A&P Unit 4: Chapter 16 Sensory Pathways and the SNS

Sensory Receptors

Specialized cells that monitor specific conditions in the body or external environment

General Senses

Temperature, pain, touch, pressure, vibration, proprioception

Special Senses

Olfaction (smell), vision (sight), gustation (taste), equilibrium (balance), hearing

Free Nerve Endings

Branching tips of dendrites, not protected by accessory structures, can be stimulated by many different stimuli

Tonic Receptors

Are always active

Phasic Receptors

Are normally inactive, become active for a short time whenever a change occurs

Adaptation

Reduction in sensitivity of a constant stimulus

Fast-Adapting Receptors

Response characteristic of phasic receptors

4 Types of General Senses

Nociceptors, Thermoreceptors, Mechanoreceptors, and Chemoreceptors

Nociceptors

Pain, common in superficial portions of skin, joint capsules, outside of bones, and around the walls of blood vessels

Fast Pain

Myelinated type A fibers, sensations reach CNS quickly and often trigger somatic reflexes

Slow Pain

Type C fibers, burning/aching pain, only have a general idea of the area affected

Slow-Adapting Receptors

Tonic, show little peripheral adaptation, remind you of an injury long after the initial damage has occured

Thermoreceptors

Temperature receptors, located in dermis, skeletal muscle, liver, and hypothalamus

Mechanoreceptors

Sensitive to stimuli that physically distort cell membrane, stretching, compression, and twisting

Receptors of Mechanoreceptors

Tactile Receptors, Baroreceptors, and Proprioceptors

Tactile Receptors

Provide the sensations of touch, pressure, and vibration

Fine Touch Receptors

Are extremely sensitive to location, shape, size, texture, and movement

Crude Touch Receptors

Provide poor localization, give little information about the stimulus

6 Types of Tactile Receptors

Free nerve endings, root hair plexus, tactile discs, tactile corpuscles, lamellated corpuscles, and ruffini corpuscles

Tactile Corpuscles

Meissner’s corpuscles, fine touch, adapt after one second

Lamellated Corpuscles

Pacinian corpuscles, sensitive to deep pressure

Ruffini Corpuscles

Distortion of the skin

Baroreceptors

Detect pressure changes in the walls of blood vessels and in portions of the digestive, reproductive, and urinary tracts

Proprioceptors

Monitor the positions of joints and muscles

Muscle Spindles

Monitors skeletal muscle length, trigger stretch reflexes

Golgi Tendon Organs

Monitors external tension developed during muscle contraction

Joint Capsule Receptors

Free nerve endings detect pressure, tension, traction, and movement of the joint

Chemoreceptors

Receptors monitor pH, carbon dioxide, and oxygen levels in arterial blood

Carotid Bodies

Near the origin of the internal carotid arteries on each side of the neck

Aortic Bodies

Between the major branches of the aortic arch

First-Order Neuron

Sensory neuron delivers sensations to CNS, from somatic receptors → spinal cord or brain stem

Second-Order Neuron

From spinal cord or brain stem → thalamus

Third-Order Neuron

If sensation is to reach our awareness, from thalamus → cerebral cortex

Somatic Sensory Pathways

Posterior Column Pathways, Anterolateral Pathway, and Spinocerebellar Pathway

Posterior Column Pathway

Carries sensations of highly localized (“fine”) touch, pressure, vibrations, and proprioception

Fasciculus Gracilis

Lower body

Fasciculus Cuneatus

Upper body

Decussation

Crossing over

Medial Lemniscus

Tract from medulla oblongata to thalamus

Sensory Homunculus

Functional map of the primary sensory cortex

Anterolateral Pathway

Anterior spinothalamic tract, lateral spinothalamic tract, and referred pain

Anterior Spinothalamic Tracts

Carries crude touch and pressure sensations

Lateral Spinothalamic Tracts

Carries pain and temperature sensations

Referred Pain

Individual feels pain in specific part of body surface

Spinocerebellar Pathway

Posterior spinocerebellar tract and anterior spinocerebellar tract

Posterior Spinocerebellar Tracts

Cerebellum receives same side trunk and lower limb proprioceptive information via the inferior cerebellar pednucle

Anterior Spinocerebellar Tracts

Cerebellum receives opposite side trunk and lower limb proprioceptive information via the superior cerebellar peduncle

Visceral Sensory Pathways

Monitors visceral tissues and organs, primarily within the thoarcic and abdominopelvic cavities

Solitary Nucleus

Large nucleus in medulla oblongata, major processing and sorting center for visceral sensory information

CN V, VII, IX, and X

Carry visceral sensory information

Somatic Nervous System

Controls contractions of skeletal muscles

Upper Motor Neuron

Cell body lies in a CNS processing center, axon synapses with lower motor neuron

Lower Motor Neuron

Axon innervates skeletal muscle

3 Integrated Motor Pathways

Corticospinal pathway, medial pathway, and lateral pathway

Corticospinal Pathway

Provides voluntary control over skeletal muscles

3 Pairs of Descending Tracts in Corticospinal Pathway

Corticobulbar tract, lateral corticospinal tract, and anterior corticospinal tract

Corticobulbar Tract

Provide conscious control over skeletal muscles (using cranial nerves)

Lateral and Anterior Corticospinal Tracts

Activates skeletal muscles on the opposite side for fast and skilled movements (writing)

Pyramids

Visable along the ventral surface of medulla oblongata as pair of thick bands

Motor Homunculus

Provides indication of degree of fine motor control available

Medial Pathway

Neck, trunk, and proximal limb muscles

Vestibulospinal Tracts

CN VIII, postion and movement of the head, maintain posture/balance

Tectospinal Tracts

Moves head and eyes towards stimuli

Reticulospinal Tracts

Controls muscle tone and visceral motor functions

Upper Motor Neurons of Medial Pathway

Vestibulospinal tracts, tectospinal tracts, and reticulospinal tracts

Lateral Pathway

Distal part of limbs

Rubospinal Tracts

Govern precise movements of distal parts of limbs

Basal Nuclei

Provide backgorun patterns of movement involved in voluntary motor activities

Cerebellum

Monitors proprioceptors (position) sensations, visual information from the eyes, and vestibular (balance) sensations from the inner ear