Download Sensory receptors and somatic sensation

Survey
yes no Was this document useful for you?
   Thank you for your participation!

* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project

Document related concepts

Hypothalamus wikipedia , lookup

Allochiria wikipedia , lookup

Psychopharmacology wikipedia , lookup

Neuropharmacology wikipedia , lookup

Neuropsychopharmacology wikipedia , lookup

Microneurography wikipedia , lookup

Transcript
Lecture: 1
Sensory Receptors and Somatic Sensation
Dr. Eyad M. Hussein
Ph.D of Neurology
Consultant in Neurology Department,
Nasser Hospital,
Assistant Professor,
Faculty of Medicine, Islamic University
Faculty of Dentistry, University of Palestine
1
‫الصامت الرجاء تحويل الجوال إلى وضع‬
‫مع الشكر‬
‫‪2‬‬
The Sensory System
The various sensations in the body include:
1. Somatic sensation: from the skin and deep tissues (e.g.
muscles, joints and bones), conducted to CNS through the
somatic nerves.
2. Special senses: reaching the CNS through specialized cranial
nerves:
a. Sense of the vision.
b. Sense of the hearing and equilibrium.
c. Sense of the smell.
d. Sense of the taste.
3. Visceral sensation: including all sensations from the internal
viscera and reaching the CNS through the autonomic nerves.
4. Organic sensations: e.g. hunger, thirst and sexual sensations.
3
The Sensory Receptors
Definition: The sensory receptors are specialized structures
located at the peripheral ends of sensory neurons.
Functions of the sensory receptors
1. Detectors: they detect energy (stimulus) changes in
both the external and internal environments.
2. Transducers: transform the stimulus into action
potential (nerve impulses).
3. Generator: generation of nerve impulses in the sensory
nerve.
4. They inform the CNS about different sensations or
changes occurring inside and outside the body.
4
Properties of the Sensory Receptors
1. Specificity (differential sensitivity).
2. Excitability (the receptor potential).
3. Discharge of impulses.
4. Adaptation.
The CNS becomes almost useless without the
sensory receptors
5
1. Specificity (Differential Sensitivity)
 Each type of receptors responds to a specific form
of energy (stimulus) called Adequate Stimulus
and produces a particular sensation.
 Some receptors can respond to other stimuli called
Inadequate Stimuli: e.g. the adequate stimulus of
the retinal receptors is light but can also
stimulated by mechanical pressure.
6
2. Excitability (Receptor Potential)
 This is the property of converting energy into
action potential.
 In the resting state (without stimuli) the sensory
nerve endings are in the polarized state.
 If it is stimulated, the terminal nerve fiber is
partially depolarized due to increase Na+ influx
secondary to Na+ channel activation.
7
 This part of partial depolarization of the sensory nerve
ending is called the receptor potential.
 The receptor potential is passively conducted to the first
node of Ranvier causing its deep depolarization and
reaching the CNS by peripheral nerve.
8
3. Discharge of Impulses
 The Weber-Fechner law states that “the frequency of
discharge (stimulus strength) from receptors is directly
proportional to the logarithm of intensity of the applied
stimuli”.
 The power law: R = KSA.
•
R- is the magnitude of sensation felt (frequency of
discharge).
•
S- is the intensity of the stimulus.
•
K and A are constant (which vary with each type of
sensation).
9
4. Adaptation
 This is decline in the frequency of discharge of action
potentials from receptors that occurs on maintained
stimulation by stimuli of a constant strength.
10
Mechanism of Adaptation
1. Accommodation of the terminal nerve fiber to the
stimulus, due to inactivation (closure) of Na+ channels
as a result of continues current flow.
2. Remodeling (Readjustment) in the structure of the
receptor itself after its initial stimulation.
11
Cutaneous Receptors
“Somatosensory Receptors in Skin”
A. Free nerve endings.
B. Fast-adapting type:
 Meissner’s corpuscles.
 Pacinian corpucles.
 Hair follicle endings.
C. Slow-adapting type:
 Merkel’s disks.
 Ruffini’s end organs.
12
Proprioceptive Receptors
•
Proprioceptors are located in muscles, tendons, joint
ligaments and in joint capsules.
•
In the skeletal (striated) muscle: two types of
encapsulated proprioceptors:
-Muscle spindles.
-Golgi tendon organs.
•
In the joints: encapsulated endings similar to those in
skin.
13
Proprioceptive Receptors
•
Muscle spindles: are found in all striated muscles.
Function: monitor muscle length (stretch).
•
Golgi tendon organs: are found in the tendons of striated
muscles near the muscle-tendon junction.
Function: monitors muscle contraction.
•
Joint receptors: are found within the connective tissue,
capsule and ligaments of joints: the Ruffini’s and Pacinian
corpuscles and the Golgi tendon organs.
14
15
16
Classification of the Receptors
1. According to site of the receptor.
2. According to site of stimulus.
3. According to specificity (physiological
classification).
4. According to adaptation.
17
A. According to Site of Receptor
1. Somatic:
- Cutaneous Senses: for pain, temperature and
touch.
- Deep Sense: from muscles, tendons and joints.
2. Special Senses: e.g. visual, hearing, smell and
taste.
3. Visceral Sense: from the internal organs.
18
B. According to Site of Stimulus
1. Exteroceptors: these include the following:
•
The cutaneous receptors.
•
The teleceptors (distance receptors) or distance
receptors of vision, hearing and smell.
2. Interceptors: these include the following:
•
The proprioceptors: for deep sensations.
•
The viceroceptors: e.g. baroreceptors, chemoreceptors,
viscerla stretch receptors for visceral pain.
•
The hypothalamic receptors: glucoreceptors,
osmoreceptors and hypothalamic thermoreceptors.
19
C. According to Specificity
(Physiological Classification)
1. Mechanoreceptors: they are stimulated by mechanical forms
energy:
• Touch or tactile receptors: present in the skin.
• The proprioceptors: in the muscles, ligaments and
joints.
• The cochlear and vestibular (equilibrium) receptors:
in the internal ear.
• Pressure receptors in the skin and deep tissue.
Baroreceptors: blood pressure (in the carotid sinus and
aortic arch).
• Stretch receptors: e.g. receptors in the wall of the
atrium, urinary bladder and the lung alveoli.
20
2. Chemoreceptors: detect the changes in the concentration of
certain chemicals:
• The peripheral (in carotid body and aortic arch) and
central chemoreceptors (in medulla): to respiration.
• The taste receptors: the taste buds.
• The smell receptors: in the olfactory mucous membrane.
• The glucoreceptors and osmoreceptors: in the
hypothalamus.
3. Nociceptors: these are free nerve endings responding to pain
sensation.
4. Thermoreceptors: these are stimulated by thermal forms of
energy and those present in the skin.
5. Photoreceptors (electromagnetic receptors): present only
in the eyes (rods and cones), which are stimulated by the
electromagnetic waves of light.
21
D. Classification According to Adaptation of
Receptors
1. Rapidly adapting receptors: include mainly the touch
receptors.
2. Moderately adapting receptors: include the smell,
taste and thermo- receptors.
3. Slowly adapting receptors: include mainly the pain
receptors, muscle spindles and the baroreceptors.
22
Classification of the Somatic Sensation
The somatic sensations can be classified in two ways:
A. According to the Site of the Sensation:
1. Superficial (exteroceptive) sensations: the sensations from
the skin (pain, touch and temperature).
2. Deep sensations: the sensations from skeletal muscles,
tendons, joints, bones and ligaments:
a. Proprioceptive sensations: sense of position and the
sense of movement (kinesthetic sensation).
b. Pressure sense.
c. Muscle tension sense.
d. Muscle sense.
Sometimes the vibration sense is included in this group.
3. Combined or synthetic senses: stereognosis , tactile
discrimination and vibration sense.
23
B. According to the Modality (type) of the Sensation:
(More common classification)
I. Mechanoreceptive sensations: these include the:
1. Touch sensations.
2. Pressure sensations.
3. Muscle tension sense.
4. Vibration sensations.
5. Tickle and itch sensations.
6. Proprioceptive sensations.
II. Thermoreceptive sensations: heat and cold
sensations.
III. Pain sensation.
24
I. The Mechanoreceptive Sensations
1. Touch (Tactile) Sensation
There are 2 types of touch sensation:
a. Crude touch: this is a poorly-localized gross tactile
sensation.
Central pathway: ventral spinothalamic tract.
Testing: by stroking the skin lightly with a piece of cotton.
b. Fine touch: this include tactile localization and
discrimination, steriognosis and the sense of texture of material.
Central pathway: the Gracile and Cuneate tracts.
25
 Tactile Localization (Topognosis): the ability to
localize a touched skin point while the eyes are closed.
 Tactile Discrimination (2 point discrimination): the
ability to distinguish 2 separated point of touch
(minimal distance between them i.e. 5 mm in finger &
5 cm over back).
 Steriognosis: the ability to recognize the nature of
object (a familiar object) by touch without using vision.
26
27
2. The Pressure Sensation
•Tested by: asking the patient to differentiate between
various weights by place them in his hand without
moving the limb or hand and with closed eyes.
3. The Muscle Tension Sense
•Tested by: asking the person to differentiate between
various weight placed in his unsupported hand.
28
4. The Vibration Sense
Pathway: Gracile and Cuneate tracts.
 Vibration is closely related to proprioception.
 Tested by: place the tuning fork on a bony prominence.
29
5. The Tickle and Itch Sensations
Tickle sensation: is a pleasurable sensation that result
from light tactile stimulation of the skin.
Itch sensation: is annoying sensation that results from
skin irritation by either moving tactile stimuli or certain
chemical substances.
Pathway: Ventral spinothalamic tract.
30
6. The Proprioceptive Sensations
These sensations arise mainly from receptors in deep
structures (tendon, ligaments, muscles and joints).
Pathway: Gracile and Cuneate tracts.
The proprioceptive sensation include two types:
a. Sense of position (static proprioception):
perception of the position of different parts of the
body
b. Sense of movement (dynamic proprioception):
sensation of movement of joints.
31
Joint Sense: "sense of movement".
Hold the big toe from each side (or
the index finger in U.L.) and
dorsiflexion or plantar flexion.
Ask the patient about which finger
and direction of movement (with
closed eyes).
Muscle Sense: done by pinching
the calf or abdominal muscles.
32
II. The Thermoreceptive Sensations
There are two types of thermoreceptors:
a. Internal thermoreceptors: located in the hypothalamus.
b. External thermoreceptors:
The warmth (Heat) receptors:
•Respond to temperature from about 30 ºC to 45 ºC.
 The cold receptors:
•Respond to temperature from about 10 ºC to 38 ºC.
The thermosensitive pain receptors: respond to
temperature below 10 ºC and above than 45 ºC.
Pathway: lateral spinothalamic tract.
33
III. Pain Sensations
34
35