Anatomy and Physiology Special Senses Unit. Sensation Conscious or subconscious awareness of external stimuli.

Anatomy and Physiology

Special Senses Unit

Sensation

Conscious or subconscious awareness of external stimuli.

Perception

Conscious awareness and interpretation of sensations.

Modality

The distinct quality that makes one sensation different from another.

Components of Sensation

1. Stimulation – Stimulus (change in the environment that can activate certain sensory neurons) occurs within the sensory neuron’s receptive field (region that can respond to stimuli).

2. Transduction – A sensory receptor or sense organ must respond to the stimulus and transduce (convert it to a generator potential).

Components of Sensation Cont’d

3. Impulse Generation and Conduction – When the generator potential reaches threshold, nerve impulses are sent.

- First Order Neurons – Sensory neurons that send messages to the CNS.

4. Integration – A region of the CNS must receive and integrate the sensory nerve impulses into a sensation. Part of the cerebral cortex is involved with each type of sensation.

Sensory Receptors

Respond vigorously to one type of stimulus, but weakly or not at all to others, a quality known as selectivity.

Classification of Receptors

Somatic Senses – Sensations of touch, pressure, vibration, warmth, cold, pain and detection of body positions and movements

Free Nerve Endings – The simplest type, having no structural specializations; examples include receptors for pain, temp. itch and tickle

Special Senses – taste, smell, vision, hearing and equilibrium

Classification by Location

1. Exteroceptors – Located at or near the surface of the body; provide information about the external environment.

2. Interoceptors – Located in the blood vessels and viscera to provide info about internal conditions

3. Proprioceptors – Located in tendons, muscles, joints and the internal ear

- Provide info about body position, muscle tension and the position and activity of our joints.

Classification by Stimulus

1. Mechanoreceptors – They detect mechanical pressure or stretching; related to touch, pressure, vibration, proprioception, hearing, equilibrium and blood pressure.

2. Thermoceptors – They detect changes in temperature.

Classification by Stimulus Cont’d

3. Nociceptors – They detect stimuli that cause physical or chemical damage to tissues.

4. Chemoreceptors – They detect chemicals in the mouth (taste), nose (smell) and body fluids.

Generator Potentials and Receptor Potentials

- The electrical response of a sensory receptor will be one of these.

- Both are graded potentials; vary in amplitude.

- They vary with the amplitude of the stimulus.

Generator Potentials

Generated by the senses of smell, touch, pressure, stretching, vibration, temperature, pain and proprioception.

Receptor Potential

* Generated by the senses of vision, hearing, equilibrium and taste.

* Have short neurons that synapse with first order sensory neurons.

* Causes an increase or decrease in exocytosis from synaptic vesicles.

Adaptation

A change in sensitivity (usually a decrease) over a long stimulation.

EX: A hot shower feels hot a first, then feels comfortable.

Receptors and Adaptation

Rapidly Adapting Receptors – Associated with pressure, smell and touch; adapt quickly.

Slowly Adapting Receptors – Associated with pain, body position and chemicals in the blood; adapt slowly.

Somatic Senses

Arise from receptors in the skin, muscles, joints, tendons and inner ear.

1. Cutaneous Sensations – Touch, pressure, vibration, thermal and pain.- receptors are in skin, mucus membranes,

mouth and anus- some areas have many receptors, others have few- receptors consist of dendrites of sensory neurons

Somatic – Cutaneous Sensations Cont’d

Tactile Sensations – Touch, pressure, vibration, itch and tickle.

Touch – Results from the stimulation of tactile receptors immediately deep to the skin.

Crude Touch – You know something has touched you, but can’t determine its location, shape, size or texture.

Somatic – Cutaneous Sensations Cont’d

Discrimitive Touch – The ability to determine the exact location and other details without seeing them.

Pressure – These sensations result from stimulation of deeper tissues.– Defined as a sustained sensation that is felt over

a larger area than touch– Receptors adapt quickly

Somatic – Cutaneous Sensations Cont’d

Vibration – Sensations result from rapidly repetitive sensory signals from tactile receptors

Itch and Tickle – Sensations result from stimulation of free nerve endings or by certain chemicals– Tickle is the only sensation that most people can’t

elicit on themselves, though why is still a mystery

Somatic – Cutaneous Sensations Cont’d

Thermal – Detects heat and cold; separate receptors for each

Somatic – Cutaneous Sensations Cont’d

Pain – Protects the body by letting you know when you’ve come in contact with something damaging.– Receptors are found in almost every body part

and respond to any type of stimulus that is strong enough to cause tissue damage.

– Receptors are slow to adapt, so pain is going to linger for long periods of time

Types of Pain

1. Acute (fast) – Occurs fast (within 0.1 second of stimulation) and is not felt in deeper tissues; aka sharp, fast and pricking pain. EX: a needle puncture or cut from a knife

2. Chronic (slow) – Begins after a second or more and increases in intensity; aka burning, throbbing, aching pain. It can be excruciating

- Can be in skin, deeper tissues viscera.

Pain

Superficial Somatic Pain – Pain that arises in the skin

Deep Somatic Pain – Arises from receptors in skeletal muscles, joints, tendons and fascia

Visceral Pain – Comes from receptors in the viscera (organs)

Referred Pain

When pain is felt in the organ as well as in some surface area far from the stimulated area.

The area to which the pain is referred and the visceral organ involved are serviced by the same area of the spinal cord.

The heart and skin on the medial portion of the upper arm enter the spinal cord between T1 and T5.

Phantom Pain

Pain experienced by patients that have had a limb amputated.

They experience itching, pressure, tingling and pain as though the limb were still there.

Possibly caused by stimulation from neurons that used to receive signals from the appendage.

Possibly from the brain and its sense of body awareness.

Proprioceptive Sensations

Awareness of body parts and movements. Informs us of how far muscles are

contracted, tension on tendons, and change in the position of a joint.

Allows us to do coordinated movements like walking and typing.

Proprioceptive Sensations Cont’d

Also allows you to estimated the amount of force needed to do something (lifting a bag of feathers or a bag of rocks).

Receptors adapt very little if at all (you constantly need that information).

Proprioceptive Receptors

Muscle spindles, tendon organs and joint kinesthetic receptors as well as hair cells of the internal ear, which provide information about balance.

1. Muscle Spindles

Special groups of muscle fibers found in regular fibers and oriented parallel to them.– They lack actin and myosin, so they can’t contract– They do contain two types of sensory receptors– They monitor changes in the length of a skeletal

muscle by responding to the rate and degree of change in length

– The information gets sent to the brain

2. Tendon Organs

Found in the junction of tendon and muscle When tension increases in a tendon, a

message is sent to the brain. Prevents damage to tendons and muscles

due to excessive tension They also monitor the force of muscle

contractions.

3. Joint Kinesthetic Receptors

Associated with the articular capsule of synovial joints

Responds to pressure and measures acceleration and deceleration of joint movement as well as joint strain.

Somatic Sensory Pathways

Pathways from receptors to the cerebrum involve three-neuron sets

1. First-Order Neurons

Carry signals from the receptor to the brain stem or spinal cord– Face, mouth, teeth and eyes; use cranial nerves– Back of the head, neck and body use spinal

nerves

2. Second Order Neurons

Carry signals from the spinal cord or brain stem to the thalamus

Second order neurons cross over from right to left and vice versa before ascending to the thalamus

3. Third Order Neurons

Run from the thalamus to the primary somatosensory area of the cerebral cortex where conscious perception of sensations result

2nd order neurons synapse with 3rd order neurons in the somatosensory area of the cerebral cortex

Impulses along this pathway give rise to several highly evolved and refined sensations:

1. Discrimitive Touch

The ability to recognize the exact location of a light touch and to make two-point discriminations

2. Sterognosis

The ability to recognize by feel the size, shape and texture of an object. Examples are reading braille or identifying (with eyes closed) a paperclip put in your hand

3. Proprioception

The awareness of the precise position of body parts, and kinesthesia, the awareness of direction of movement

4. Weight Discrimination

The ability to assess the weight of an object.

5. Vibratory Sensations

The ability to sense rapidly fluctuating touch.