The field of general sensory physiology is concerned with the principles underlying the sensory abilities of humans and animals. This general approach is possible and useful because the various sense organs closely resemble one another in organization and function, in their connections to centers in the brain, and in the reactions they elicit. Moreover, for all sense organs there is the problem of “objective” and “subjective” aspects. We can observe and analyze the performance of a human or animal sense organ by the same procedures as in research on the circulatory system, for example; in so doing, we are studying objective sensory physiology. But we can go further and apply scientific analysis to our own sensations, produced by environmental phenomena with the mediation of the sense organs, and in this we can also draw upon the analogous experiences reported by other people. Here we are in the area of subjective sensory physiology. One of the basic concerns in general sensory physiology is to analyze these two aspects and evaluate their interdependence.

1. SPECIAL TOPICS IN SENSORY
PHYSIOLOGY

2. THALAMUS
• Thalamus is a large ovoid mass of gray matter,
situated bilaterally in diencephalon.
• Both thalami form 80% of diencephalon.
• Thalami on both sides are connected in their
rostral portions by means of an intermediate mass
„THALAMIC NUCLEI
• Thalamic nuclei are classified by two methods:
A. Anatomical classification
B. Physiological classification.

5. FUNCTIONAL GATEWAY FOR CEREBRAL CORTEX
• Almost all the sensations are processed in
thalamus before reaching cerebral cortex.
• Very little information of somatosensory
function is sent directly to cerebral cortex
without being processed by the thalamic
nuclei
• Because of this function, thalamus is usually
called ‘functional gateway’ for cerebral cortex

7. ANATOMICAL CLASSIFICATION
• Thalamus on each side is divided into five
main nuclear groups by ‘Y’-shaped internal
medullary lamina
• 1. Midline nuclei
• 2. Intralaminar nuclei
• 3. Medial mass of nuclei
• 4. Lateral mass of nuclei
• 5. Posterior group of nuclei

10. PHYSIOLOGICAL CLASSIFICATION
• On the basis of functions and their projections,
thalamic nuclei are classified into five groups
• This classification aka Bondok classification
• 1. Specific sensory relay nuclei
• 2. Specific motor nuclei
• 3. Association or less specific nuclei
• 4. Non-specific nuclei
• 5. Limbic system nuclei

12. FUNCTIONS OF THALAMUS
1. RELAY CENTER
• Thalamus forms the relay center for
sensations.
• Impulses of almost all the sensations reach
the thalamic nuclei, particularly in the VPL
nucleus
• After being processed, impulses carried to
cerebral cortex through thalamocortical
fibers

13. FUNCTIONS OF THALAMUS
2. CENTER FOR PROCESSING OF SENSORY
INFORMATION
• Thalamus forms the major center for processing
the sensory information
• VPL, VPM, lateral and medial geniculate nuclei.
• All peripheral sensory impulses are integrated
and modified before being sent to specific areas
of cortex
• This function of thalamus called the processing
of sensory information.

14. 3. CENTER FOR DETERMINING QUALITY OF SENSATIONS
• determine the affective nature of sensations. Usually the
sensations have two qualities:
• i. Discriminative nature
• ii. Affective nature.
i. Discriminative Nature
• Discriminative nature is the ability to recognize the type,
• location and other details of the sensations and it is the
• function of cerebral cortex.
ii. Affective Nature
• Affective nature is the capacity to determine whether
• a sensation is pleasant or unpleasant and agreeable
• or disagreeable

15. FUNCTIONS OF THALAMUS
• 4. CENTER FOR SEXUAL SENSATIONS
Thalamus forms the center for perception of sexual
sensations
• „5. ROLE IN AROUSAL & ALERTNESS REACTIONS
Because of its connections with nuclei of
reticular formation, thalamus plays an important
role in arousaland alertness reactions
• „6. CENTER FOR REFLEX ACTIVITY
Since the sensory fibers relay here, thalamus forms
the center for many reflex activities

16. FUNCTIONS OF THALAMUS
• 7. CENTER FOR INTEGRATION OF MOTOR
ACTIVITY
recieves motor inputs from cerebellum and
basal ganglia and projects to premotor and
primary motor cortices.

17. LESION OF THALAMUS
• Following are the symptoms of thalamic syndrome:
• 1. Loss of Sensations(anesthesia)- occurs as the
sensory relay system in thalamus is affected.
• 2. Astereognosis- the loss of ability to recognize a
known object by touch with closed eyes. It is due to
the loss of tactile and kinesthetic sensations
• 3. Ataxia- refers to in-coordination of voluntary
movements. This type of ataxia due to loss of sensation
is called sensory ataxia.
• 4. Thalamic Phantom Limb- unable to locate position
of a limb with closed eyes. The patient may search for
the limb in air or may have the illusion that the limb is
lost. This is called thalamic phantom limb.

18. 5. SPONTANEOUS PAIN AND THALAMIC OVER-REACTION
• Spontaneous pain occurs often. Pain stimulus is felt
more acutely than normal (hyperalgesia)
• Threshold for pain is very much reduced
• May be so intense, to resist action of powerful
sedatives like morphine
• Sometimes, pain felt in the absence of pain
stimulus- becomes worst in conditions such as
emotional disturbance

19. • Pain is due to over activity of medial mass of
nuclei of thalamus, which escape the lesion
• Abnormal reaction to various stimuli is called
thalamic over-reaction

20. 6. INVOLUNTARY MOVEMENTS
• Athetosis - slow writhing and twisting
movements
• Chorea - quick, jerky, involuntary movements.
• Intention tremor
Tremor is defined as rapid alternate rhythmic and
involuntary movement of flexion and extension in
the joints of fingers and wrist or elbow.
• Intention tremor is the tremor that develops
while attempting to do any voluntary act

21. THE RETICULAR FORMATION
• Set of interconnected nuclei that are
located throughout the brainstem
• Not anatomically well defined because it
includes neurons located in diverse parts
of the brain
• Neurons play a crucial role in maintaining
behavioral arousal and consciousness

23. THE RETICULAR FORMATION
• The reticular formation has two components:
1. The ascending reticular formation is-
aka Reticular Activating System
• Responsible for the sleep-wake cycle, thus
mediating various levels of alertness.
• This part projects to the mid-line group of
the thalamus, which also plays a role in
wakefulness. From there, information is sent
to the cortex.

24. 2. DESCENDING RETICULAR FORMATION
• Involved in posture and equilibrium as well
as autonomic nervous system activity
• Also plays a role in motor movement.
• Receives information from hypothalamus.
• The descending reticular nuclei in the brain
are involved in reflexive behavior such as
coughing, chewing, swallowing and
vomiting

26. FUNCTION
• Consists of more than 100 small neural networks,
with varied functions including the following:
• Pain modulation –one means by which pain signals
reach the cerebral cortex
• Also origin of descending analgesic pathways
• Sleep and consciousness –projections to
thalamus and cortex allow it to exert some control
over which sensory signals reach the cerebrum
• Plays a central role in states of consciousness like
alertness and sleep
• Injury can result to irreversible coma

27. FUNCTION
• Somatic motor control –function in maintaining tone,
balance and posture—esp. during body movements
• Also relays eye and ear signals to cerebellum so that
the cerebellum can integrate visual, auditory and
vestibular stimuli in motor coordination
• Cardiovascular control – Reticular formation includes
cardiac and vasomotor centers of medulla oblongata
• Habituation– process in which brain learns to ignore
repetitive, meaningless stimuli while remaining
sensitive to others; - person who can sleep through
loud traffic in a large city, but is awakened promptly
due to the sound of an alarm or crying baby

28. CLINICAL SIGNIFICANCE
• Mass lesions in brainstem cause severe
alterations in level of consciousness (eg coma)
because of effects on reticular formation
• Bilateral damage to the reticular formation of
the midbrain may lead to coma or death
• Imaging studies have shown abnormal activity
in this area in people with chronic fatigue
syndrome, likelihood that damage to the
reticular formation is responsible for the fatigue
associated with these syndromes

29. The Somatosensory Cortex
• Location:
• Primary somatosensory cortex: Located on the
postcentral gyrus (anterior part of the parietal
lobe)
• Secondary somatosensory cortex: behind the
primary. An association area

30. function
• Involved in somatic sensation, visual stimuli
and movement planning.
• Regions requiring precise sensation occupy
larger areas.
• Secondary somatosensory area: involved in
episodic memory, visuospatial processing,
aspects of conc.iousness