Sensation- conscious (perception) Sensation- conscious (perception) or subconscious awareness or subconscious awareness of changes in environmentof changes in environment

Sensory modality- unique type of sensation• general senses- somatic (tactile, thermal, pain, proprioceptive) and visceral (internal organs)• special senses- smell, taste, vision, hearing, equilibrium

Process of sensation• begins with receptor (selective)- stimulus produces potential at threshold• CNS integrates impulse

Three types of receptors: free nerve endings, encapsulated nerve endings, separate cells- see figure 16.1

Can be grouped on location

• exteroceptors (external surface of body), • interoceptors (internal environment), • proprioceptors (muscles, tendons, joints)

Can be grouped by stimulus• mechanoreceptors (mechanical stimuli)• thermoreceptors (heat) • nociceptors (pain)• photoreceptors (light) • chemoreceptors (chemicals)• osmoreceptors (osmotic pressure)

Adaptation to maintained, constant stimulus (can be rapid or slow)

A dog (red line) tracks a pheasant (yellow line). As the dog keeps leaving the odour to prevent receptor adaptation, it zigzags.

 

Somatic sensations• Tactile: touch, tickle, pressure, vibration, itch (mechanoreceptors)

• touch: Meissner corpuscles and hair root plexuses are rapidly adapting, Merkel discs and Ruffini corpuscles are slowly adapting• pressure and vibration: Meissner corpuscles, Merkel discs, and lamellated corpuscles • itch and tickle: stimulation of free nerve endings

• Thermal: free nerve endings, cold in epidermis, warmth in dermis• Pain: free nerve endings everywhere except the brain- very little adaptation

• two types: fast ( sharp or prickling) and slow (aching or throbbing)

• Proprioceptive: know where body parts are and control equilibrium

Somatic Sensory Pathways- from somatic Somatic Sensory Pathways- from somatic receptors to cerebral cortex receptors to cerebral cortex (somatosensory area)(somatosensory area)

• First-order neuron- from somatic First-order neuron- from somatic receptor to brain stem (cranial) or spinal receptor to brain stem (cranial) or spinal cord (spinal nerves)cord (spinal nerves)• Second-order neuron- from brain stem Second-order neuron- from brain stem or spinal cord to thalamus- decussate or spinal cord to thalamus- decussate (cross over to other side)(cross over to other side)• Third-order neuron- from thalamus to Third-order neuron- from thalamus to primary somatosensory areaprimary somatosensory area

Three pathways to cerebrum and Three pathways to cerebrum and cerebellum- table 16.3cerebellum- table 16.3

• posterior column-medial posterior column-medial lemmniscus: fine touch, lemmniscus: fine touch, stereognosis (recognize by feel), stereognosis (recognize by feel), proprioception, vibrationproprioception, vibration

• anterolateral: crude touch impulses, anterolateral: crude touch impulses, pain, temperaturepain, temperature

• spinocerebellar tracts: spinocerebellar tracts: proprioceptive to cerebellumproprioceptive to cerebellum

can map somatosensory areas (lips and hands large area, trunk and limbs small area)

Somatic Motor pathways- provide imput to lower Somatic Motor pathways- provide imput to lower motor neuronsmotor neurons

• Local circuit neurons- coordinate rhythmic Local circuit neurons- coordinate rhythmic activityactivity• Upper motor neurons- planning, initiating, Upper motor neurons- planning, initiating, and directing sequences of voluntary and directing sequences of voluntary movementsmovements• Basal ganglia neurons- initiate and terminate Basal ganglia neurons- initiate and terminate movements, suppress unwanted movement, movements, suppress unwanted movement, establish muscle toneestablish muscle tone• Cerebellar neurons- monitor movement Cerebellar neurons- monitor movement (posture and balance)(posture and balance)• Direct (cerebral cortex- voluntary) and Direct (cerebral cortex- voluntary) and indirect (brain stem) pathwaysindirect (brain stem) pathways

Integrative functions of the Cerebrum• Wakefulness and sleep (circadian rhthyms): recticular formation to cerebral cortex• Learning and memory: due to plasticity, occurs in stages over time