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Classification by Stimulus Type Mechanoreceptors — respond to touch, pressure, vibration, stretch, and itch Thermoreceptors — sensitive to changes in temperature Photoreceptors — respond to light energy (e.g., retina) Chemoreceptors — respond to chemicals (e.g., smell, taste, changes in blood chemistry) Nociceptors — (noci = harm) sensitive to pain- causing stimuli (e.g. extreme heat or cold, excessive pressure, inflammatory chemicals)
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The somatic sensory system
Sensory stimuli that reach the conscious level of perception Somatic senses of touch, temperature, pain, itch
and proprioception. Special senses
Receptor react to specific forms of energy
• Classification of receptors based on:– Stimulus type– Location– Structural complexity
Classification by Stimulus Type
• Mechanoreceptors — respond to touch, pressure, vibration, stretch, and itch
• Thermoreceptors — sensitive to changes in temperature
• Photoreceptors — respond to light energy (e.g., retina)
• Chemoreceptors — respond to chemicals (e.g., smell, taste, changes in blood chemistry)
• Nociceptors — (noci = harm) sensitive to pain-causing stimuli (e.g. extreme heat or cold, excessive pressure, inflammatory chemicals)
Classification by Location1. Exteroceptors
– Respond to stimuli arising outside the body– Receptors in the skin for touch, pressure, pain, and
temperature– Most special sense organs
2. Interoceptors (visceroceptors)– Respond to stimuli arising in internal viscera and blood
vessels– Sensitive to chemical changes, tissue stretch, and
temperature changes
Classification by Location
3. Proprioceptors– Respond to stretch in skeletal muscles, tendons,
joints, ligaments, and connective tissue coverings of bones and muscles
– Inform the brain of one’s movements
Classification by Structural Complexity
1. Complex receptors (special sense organs – will be discussed separately)
– Vision, hearing, equilibrium, smell, and taste 2. Simple receptors for general senses:
– Tactile sensations (touch, pressure, stretch, vibration), temperature, pain, and muscle sense• Simple receptors can be
– Unencapsulated (free)– Encapsulated dendritic endings
Unencapsulated dendritic endings Found all over the body Abundant in the ET and CT Unmyelinated Respond to temperature changes and pain and some to pressure
changes Cold response are more superficial and receptors that
respond to heat – deeper Temperature out of the range of the thermoreceptors will
activate nociceptors Other receptors respond to itch (respond among other to
histamine) and light touch (detect changes in shape like bending)
Encapsulated dendritic endings
One or more fiber terminals of sensory neurons enclosed in connective tissue capsule
All are mechanoreceptors: Touch proprioceptors
Touch receptors
Meissner’s corpuscle
Pacinian corpuscle
Ruffini's endings
http://www.neurobiography.info/teaching/lecture.php?lectureid=1&mode=handout
Receptor type Structure Location Function
Meissner’s corpuscle/tactile corpuscle
Few spiral terminals surrounded by Schwann cell and CT capsule
Between dermal papillae in hairless skin
Touch, pressure
Pacinian corpuscle/lamellated corpuscle
Single dendrite surrounded by capsule with up to 60 layers of collagen fibers
Skin, interosseous membrane, viscera
Deep pressure. Respond only when the pressure is first applied (on/off pressure stimulation)
Ruffini’s corpuscle
Receptor endings enclosed by flatten capsule
All skin, joint capsule
Stretching of skin – continuous pressure
Proprioceptors
Muscle receptors Joint receptors
Tendon receptors
http://www.neurobiography.info/teaching/lecture.php?lectureid=1&mode=handout
Receptor type Structure Location Function
Muscle spindles
Spindle-shape proprioceptors. Modified skeletal muscle fibers enclosed in CT capsule
Perimysium of skeletal muscles
Detect muscle stretch and initiate reflex that resist stretch
Golgi tendon organs
Proprioceptors. Consist of bundle of collagen fibers enclosed in CT capsule with sensory endings coiling between and around the fibers
In tendons close to skeletal muscle insertion
When tendon fibers are stretched by muscle contraction the nerve endings are activated by compression. When activated, the contraction of the muscle is inhibited which causes relaxation
Joint receptors
Proprioceptors (combination of several receptors types – Pacinian, Raffini, free ending and Golgi tendon)
Joints’ CT capsule Monitor stretch in in the articular capsule and provide information on the position and motion of the joint (conscious)
From Sensation to Perception
• Sensation: the awareness of changes in the internal and external environment
• Perception: the conscious interpretation of those stimuli
Processing of the sensory information
• Levels of neural integration in sensory systems:
1. Receptor level — the sensor receptors2. Circuit level — ascending pathways in the CNS3. Perceptual level — neuronal circuits in the
cerebral cortex
Processing at the Receptor Level
• The receptor must have specificity for the stimulus energy (as
previously discussed)
• The receptor’s receptive field must be stimulated
• The stimulus need to be converted to a nerve impulse
• Receptors have different levels of adaptation
• Information is encoded in the frequency of the stimuli – the
greater the frequency, the stronger is the stimulus.
The stimulation of the receptive field affects the discharge of the sensory neurons
The receptive field is the a specific physical area that, when stimulated, affect the discharge of the stimulus.
Most receptive fields activation will result in message sending – excitatory receptive field
Sensory receptors in the CNS can have inhibitory receptive field (we will mention some examples later when talking about vision).
Sensory neurons of neighboring receptive field may exhibit convergence (many sub-threshold stimuli to sum in the
postsynaptic neuron) Overlapping with another receptor’s receptive field – sending
2 sensations from the same area (pressure and pain) The smaller the receptive field the greater the ability of the brain
to localize the site
Transduction allows sensory receptors to respond to stimuli – converting sensation into a nerve impulse
Sensory transduction – the process that enables a sensory receptor to
respond to a stimulus.
The sensory transduction induces a receptor potential in the peripheral
terminal of the sensory neuron
A receptor potential is a depolarization event that if brings the membrane
to a threshold, will become a nerve impulse (AP)
The conversion from receptor potential to AP happens in the trigger zone
that can be in the first node of Ranvier.
In some cases, the peripheral terminal is a separate sensory cell (ex. Photo
receptors). In this case there is an involvement of a synapse and NT
Receptors adaptation The duration of a stimulus is coded by duration of action
potentials. A longer stimulus generates longer series of APs. If a stimulus persists, some receptors adapt or stop responding There are 2 classes of receptors according to how they adapt:
Tonic receptors – slowly adapting – they fire rapidly when first activated, than they slow and maintain firing as long as the stimulus is present (baroreceptors, proprioceptors)
Phasic receptors – rapidly adapting receptors – rapidly firing when first activated but stop firing if the strength of stimulus remains constant
This type of reaction allows the body to ignore information that was evaluated and found not to be a threat to homeostasis (smell)
Receptors adaptation
The mechanisms for receptors’ adaptation depends on the receptors: Potassium channels in the receptor’s membrane
open causing the membrane repolarization Sodium channels inactivated stopping
depolarization Accessory structure may contribute to decrease
sensitivity (muscle in the ear contract and limit the movement of the auditory oscicles)
Processing at the circuit Level• A sensory pathway is a set of neurons arranged in series.• The circuit level role is to deliver the impulses to the appropriate
region in the cerebral cortex.• The ascending tract typically consists of 3 neurons• First order neurons
– cell bodies in a ganglion (dorsal or cranial)– Impulses from skin and proprioceptors to spinal cord or brain
stem to a 2nd order neuron• Second order neuron
– In the dorsal horn of the spinal cord or in the medulary nuclei– Transmit impulses to thalamus or cerebellum
• Third order neurons– Cell bodies in the thalamus (no 3rd-order neurons in the
cerebellum)– Transmit signals to the somatosensory cortex of the cerebrum
The neural pathway of nociception from primary afferent neurons (PANs) to the superficial lamina in the dorsal horn of the spinal cord.
White F A et al. PNAS 2007;104:20151-20158
©2007 by National Academy of Sciences
1st order neuron
2nd order neuron
3rd order neuron
Processing at the circuit Level
• Impulses ascend in :– Non specific pathway that in general transmit pain,
temperature and touch– Give branches to reticular formation and thalamus on
the way up– Sends general information that is also involved in
emotional aspects of perception– Specific ascending pathways involve in more precise
aspect of sensation
Processing at the Perceptual Level
• Interpretation of sensory input occurs in the cerebral cortex
• The ability to identify the sensation depends on the specific location of the target neurons in the sensory cortex not on the nature of the message (all messages are action potentials)
The CNS integrate sensory information
Most of the somatic sensory information enters the spinal cord and travels via ascending pathways to the brain
Some information goes directly to the brain through the cranial nerves
Autonomic sensory information does not arrive conscious perception
Each area of the brain is processing different information
Main Aspects of Sensory Perception• Perceptual detection – detecting that a stimulus has
occurred and requires summation• Magnitude estimation – the ability to detect how intense
the stimulus is• Spatial discrimination – identifying the site or pattern of the
stimulus• Feature abstraction – used to identify a substance that has
specific texture or shape• Quality discrimination – the ability to identify
submodalities of a sensation (e.g., sweet or sour tastes)• Pattern recognition – ability to recognize patterns in stimuli
(e.g., melody, familiar face)
Properties of the sensory system - summary Stimulus – works on a receptor
The receptor is a transducer that converts the stimulus into a change of membrane potential
The message from the receptor will be sent in the form of action potential to the CNS
Stimuli that will reach the cerebral cortex will be come conscious
Somatosensory information ascends the spinal column along several pathways, which synapse at the midbrain &/or thalamus before reaching the cortex
Sensory processes have different sub-modalities of somatosensory information
Later stages of processing combine information across the sub-modalities, & with information from other senses
What sensory pathways we will discuss?
Pathways for somatic perception that project to the somatosensory cortex and cerebellum
Somatovisceral sensations Touch receptors Temperature receptors Pain and itching receptors Hearing Equilibrium vision