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

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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

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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