SENSES GENERAL

• Senses


  Means by which the brain receives information about the environment and the body
• General senses


  • Distributed over large part of body
  • Touch, pressure, temperature, proprioception, pain
  • Internal organs and consist mostly of pain and pressure
• Special senses


  • Smell
  • Taste
  • Sight
  • Hearing
  • Balance
• Sensation


  The conscious awareness of stimuli received by sensory receptors
• Perception


  Understanding what is happening around us. Awareness of something and knowing that something exists or is actually happening
• For a number of years, the sense of touch was considered a special sense. New evidence suggests it is part of the general senses
• Action and equilibrium or balance is now considered the fifth special sense to replace the sense of touch
• Sensory receptors


  Nerve endings, or specialized cells capable of responding to stimuli by developing action potentials
• Types of sensory receptors

  • Mechanoreceptors (Compression, bending, stretching of cells)
  • Chemoreceptors (Smell and taste)
  • Thermoreceptors (Temperature)
  • Photoreceptors (Light as vision)
  • Nociceptors (Pain)
  • Exteroreceptors (Associated with skin)
  • Visceroreceptors (Associated with organs)
  • Proprioceptors (Associated with joints, tendons)
• Sensory receptors are associated with both the general and special senses, and respond to different types of stimuli
• Sensory receptors can be large complex organs (eyes, ears) or localized clusters of receptors (taste buds, olfactory epithelium)
• Free nerve endings


  Structurally the simplest and most common receptors, similar to dendrites, distributed throughout the body, can respond to painful stimuli, temperature, itch, and movement
• Cold receptors


  • Respond to decreasing temperatures, stop responding below 12°C (54°F)
• Warm receptors


  • Respond to increasing temperatures, stop responding above 47°C (170°F)
• At temperatures below 12°C or above 47°C, only pain receptors are stimulated, so it is difficult to distinguish cold from warm
• Touch receptors


  Structurally more complex than free nerve endings, many enclosed by capsules
• Types of touch receptors

  • Merkel's disk
  • Hair follicle receptor
  • Pacinian corpuscle
  • Meissner's corpuscle
  • Ruffini's end organ
  • Muscle spindle
  • Golgi tendon organ
• Merkel's disk


  Small, superficial nerve endings, respond to light touch and superficial pressure
• Hair follicle receptor


  Associated with hairs, respond to light touch and bending of hair
• Light touch receptors


  Very sensitive but not very discriminative, cannot precisely locate the point of touch
• Pacinian corpuscle


  Deepest receptors, associated with tendons and joints, relay information about deep pressure, vibration, and proprioception
• Proprioception


  The perception or self-awareness of the position and movement of the body, also called kinesthesia
• Meissner's corpuscle


  Receptors for fine discriminative touch, located just deep to the epidermis, more numerous in the tongue and fingertips, allow two-point discrimination
• Two-point discrimination


  The body's ability to detect simultaneous stimulation of two points on the skin
• Ruffini's end organ

  Deeper tactile receptors, respond to continuous touch or pressure, located in the dermis of the skin, primarily in the fingers
• Muscle spindle


  Proprioceptive receptors in skeletal muscles, provide information about muscle stretch and control of muscle tone, activate stretch reflex
• Golgi tendon organ

  Proprioceptive receptors associated with tendon fibers, activated by increased tendon tension from muscle contraction or passive stretch
• Sensory receptors


  Produce a response or series of responses, which we call sensations
• Types of receptors

  • Primary
  • Secondary
• Primary receptors


  • Have axons that conduct action potential in response to receptor potential
  • Many primary receptors, such as mechanoreceptors, photoreceptors, thermoreceptors, and the others, activate the receptors immediately, causing an action potential
  • Immediate action potential in response to the receptor potential
• Secondary receptors


  • Have no axons and receptor potentials produced do not result in action potentials but cause release of neurotransmitters
  • Examples would involve the chemoreceptors such as those involved in the taste and the smell sensations
  • When taste cells receive chemical information or chemical stimuli, they would now release neurotransmitters in the synaptic cleft that stimulates the sensory neuron, and when the stimulation reaches threshold, the neurons will generate action potentials that will be propagated towards the CNS and where it is processed. Hence sensation is felt.
  • Will have first to release neurotransmitters in order for the potentials to be propagated towards the CNS.
• Accommodation or adaptation


  Decreased sensitivity to a persistent or continued stimulus
• Accommodation or adaptation


  • When we get dressed, initially the texture of the clothes is discernible, but as we continue wearing them throughout the day, we do not necessarily get bothered by the texture anymore
• Types of proprioceptors

  • Tonic
  • Phasic
• Tonic proprioceptors


  • An example is you know where little finger is without looking
  • Slow in adaptation
• Phasic proprioceptors

  • An example is you know where hand is as it moves
  • Fast adapting and are most sensitive to changes in stimuli
• We are not really conscious about tonic and phasic proprioception, because the higher brain centers would ignore them most of the time. However, through selective and deliberate awareness, we can actually call up the information when we wish.
• Ascending pathways


  • Transmit action potentials from the periphery to brain
  • Each pathway is involved with specific modalities
• Anterior Spinothalamic Tract


  Front side and arising from the spine going into the thalamus, and then later on into the cerebral cortex
• Spinothalamic System


  • Conveys cutaneous sensory information to the brain
  • Unable to localize source of stimulus because there are many nerve fibers that overlap within this tract