The somatic sensory system

 Sensory stimuli that perception  and proprioception.

reach the conscious level of Somatic senses of touch, temperature, pain, itch  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 temperature — sensitive to changes in Photoreceptors retina) — respond to light energy (e.g., 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 Location

1.

  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 vessels internal viscera and blood  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 discussed separately) (special sense organs – will be 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 changes and pain  and some to pressure 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 http://www.neurobiography.info/teaching/lecture.php?lectureid=1&mode=handout Ruffini's endings

Receptor type Structure Location Meissner’s corpuscle/tacti le corpuscle

Few spiral terminals surrounded by Schwann cell Between dermal papillae in hairless skin

Pacinian

and CT capsule Single dendrite

corpuscle/lame

surrounded by

llated corpuscle Ruffini’s corpuscle

capsule with up to 60 layers of enclosed by flatten capsule Skin, interosseous membrane, viscera collagen fibers Receptor endings All skin, joint capsule

Function

Touch, pressure Deep pressure. Respond only when the pressure is first applied (on/off pressure stimulation) Stretching of skin – continuous pressure

Proprioceptors

Muscle receptors http://www.neurobiography.info/teaching/lecture.php?lectureid=1&mode=handout Joint receptors Tendon receptors

Receptor type Structure Muscle spindles Golgi tendon organs

Spindle-shape proprioceptors. Modified skeletal muscle fibers enclosed in CT capsule

Location

Perimysium of skeletal muscles 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

Joint receptors

Proprioceptors (combination of several receptors types – Pacinian, Raffini, free ending and Golgi tendon) Joints’ CT capsule

Function

Detect muscle stretch and initiate reflex that resist stretch 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 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

 1.

Levels of neural integration in sensory systems: Receptor level — the sensor receptors 2.

3.

Circuit level — ascending pathways in the CNS 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 The stimulus need to be converted to a Receptors have different levels of

stimulated nerve impulse 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 stimulated, affect the discharge of the stimulus.

that, when 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  convergence neighboring receptive field postsynaptic neuron) may exhibit (many sub-threshold stimuli to sum in the  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 that can be in the first node of Ranvier.

trigger zone  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 depolarization inactivated stopping  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 2 nd 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 3 rd -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.

3 rd order neuron 2 nd order neuron 1 st order neuron

White F A et al. PNAS 2007;104:20151-20158

©2007 by National Academy of Sciences

Processing at the circuit Level

  Impulses ascend in : Non specific pathway temperature and touch  the way up that in general transmit pain, Give branches to reticular formation and thalamus on  Sends general information that is also involved in emotional aspects of perception  Specific ascending pathways aspect of sensation involve in more precise

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 occurred – detecting that a and requires summation Magnitude estimation the stimulus is stimulus has – the ability to detect how intense Spatial discrimination the stimulus – identifying the site or pattern of 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 somatosensory information different sub-modalities of 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