Sensory Pathways and the Somatic Nervous System Flashcards
Neural pathways
- Information arrives by sensory receptors
- Information ascends within the afferent division
- Motor commands descend and are distributed by the efferent division
Sensory receptor
Specialised cell or cell process that monitors the specific conditions within the body or in the external environment
Sensation
Arriving information
Perception
Awareness of a sensation
General senses
Sensitivity to temperature, pain, touch, pressure, vibration, and proprioception
Special senses
Located in specific sense organs
Structurally more complex than general senses
Receptive field
Each receptor cell monitors a specific receptive field
Transduction
Begins when a large enough stimulus depolarises the receptor potential or generator potential to the point where action potentials are produced
Labeled line
A link between a peripheral receptor and a cortical neuron
Tonic receptors
- Always active
- Slow-adapting receptors
Phasic receptors
Provide information about the intensity and rate of change of a stimulus
Adaptation
- A reduction in sensitivity in the presence of a constant stimulus
- Fast-adapting receptors
Nociceptors
Provide information of pain as related to extremes of temperature, mechanical damage and dissolved chemicals
Myelinated type A fibres
Carry fast pain
Unmyelinated type C fibres
Carry slow pain
Thermoreceptors
Found in the dermis
Mechanoreceptors
- Sensitive to distortion of their membranes
- Include tactile receptors (6 types), baroreceptors, proprioceptors (3 types)
Chemoreceptors
Include carotid bodies and aortic bodies
First-order neurons
Sensory neurons that deliver sensation to the CNS
Second-order neurons
In the brainstem and spinal cord
Third-order neurons
In the thalamus
Spinothalmic pathway
Carries poorly localised (“crude”) sensations of touch, pressure, pain and temperature
Axons involved decussate in the spinal cord and ascend within the anterior and lateral spinothalmic tracts to the ventral nuclei of the thalamus
Phantom limb syndrome
Caused by abnormalities along spinothalmic pathway
Painful sensations that are perceived despite a missing limb and referred pain
Referred pain
Inaccurate localisations of the source of pain
Posterior column pathway
Carries fine touch, pressure and proprioceptive sensations
Axons ascend within the gracile fasciculus and cuneate fasciculus and relay information to the thalamus by the medical lemniscus
Decussation
Before the axons enter the medical lemniscus, they cross over to the opposite side of the brainstem
Spinocerebellar pathway
- Includes posterior and anterior spinocerebellar tracts
- Carries sensations to the cerebellum concerning the position of muscles, tendons, and joints
Visceral sensory pathways
- Carry information collected by interocceptors
- Sensory information from cranial nerves V, VII, IX, and X is delivered to the solitary nucleus in the medulla oblongata
- Posterior roots of spinal nerves T1-L2 carry visceral sensory information from organs between the diaphragm and the pelvic cavity
- Posterior roots of spinal nerves S2-S4 carry sensory information from more inferior structures
Somatic motor
Involve an upper motor neuron (whose cell body lies in a CNS processing centre) and a lower motor neuron (whose cell body is located in a nucleus of the brainstem or spinal cord)
Pyramidal cells
Neurons of the primary motor cortex
Corticospinal pathway
Provides voluntary skeletal muscle control
Corticobulbar tracts
- End at the cranial nerve nuclei
- Synapse on lower motor neurons in the anterior horns of the spinal cord
- Visible along the medulla oblongata as a pair of thick bands called pyramids, where most of the axons decussate to enter lateral corticospinal tracts. Those that do not cross over enter the anterior corticospinal tracts
- Provides a rapid, direct mechanism for controlling skeletal muscles
Medial and lateral pathways
Include several other centres that issue motor commands as a result of processing performed at subconscious level
Medial pathways
- Primarily controls gross movements of the neck, trunk and proximal limbs
- Includes the vestibulospinal, tectospinal, and medial and lateral reticulospinal tracts
Vestibulospinal tracts
Carry information related to maintaining balance and posture
Tectospinal tracts
Commands carried by tectospinal tracts change position of the head, neck, and upper limbs in response to bright lights, sudden movements, or loud noises
Reticulospinal tracts
- Makes up the lateral pathway
- Primarily control muscle tone and movements of the distal muscles of the upper limbs
Basal nuclei
Adjust the motor commands issued in other processing centres and provide background patterns of movement involved in voluntary motor activities
Cerebellum
- Monitors proprioceptive sensations, visual information, and vestibular (balance) sensations
- Precise control of movement
