Neural Integration : Sensory Pathways and the Somatic Nervous System

An Overview of Sensory Pathways and the Somatic Nervous System

Neural pathways

• Afferent pathways

• Sensory information coming from the sensory receptors through peripheral nerves to the spinal cord and on to the brain

• Efferent pathways

• Motor commands coming from the brain and spinal cord, through peripheral nerves to effecter organs

Sensory Receptors and their Classification

Sensory receptor

• Specialized cell or cell process that monitors specific conditions

• Arriving information is a sensation

• Awareness of a sensation is a perception

Senses

• General senses

• Pain

• Temperature

• Physical distortion

• Chemical detection

• Receptors for general senses scattered throughout the body

• Special senses

• Located in specific sense organs

• Structurally complex

Sensory receptors

• Each receptor cell monitors a specific receptive field

• Transduction

• A large enough stimulus changes the receptor potential, reaching generator potential

Receptors

• Tonic receptors

• Always active

• Slow acting receptors

• Phasic receptors

• Provide information about the intensity and rate of change of a stimulus

• Fast acting receptors

• Adaptation

• Reduction in sensitivity in the presence of a constant stimulus

The general senses

• Three types of nociceptor

• Provide information on pain as related to extremes of temperature

• Provide information on pain as related to extremes of mechanical damage

• Provide information on pain as related to extremes of dissolved chemicals

• Myelinated type A fibers carry fast pain

• Slower type C fibers carry slow pain

Thermoceptors and mechaniceptors

• Found in the dermis

• Mechaniceptors

• Sensitive to distortion of their membrane

• Tactile receptors (six types)

• Baroreceptors

• Proprioceptors (three groups)

Chemoreceptors

• Chemoreceptors

• Carotid bodies

• Aortic bodies

Neural Integration I: Sensory Pathways and the Somatic Nervous System

The Organization of Sensory Pathways

First, second, and third order neurons

• First order neurons

• Sensory neurons that deliver sensory information to the CNS

• Second order neurons

• First order neurons synapse on these in the brain or spinal cord

• Third order neurons

• Found in the thalamus

• Second order neurons synapse on these

Somatic sensory pathways

• Three major pathways carry sensory information

• Posterior column pathway

• Anterolateral pathway

• Spinocerebellar pathway

Posterior column pathway

• Carries fine touch, pressure and proprioceptive sensations

• Axons ascend within the fasciculus gracilis and fasciculus cuneatus

• Relay information to the thalamus via the medial lemniscus

• Decussation

Anterolateral pathway

• Carries poorly localized sensations of touch, pressure, pain, and temperature

• Axons decussate in the spinal cord and ascend within the anterior and lateral spinothalamic tracts

• Headed toward the ventral nuclei of the thalamus

Spinocerebellar pathway

• Includes the posterior and anterior spinocerebellar tracts

• Carries sensation to the cerebellum concerning position of muscles, tendons and joints

Visceral sensory pathways

• Carry information collected by interoceptors

• Information from cranial nerves V, VII, IX and X delivered to solitary nucleus in medulla oblongata

• Dorsal roots of spinal nerves T1 – L2 carry visceral sensory information from organs between the diaphragm and pelvis

• Dorsal roots of spinal nerves S2 – S4 carry sensory information below this area

The Somatic Nervous System

Somatic motor pathways

• Upper motor neuron

• Cell body lies in a CNS processing center

• Lower motor neuron

• Cell body located in a motor nucleus of the brain or spinal cord

The corticospinal pathway

• Provides voluntary skeletal muscle control

• Corticobulbar tracts terminate at cranial nerve nuclei

• Corticospinal tracts synapse on motor neurons in the anterior gray horns of the spinal cord

• Visible along medulla as pyramids

Pyramids

• Most of the axons decussate to enter the descending lateral corticospinal tracts

• Those that do not cross over enter the anterior corticospinal tracts

• Provide rapid direct method for controlling skeletal muscle

medial and lateral pathways

• The medial and lateral pathways

• Issue motor commands as a result of subconscious processing

• Medial pathway

• Primarily controls gross movements of the trunk and proximal limbs

• Includes the vestibulospinal tracts, tectospinal tracts and reticulospinal tracts

lateral pathways

• Lateral pathway

• Controls muscle tone and movements of the distal muscles of the upper limbs

• Rubrospinal tracts

The basal nuclei and cerebellum

• Basal nuclei adjust motor commands issued in other processing centers

• Provide background patterns of movement involved in voluntary motor movements

• Cerebellum monitors proprioceptive information, visual information and vestibular sensations

control and responses

• Levels of processing and motor control

• Spinal and cranial reflexes provide rapid, involuntary, preprogrammed responses

• Voluntary responses

• More complex

• Require more time to prepare and execute

During development

• Spinal and cranial reflexes are first to appear

• Complex reflexes develop as CNS matures and brain grows