This document defines and describes different types of sensation, including somatic, visceral, and special sensations. It focuses on somatic sensation, which is initiated by receptors in the skin, muscles, bones and joints. Somatic sensation includes tactile sensations like touch and pressure, thermal sensations, and pain sensation. It discusses the receptors involved and pathways for transmitting different types of somatic sensation, including the dorsal column medial lemniscus system and anterolateral system. Proprioceptive sensation and vibration sense are also defined and their receptors and pathways described.

1. Sensation
Dr. Hussein Farouk Sakr

2. Sensation
• Definition: conscious perception of particular feeling initiated by
stimulation of certain type of receptors by its adequate stimulus.
General Special
Somatic Visceral Organic - Vision.
- Hearing.
- Taste.
- Smell.
- Sense of
equilibrium.
- Touch and pressure
sensation.
- Position and movement
sense.
- Pain sensation.
- Thermal sensation
1. Mechanoreceptive
2. Chemoreceptive
sensation.
3. Pain sensation.
4. Thermal sensation.
- Hunger.
- Thirst.
- Sexual desire.

3. Somatic Sensation
Sensation from the skin, muscles, bones, tendons and
joints, or somatic sensation, is initiated by a variety of
sensory receptors collectively called somatic receptors.
These receptors respond to:
• Touch and pressure
• Sense of posture and movement
• Temperature
• Pain
3

4. Somatic sensation
Tactile sensations: (touch, pressure vibration, itch tickle)
Transmitted by the anterolateral system
Thermal sensation:
Free nerve endings, two type cold and warm receptors
Transmitted by the anterolateral system
Pain sensation (nociception):
Carried by free nerve endings, pain receptors
Two types of pain:
1. Fast pain (acute, pricking pain), medium diameter
myelinated A fibers)
2. Slow pain( chronic, burning, aching throbbing) C
fibers
Transmitted by the anterolateral system
Proprioceptive sensation: sense of our position and
movement
1. Muscle spindles
2. Golgi tendon bodies
3. Joint kinesthetic receptors
Transmitted by dorsal column-medial lemniscus
system

5. Sensory pathways
• The sensory pathways convey the type
and location of the sensory stimulus
• The type: because of the type of
receptor activated
• The location: because the brain has a
map of the location of each receptor
• Pathway:
• First-order neurons: conduct
impulses from somatic receptors
into the brainstem or the spinal
cord.
• Second-order neurons: conduct
impulses from the brainstem and
spinal cord to the thalamus.
• Third-order neurons: conduct
impulses from the thalamus to the
primary somatosensory area of the
cortex.

6. Touch (pressure)

7. Types of touch:
1) Fine Touch:
• Enables the subject to detect the
shape and texture of objects.
• Receptors are capsulated and
hair end organs.
• Carried by the dorsal column-
medial lemniscus system.
2)Crude Touch:
• Poorly localized sensation which
needs a relatively strong
stimulus.
• Emotionally important.
• Receptors are less differentiated
free nerve endings,
• Carried by the anterolateral
system.

8. Types of fine Touch:
1-Tactile Localization: the ability of the person to localize exactly
the point touched with closed eyes.
2-Tactile Discrimination (Two point discrimination): the ability of
the person to perceive 2 stimuli applied at the same time as two
separate points provided that with closed eyes and the distance
between the two points is more than the threshold distance.
• Threshold (minimal) Distance: it is the least distance between
two points at which the person can discriminate between 2
stimuli applied at the same time.
• Factors affecting threshold distance:
a) Number of Receptors per unit area of the skin.
b) Number of afferents with minimal convergence.
c) Area of presentation in the cerebral cortex.
• the threshold distance is small in areas with high sensory
function as tip of the tongue 1 mm, and tip of the finger 3 mm.
but in the back it is about 70 mm.
The two-point touch threshold test. If each point touches the
receptive fields of different sensory neurons, two separate points of
touch will be felt.
If both caliper points touch the receptive field of one sensory
neuron,only one point of touch will be felt.

9. Types of fine Touch:
3- Streognosis:
• The Ability of the subject to identify the name of the object with
closes eyes.
• It depends on the somatic association area of the (area 5,7) of the
cerebral cortex which is responsible for the understanding of the
meaning somatic sensation.
• Lesion in area (5,7)  Astreognosis.
4- Texture of Material:
• The Ability of the subject to identify the texture of material of clothes
with closed eyes.

10. Somatic sensory pathways
Dorsal Column Medial Leminscus system:
Concerned with transmission of fine
touch, vibration, proprioceptive and
pressure sense
Anterolateral system:
Concerned with transmission of tickle,
crude touch, thermal and pain sensation

11. Dorsal Column Medial
Leminscus system
Anterolateral system
1stOrderNeuron
Receptors  Afferent fibers Aβ fibers
 enters the spinal cord  ascend
in the dorsal column as Gracil (carries
sensation below thoracic 6) and
Cuneate (carries sensation above
thoracic 6) to the level of the
medulla oblongata where they in the
Gracil and cuneate nuclei at the level
of the medulla oblongata.
Receptors  impulses are carried by
two afferents:
1) Aδ fibers.
2) C- unmyelinated fibers.
Both afferent terminate at he dorsal
horn synapsing with neurons in the
gray matter (Main sensory nucleus).

12. Dorsal Column Medial
Leminscus system
Anterolateral system
2ndOrderNeuron
From the Gracil & Cuneate nuclei
fibers arches downward crossing to
the opposite side to ascend on the
opposite site in the medial lemniscus
to terminate at the Postero-Ventral
Nucleus of the thalamus.
Axons of these fibers cross to the
opposite side in front of the central
canal of the spinal cord to ascend on
the opposite side in the ventral
column.
Ventral Spinothalamic tract carrying
impulses of Aδ fibers to the PVNT.
Ventral spinoreticular tract carrying
impulses of the C fibers to the
reticular formation.

13. Dorsal Column Medial
Leminscus system
Anterolateral system
3rdOrderNeuron
From the PVNT  sensory
radiation passing through the
posterior limb of the internal
capsule terminate in the
primary somatic sensory area.
From the PVNT  sensory
radiation passing through the
posterior limb of the internal
capsule terminate in the
primary somatic sensory area.
From the Reticular formation
 interlaminar nucleus of the
thalamus  diffuse areas in
the C.C.

15. Vibration Sense
• Definition: Certain type of Mechanoreceptive sensation in which the
stimulus is rapid and receptive.
• Receptors:
1- Pacinian corpuscles: they are stimulated up to 700 Cycle/sec.
2- Meissner's corpuscles: they are stimulated up to 200 Cycle/sec.

16. Proprioceptive sensation
• Conscious perception of the position of the different parts of the
body in relation to each other during rest and movement.
• This type of sensation allows our brain to identify the position of the
different parts of the body during rest and movement.
• Classification:
1- Static position sense: Perception of the position of the body during
rest
2- Movement sense (Kinesthetic sense): Provides information about
the position of the body during movement

17. Proprioceptors
Muscle Proprioceptors
• Muscle spindle: detect muscle
length and change in muscle
length.
• Golgi Tendon Organ: detect muscle
tension and force of contraction.
• According to their inputs the
higher centers can detect position
of limbs and the change in their
position during movement
Joint Proprioceptors
• Rapidly adapting: detect the onset
and offset of the movement.
• Slowly adapting receptors: detect
the stationary position of the joint
by their continuous discharge.
• Both receptors can detect the
direction of the movement by
changing their discharge i.e. some
receptors discharge during flexion
of the joint and other responds
only with extension.

18. Pathways:
1- Conscious pathway: to
the cerebral cortex via the
dorsal column medial
lemniscus system
2- Subconscious pathway:
to the cerebellum via the
spinocerebellar tracts.
these two pathways are
critical for posture,
balance, and coordination
of skilled movements
Spinocerebellar tracts