwanted movements
    • Monitor muscle tone

Reflex Arc

  • Five Components:
    • Receptor: Site of stimulus.
    • Sensory Neuron: Transmits the afferent impulse to the CNS.
    • Integration Center: Monosynaptic/Polysynaptic region within the CNS.
    • Motor Neuron: Conducts efferent impulses from the integration center to the effector.
    • Effector: Muscle fiber or gland that responds to the efferent impulse.

Muscle Spindle and Golgi Tendon Organs

  • Muscle Spindle:
    • Stretching muscles activates the muscle spindle.
      • Lengthen muscle or shorten antagonistic muscle.
    • Activating motor neurons that stimulate distal ends of intrafusal fibers to contract.
    • Length of muscle
      • Intrafusal fibers are noncontractile and receptive surfaces of spindles.
  • Golgi tendon organ:
    • Amount of tension in the muscle and associated tendon.

Muscle Stretch Reflex

  • When muscle spindles are activated by stretch, the associated sensory neurons transmit afferent impulses at a higher frequency to the spinal cord.
  • The sensory neurons synapse directly with alpha motor neurons, which excite extrafusal fibers of the stretched muscle. Afferent fibers also synapse with interneurons that inhibit motor neurons controlling antagonistic muscles.
  • Efferent impulses of the alpha motor neurons cause the stretched muscle to contract, which resists or reverses the stretch.
  • Efferent impulses of alpha motor neurons to antagonist muscles are reduced (reciprocal inhibition).

Golgi Tendon Reflex

  • Quadriceps strongly contracts. Golgi tendon organs are activated.
  • Afferent fibers synapse with interneurons in the spinal cord.
  • Efferent impulses to muscle with stretched tendon are damped. Muscle relaxes, reducing tension.
  • Efferent impulses to the antagonist muscle cause it to contract.

Flexor (Withdrawal) Reflex

  • A noxious stimulus causes a flexor reflex on the same side, withdrawing that limb.
  • Site of reciprocal activation: At the same time, the extensor muscles on the opposite side are activated.