THALAMUS
&
LIMBIC SYSTEM
Lecture 27
Functions of thalamus and limbic system
At the end of the session the students should be able to:
a
Classify the nuclei of thalamus
b
Describe the connections of thalamus and relate them to the thalamic functions.
c
Discuss the applied part including thalamic syndrome.
d
Discuss briefly the structures and functions of limbic system.
 Guyton and hall textbook of medical physiology 12th edition, chapter 60 page 714 -720, 986.
THE THALAMUS
 It is an ovoid gray mass located in the diencephalon. The two thalami of both sides are
separated by the 3rd ventricle with interconnection by a narrow band called interthalamic
adhesion.
 The gray matter of the thalamus is divided internally by laminae of white matter into
many separate nuclei. The thalamic nuclei are functionally divided into two groups:
Specific i.e. receive well defined afferents and project to
specific areas in the cerebral cortex and they include:
- Anterior nuclei.
- Medial nuclei.
- Lateral nuclei (dorsal lateral + ventral lateral).
Non-specific i.e. receive input mainly from R.A.S, and
project diffusely to all parts of the neocortex and they
include:
- Intralaminar nuclei.
- Midline nuclei.
- Reticular nuclei.
The afferent (input) and efferent
(projections)
nuclei
are
of
the
illustrated
following figure:
thalamic
in
the
THALAMIC FUNCTIONS
I-
Sensory functions.
II-
Motor functions.
III- Integration functions.
IV- Arousal function.
I) Sensory functions
a- It is a sensory relay station:
• All ascending sensory pathways before reaching, the cerebral cortex relay
in the thalamus specially those carrying fine epicritic sensations from
opposite side of the body.
i) Somatic sensory:
Pathways relay in V.P.L and VP.M nuclei of the opposite side, where those
from the body relay in V.P.L and those from the head and the face plus taste
relay in V.P.M.
ii) Special sensations:
- Vision: the optic fibers from both retinae reach L.G.B. and projected to the
visual cortex.
- Hearing: auditory fibers from both ears relay in M.G.B. before reaching the
auditory cortex.
- Olfaction: recently, olfactory pathway has found to pass through the
dorsomedial nucleus to the orbitofrontal cortex.
b- Gating of the ascending sensory information:
 Coticofugal impulses are transmitted back from the cerebral cortical sensory
areas to the thalamic relay nuclei which already project to these areas.
 This cortical-feedback is inhibitory, and it decreases transmission through
these nuclei particularly when the sensory input is very high and the sensory
system is overloaded with sensory input
c- Thalamus as a sensory center:
 The discrimination of many sensory impulses occurs in the thalamus but
the sensations felt are of crude protopathic nature e.g: diffuse pain, crude
touch & extremes of temperature change.
II) Motor functions
 The V.A. and V.L. nuclei relay motor signals from the B.G. and
cerebellum to the motor and premotor areas of the frontal lobe to control
their functions.
 The V.P.L nucleus relays tactile and proprioceptive signals to the motor
cortex. This provides sensory information about position and movements
of the different parts of the body.
 The non-specific nuclei adjust the general level of excitability of the
motor cortex.
III Association and integration function
 The anterior and medial nuclei together with hypothalamus and limbic
system play a role in integrating the visceral and somatic motor responses
evoked during emotional reactions with incoming sensory signals.
 The reciprocal connection between the D.M. nucleus of the thalamus and
prefrontal areas may play a role in the coding, storing and recalling of
memory.
IV) Arousal function
 The non-specific thalamic nuclei receive excitatory signals from the R.A.S
of the brain stem, and project it to almost all areas of the cerebral cortex,
producing arousal and wakefulness
Limbic System
 The limbic system consist of those parts of the brain that are of great
importance in:
1. Initiation of emotions.
2. Regulation of emotional behavior

The limbic system is interconnected group of cortical and subcortical
nuclei within the brain. It includes both;
1- Limbic cortex:
2- Subcortical limbic structures:
A ring of old cortex:
1. Beginning in orbitofrontal area on the
2. Extending upward infront and over the
corpus callosum on the medial aspect of
cerebral hemispheres to cingulate gyrus.
3. Passing posteriorly to the corpus callosum
and downward on the ventromedial surface
the
temporal
lobe,
to
includes
the
structures:
ventral surface of the frontal lobe.
of
It
the
para-
hippocampal gyrus, pyriform area and uncus.
 Hypothalamus
 Thalamus
 Paraolfactory areas
 Amygdala
 Hippocampus
 Septal nucleus
 Parts of basal ganglia.
following
Connections of limbic system:
A- With cerebral cortex:
Through connection with cerebral cortex, the limbic system receives processed
sensory information from the cortical association area about various stimuli in the internal
and external environments, for deciding the nature of appropriate internal and external
response initiated by the limbic system.
B- With reticular formation and autonomic centers in the brian stem:
The hypothalamus as well as the amygdala which is the major output pathway for
the limbic system, have numerous connections with the R.F and the autonomic centers of
the brain stem, then through,
 Descending projections to motor neurons of cranial and spinal nerves  control of
skeletal muscle contraction and movements.
 Descending projections to autonomic neurons in brian stem and spinal cord  activation
of smooth muscles and exocrine glands.
 In addition to the controlling effect of the hypothalamus on the endocrine system.
C- Internal connections:
1. A closed circuit called Papez circuit connects the thalamus and hypothalamus with the
limbic system as follow:
The hippocampus is connected to mamillary bodies (by means of fornix), which are
connected to the anterior thalamic nuclei (by means of mamillo-thalamic tract), which
projects to cingulated gyrus, and this finally discharges to the hippocampus again.
2. Stria-terminalis which connects the amygdaloid with the hypothalamus.
3. Few connections exist between limbic system and the neocortex (cortical tissues other
than the limbic cortex) specially from the frontal lobe to the adjacent limbic areas.
 Papez circuit have prolonged after discharge  allows the various component of the
limbic sysem to co-ordinate various aspects of somatic, autonomic and hormonal
responses that associate the emotional reaction.
 Connection of limbic system with the neocortex enables the neocortex to modify the
emotional behavior.
Functions of limbic system
- Limbic system is primarily concerned with emotions, motivation, and emotional behavior
and plays an essential role in learning and memory.
Emotions: complex feeling states having two components:
a) Sensory component:
Awareness of a particular inner feeling and sense of motivation or urge to take an
action in response to these feelings.
b) Behavior (physical component):
Internal
- neural by A.N.S.  HR, sweating
- hormonal by endocrine glands.
External
- motor response
* Motivation: means the desire or drive for a particular behavior to achieve certain goal.
* Emotional behavior: refers to the internal and external responses which participate in
the physiological functions aiming in maintaining the survival of the individual and
preservation of species.
Role of limbic system in motivation and emotional behavior:
The limbic system regulates patterns of behavior related to the following physiological
functions:
1- Control of the feeding behaviour:
This is one of the functions of the amygdaloid nuclei. The amygdaloid nuclei seems
to differentiate food into edible and inedible on the basis of past experience. The
stimulation of these nuclei causes chewing and licking movements, while its damage leads
to hyperphagia associated with ingestion of all kinds of food and objects.
2- Fear and rage reactions:
 The limbic system receives information from different association areas of the cortex
about various environmental stimuli. Many limbic centers especially the amygdale,
evoke protective emotional reactions (.e.g. fear or rage reactions).
 Manifestation of fear reaction are: autonomic responses as pupillary dilatation, sweating
and escape reaction.
 The emotional behavior associated the fear or rage reactions are:
1. Feeling of general excitement and arousal.
2. Autonomic and endocrine responses  support somatic motor reactions.
3- Sexual behavior and reproduction:
A) Hypothalamus plays a control role on the onset of puberty as well as it contains
sensory neurons that respond to gonadal hormones  powerful control upon the sexual
function and activity.
B) The behavior components that accompany the sexual act are regulated by the limbic
cortex and the hypothalamus:
* Bilateral damage of amygdale  hypersexuality.
* Removal of limbic cortex  loss of interest in other sex.
4- Punishment and reward response:
A) Stimulation of certain areas in limbic system and hypothalamus e.g. lateral,
ventromedial nuclei of hypothalamus, amygdale and septal nuclei  pleasure and
satisfaction (reward effect). The transmitter that is responsible for this sense is
dopamine. The cocaine seems to produce its euphoric effect by  dopaminergic
activity in the reward system.
B) Stimulation of certain areas in limbic system and hypothalamus e.g. periaquiductal
area and periventricular area in hypothalamus  displeasure, rage, fear, escape
(punishment effect), leading to avoidance of these punishing stimuli.
So, there are 2 mechanisms in the hypothalamus and limbic systems.
* One promoting pleasure, comfort and placidity called reward or approach system.
* Other promoting anxiety, fear and rage called punishment or avoidance system.
So, emotional state is determined by the balance between the two systems.
5- Role (importance) of reward and punishment in memory & learning
Every thing we do is related to reward and punishment i.e.
1. If we do something rewarding, we continue doing it and if we do something
punishing, we stop doing it.
So, reward and punishment are very important in motivation for learning.
2. If the stimulus causes neither reward nor punishment experience can't be remembered
easily i.e. no transformation into long-term memory and become hardly to be
remembered.
- If the stimulus causes either reward or punishment  leads to transformation of
short-term memory into long-term memory and become hardly to be forgotten.
That is to say the limbic system differentiates the sensory signals into significant
ones which are stored in memory and insignificant ones which are ignored.
6- Olfaction:
The limbic system is concerned with perception and discrimination of different odours. It
stores the olfactory memories and controls the emotional responses to olfactory stimuli.
Thank you