THE LIMBIC SYSTEM
- The word limbic means borde r or margin, and the term limbic system
was loosely used to include a group of structures that lie in the border
zone between the cerebral cortex and the hypothalamus.
- Now it is recognized, as the result of research, that the limbic system is
involved with many other structures beyond the border zone in the
control of emotion, behavior, and drive; it also appears to be important to
memory.
- Anatomically, the limbic structures include:
1- The subcallosal, the cingulate, and the parahippocampal gyri
2- The hippocampal formation
3- The amygdaloid nucleus
4- The mammillary bodies
5- The anterior thalamic nucleus
6- The olfactory lobe which consists of; O. bulb, tract, trigone & anterior
perforated substance
- The alveus, the fimbria, the fornix, the mammillothalamic tract, and the
stria te rminalis constitute the connecting pathways of this system.
Hippocampal Formation:
- The hippocampal formation consists of the hippocampus, the dentate
gyrus, and the parahippocampal gyrus.
- The hippocampus is a curved elevation of gray matter that extends
throughout the entire length of the floor of the infe rior horn of the lateral
ventricle
- Its anterior end is expanded to form the pes hippocampus.
- It is named hippocampus because it resembles a sea horse in coronal
section.
- The convex ventricular surface is covered with ependyma, beneath which
lies a thin layer of white matter called the alveus
- The alveus consists of nerve fibers that have originated in the
hippocampus, and these converge medially to form a bundle called the
fimbria
- The fimbria, in turn, becomes continuous with the crus of the fornix
- The hippocampus terminates posteriorly beneath the splenium of the
corpus callosum.
The dentate gyrus:
- A narrow, notched band of gray matter that lies between the fimbria of
the hippocampus and the parahippocampal gyrus
- Posteriorly, the gyrus accompanies the fimbria almost to the splenium of
the corpus callosum and becomes continuous with the indusium griseum .
The indusium griseum:
- A thin, vestigial layer of gray matter that covers the supe rior surface of the
corpus callosum
- Embedded in the superior surface of the indusium griseum are two
slender bundles of white fibers on each side called the medial and lateral
longitudinal striae.
- The striae are the remains of the white matter of the vestigial indusium
griseum.
- Anteriorly, the dentate gyrus is continued into the uncus.
The parahippocampal gyrus:
- The word limbic means borde r or margin, and the term limbic system
was loosely used to include a group of structures that lie in the border
zone between the cerebral cortex and the hypothalamus.
- Now it is recognized, as the result of research, that the limbic system is
involved with many other structures beyond the border zone in the
control of emotion, behavior, and drive; it also appears to be important to
memory.
- Anatomically, the limbic structures include:
1- The subcallosal, the cingulate, and the parahippocampal gyri
2- The hippocampal formation
3- The amygdaloid nucleus
4- The mammillary bodies
5- The anterior thalamic nucleus
6- The olfactory lobe which consists of; O. bulb, tract, trigone & anterior
perforated substance
- The alveus, the fimbria, the fornix, the mammillothalamic tract, and the
stria te rminalis constitute the connecting pathways of this system.
Hippocampal Formation:
- The hippocampal formation consists of the hippocampus, the dentate
gyrus, and the parahippocampal gyrus.
- The hippocampus is a curved elevation of gray matter that extends
throughout the entire length of the floor of the infe rior horn of the lateral
ventricle
- Its anterior end is expanded to form the pes hippocampus.
- It is named hippocampus because it resembles a sea horse in coronal
section.
- The convex ventricular surface is covered with ependyma, beneath which
lies a thin layer of white matter called the alveus
- The alveus consists of nerve fibers that have originated in the
hippocampus, and these converge medially to form a bundle called the
fimbria
- The fimbria, in turn, becomes continuous with the crus of the fornix
- The hippocampus terminates posteriorly beneath the splenium of the
corpus callosum.
The dentate gyrus:
- A narrow, notched band of gray matter that lies between the fimbria of
the hippocampus and the parahippocampal gyrus
- Posteriorly, the gyrus accompanies the fimbria almost to the splenium of
the corpus callosum and becomes continuous with the indusium griseum .
The indusium griseum:
- A thin, vestigial layer of gray matter that covers the supe rior surface of the
corpus callosum
- Embedded in the superior surface of the indusium griseum are two
slender bundles of white fibers on each side called the medial and lateral
longitudinal striae.
- The striae are the remains of the white matter of the vestigial indusium
griseum.
- Anteriorly, the dentate gyrus is continued into the uncus.
The parahippocampal gyrus:
- Lies between the hippocampal fissure and the collateral sulcus
- It is continuous with the hippocampus along the medial edge of the
temporal lobe
Amygdaloid Nucleus:
- The amygdaloid nucleus is so named because it resembles an almond.
- It is situated partly anterior and partly superior to the tip of the infe rior
horn of the late ral ventricle
- It is fused with the tip of the tail of the caudate nucleus, which has passed
anteriorly in the roof of the inferior horn of the lateral ventricle.
- The stria terminalis emerges from its posterior aspect.
- Mamillary bodies (hypothalamus)
- Anterior thalamic nuclei (thalamus)
Connecting Pathways of the Limbic System:
- The alveus consists of a thin layer of white matte r that lies on the superior or
ventricular surface of the hippocampus, it is composed of nerve fibers that
originate in the hippocampal cortex.
- The fimbria is the fibe rs of the alveus after they converge on the medial surface
of the hippocampus
- The fimbria now leaves the posterior end of the hippocampus as the crus
of the fornix which from each side curves posteriorly and superiorly
beneath the splenium of the corpus callosum and around the posterior
surface of the thalamus.
- The two crura now converge to form the body of the fornix, which is
applied closely to the unde rsurface of the corpus callosum
- As the two crura come togethe r, they are connected by transverse fibers
called the commissure of the fornix
- These fibers decussate and join the hippocampi of the two sides.
- Anteriorly, the body of the fornix is connected to the undersurface of the
corpus callosum by the septum pellucidum.
- Infe riorly, the body of the fornix is related to the tela choroidea and the
ependymal roof of the third ventricle.
- The body of the fornix splits anteriorly into two anterior columns of the
fornix, each of which curves anteriorly and inferiorly over the
interventricular foramen, then, each column disappears into the lateral
wall of the third ventricle to reach the mammillary body
- The mammillothalamic tract provides important connections between the
mammillary body and the anterior nuclear group of the thalamus.
- The stria terminalis emerges from the posterior aspect of the amygdaloid
nucleus and runs as a bundle of nerve fibers posteriorly in the roof of the
inferior horn of the lateral ventricle on the medial side of the tail of the
caudate nucleus. It follows the curve of the caudate nucleus and comes to
lie in the floor of the body of the lateral ventricle.
Afferent Connections of the Hippocampus:
1. Fibers from the cingulate gyrus
2. Fibers from the septal nuclei (nuclei lying within the midline close to the
anterior commissure) pass posterior in the fornix to the hippocampus .
3. Fibers from one hippocampus pass across the midline to the opposite
hippocampus in the commissure of the fornix.
4. Fibers from the indusium griseum pass posteriorly in the longitudinal
striae
- It is continuous with the hippocampus along the medial edge of the
temporal lobe
Amygdaloid Nucleus:
- The amygdaloid nucleus is so named because it resembles an almond.
- It is situated partly anterior and partly superior to the tip of the infe rior
horn of the late ral ventricle
- It is fused with the tip of the tail of the caudate nucleus, which has passed
anteriorly in the roof of the inferior horn of the lateral ventricle.
- The stria terminalis emerges from its posterior aspect.
- Mamillary bodies (hypothalamus)
- Anterior thalamic nuclei (thalamus)
Connecting Pathways of the Limbic System:
- The alveus consists of a thin layer of white matte r that lies on the superior or
ventricular surface of the hippocampus, it is composed of nerve fibers that
originate in the hippocampal cortex.
- The fimbria is the fibe rs of the alveus after they converge on the medial surface
of the hippocampus
- The fimbria now leaves the posterior end of the hippocampus as the crus
of the fornix which from each side curves posteriorly and superiorly
beneath the splenium of the corpus callosum and around the posterior
surface of the thalamus.
- The two crura now converge to form the body of the fornix, which is
applied closely to the unde rsurface of the corpus callosum
- As the two crura come togethe r, they are connected by transverse fibers
called the commissure of the fornix
- These fibers decussate and join the hippocampi of the two sides.
- Anteriorly, the body of the fornix is connected to the undersurface of the
corpus callosum by the septum pellucidum.
- Infe riorly, the body of the fornix is related to the tela choroidea and the
ependymal roof of the third ventricle.
- The body of the fornix splits anteriorly into two anterior columns of the
fornix, each of which curves anteriorly and inferiorly over the
interventricular foramen, then, each column disappears into the lateral
wall of the third ventricle to reach the mammillary body
- The mammillothalamic tract provides important connections between the
mammillary body and the anterior nuclear group of the thalamus.
- The stria terminalis emerges from the posterior aspect of the amygdaloid
nucleus and runs as a bundle of nerve fibers posteriorly in the roof of the
inferior horn of the lateral ventricle on the medial side of the tail of the
caudate nucleus. It follows the curve of the caudate nucleus and comes to
lie in the floor of the body of the lateral ventricle.
Afferent Connections of the Hippocampus:
1. Fibers from the cingulate gyrus
2. Fibers from the septal nuclei (nuclei lying within the midline close to the
anterior commissure) pass posterior in the fornix to the hippocampus .
3. Fibers from one hippocampus pass across the midline to the opposite
hippocampus in the commissure of the fornix.
4. Fibers from the indusium griseum pass posteriorly in the longitudinal
striae
5. Fibers from the entorhinal area or olfactory-associated cortex
6. Fibers from the dentate and parahipp. gyri
Effe rent Connections of the Hippocampus:
1. Fibers pass posterior to the anterior commissure to enter the mammillary
body.
2. Fibers pass posterior to the anterior commissure to end in the anterior
nuclei of the thalamus.
3. Fibers pass posterior to the anterior commissure to enter the tegmentum
of the midbrain.
4. Fibers pass anterior to the anterior commissure to end in the septal
nuclei, the lateral preoptic area, and the anterior part of the
hypothalamus.
5. Fibers join the stria medullaris thalami to reach the habenular nuclei.
6. The limbic system not only are interconnected but also send projection
fibers to many different parts of the ne rvous system
7. Physiologists now recognize the importance of the hypothalamus as being
the major output pathway of the limbic system
Functions of the Limbic System:
1- The limbic system, via the hypothalamus and its connections with the outflow
of the autonomic nervous system and its control of the endocrine system, is able
to influence many aspects of emotional behavior. These include particularly the
reactions of fear and anger and the emotions associated with sexual behavior.
2- There is also evidence that the hippocampus is concerned with converting
recent memory to long-term memory. A lesion of the hippocampus results in the
individual being unable to store long-term memory. Memory of remote past
events before the lesion developed is unaffected(ante rograde amnesia).
3- There is no evidence that the limbic system has an olfactory function. The
various affe rent and efferent connections of the limbic system provide pathways
for the integration and effective homeostatic responses to a wide variety of
environmental stimuli.
THE VISUAL PATHWAY
- Axons of the retinal ganglionic cell layrer converge on the optic disc and
exit from the eye, about 3 or 4 mm to the nasal side of its center, as the
optic nerve
- The optic ne rve leaves the orbital cavity through the optic canal and
unites with the optic nerve of the opposite side to form the optic chiasma.
- The optic chiasma is situated at the junction of the anterior wall and floor
of the third ventricle.
- In the chiasma, the fibers from the nasal (medial) half of each retina,
including the nasal half of the macula cross the midline and enter the
optic tract of the opposite side, while the fibe rs from the temporal
(lateral) half of each retina, including the temporal half of the macula,
pass posteriorly in the optic tract of the same side.
- The optic tract emerges from the optic chiasma and passes
posterolate rally around the cerebral peduncle.
- Most of the fibers now terminate by synapsing with ne rve cells in the
lateral geniculate body, which is a small projection from the posterior
part of the thalamus. A few of the fibers pass to the pretectal nucleus and
the supe rior colliculus of the midbrain and are concerned with light
reflexes
6. Fibers from the dentate and parahipp. gyri
Effe rent Connections of the Hippocampus:
1. Fibers pass posterior to the anterior commissure to enter the mammillary
body.
2. Fibers pass posterior to the anterior commissure to end in the anterior
nuclei of the thalamus.
3. Fibers pass posterior to the anterior commissure to enter the tegmentum
of the midbrain.
4. Fibers pass anterior to the anterior commissure to end in the septal
nuclei, the lateral preoptic area, and the anterior part of the
hypothalamus.
5. Fibers join the stria medullaris thalami to reach the habenular nuclei.
6. The limbic system not only are interconnected but also send projection
fibers to many different parts of the ne rvous system
7. Physiologists now recognize the importance of the hypothalamus as being
the major output pathway of the limbic system
Functions of the Limbic System:
1- The limbic system, via the hypothalamus and its connections with the outflow
of the autonomic nervous system and its control of the endocrine system, is able
to influence many aspects of emotional behavior. These include particularly the
reactions of fear and anger and the emotions associated with sexual behavior.
2- There is also evidence that the hippocampus is concerned with converting
recent memory to long-term memory. A lesion of the hippocampus results in the
individual being unable to store long-term memory. Memory of remote past
events before the lesion developed is unaffected(ante rograde amnesia).
3- There is no evidence that the limbic system has an olfactory function. The
various affe rent and efferent connections of the limbic system provide pathways
for the integration and effective homeostatic responses to a wide variety of
environmental stimuli.
THE VISUAL PATHWAY
- Axons of the retinal ganglionic cell layrer converge on the optic disc and
exit from the eye, about 3 or 4 mm to the nasal side of its center, as the
optic nerve
- The optic ne rve leaves the orbital cavity through the optic canal and
unites with the optic nerve of the opposite side to form the optic chiasma.
- The optic chiasma is situated at the junction of the anterior wall and floor
of the third ventricle.
- In the chiasma, the fibers from the nasal (medial) half of each retina,
including the nasal half of the macula cross the midline and enter the
optic tract of the opposite side, while the fibe rs from the temporal
(lateral) half of each retina, including the temporal half of the macula,
pass posteriorly in the optic tract of the same side.
- The optic tract emerges from the optic chiasma and passes
posterolate rally around the cerebral peduncle.
- Most of the fibers now terminate by synapsing with ne rve cells in the
lateral geniculate body, which is a small projection from the posterior
part of the thalamus. A few of the fibers pass to the pretectal nucleus and
the supe rior colliculus of the midbrain and are concerned with light
reflexes
- Lateral geniculate body is a small, oval swelling projecting from the
pulvinar of the thalamus. It consists of six layers of cells, on which
synapse the axons of the optic tract. The axons of the nerve cells within
the geniculate body leave it to form the optic radiation
- The optic radiation passes posteriorly through the retrolenticular part of
the internal capsule and te rminates in the visual cortex (area 17), which
occupies the upper and lowe r lips of the calcarine sulcus on the medial
surface of the cerebral hemisphere
- The visual association cortex (areas 18 and 19) is responsible for
recognition of objects and pe rception of color
Visual Reflexes:
Direct and Consensual light reflexes:
- If a light is shone into one eye, the pupils of both eyes normally constrict.
- The constriction of the pupil on which the light is shone is called the
direct light reflex ;the constriction of the opposite pupil, even though no
light fell on that eye, is called the consensual light reflex
- The afferent impulses travel through the optic nerve, optic chiasma, and
optic tract
- Here, a small number of fibers leave the optic tract and synapse on nerve
cells in the pretectal nucleus ,which lies close to the superior colliculus.
- The impulses are passed by axons of the pretectal nerve cells to the
parasympathetic nuclei (Edinger-Westphal nuclei) of the third cranial
nerve on both sides .
- Here, the fibers synapse and the parasympathetic ne rves travel through
the third cranial nerve to the ciliary ganglion in the orbit
- Finally, postganglionic parasympathetic fibers pass through the short
ciliary ne rves to the eyeball and the constrictor pupillae muscle of the iris.
- Both pupils constrict in the consensual light reflex because the pretectal
nucleus sends fibers to the parasympathetic nuclei on both sides of the
midbrain
- The fibe rs that cross the median plane do so close to the cerebral
aqueduct in the posterior commissure.
Accommodation reflex:
- When the eyes are directed from a distant to a near object, contraction of
the medial recti brings about convergence of the ocular axes; the lens
thickens to increase its refractive powe r by contraction of the ciliary
muscle; and the pupils constrict to restrict the light waves to the thickest
central part of the lens.
- The afferent impulses travel through the optic nerve, the optic chiasma,
the optic tract, the lateral geniculate body, and the optic radiation to the
visual cortex.
- The visual cortex is connected to the eye field of the frontal cortex
- From here, cortical fibers descend through the internal capsule to the
oculomotor nuclei in the midbrain.
- The oculomotor nerve travels to the medial recti muscles.
- Some of the descending cortical fibers synapse with the parasympathetic
nuclei (Edinger-Westphal nuclei) of the third cranial nerve on both sides .
- Here, the fibers synapse, and the parasympathetic ne rves travel through
the third cranial nerve to the ciliary ganglion in the orbit
pulvinar of the thalamus. It consists of six layers of cells, on which
synapse the axons of the optic tract. The axons of the nerve cells within
the geniculate body leave it to form the optic radiation
- The optic radiation passes posteriorly through the retrolenticular part of
the internal capsule and te rminates in the visual cortex (area 17), which
occupies the upper and lowe r lips of the calcarine sulcus on the medial
surface of the cerebral hemisphere
- The visual association cortex (areas 18 and 19) is responsible for
recognition of objects and pe rception of color
Visual Reflexes:
Direct and Consensual light reflexes:
- If a light is shone into one eye, the pupils of both eyes normally constrict.
- The constriction of the pupil on which the light is shone is called the
direct light reflex ;the constriction of the opposite pupil, even though no
light fell on that eye, is called the consensual light reflex
- The afferent impulses travel through the optic nerve, optic chiasma, and
optic tract
- Here, a small number of fibers leave the optic tract and synapse on nerve
cells in the pretectal nucleus ,which lies close to the superior colliculus.
- The impulses are passed by axons of the pretectal nerve cells to the
parasympathetic nuclei (Edinger-Westphal nuclei) of the third cranial
nerve on both sides .
- Here, the fibers synapse and the parasympathetic ne rves travel through
the third cranial nerve to the ciliary ganglion in the orbit
- Finally, postganglionic parasympathetic fibers pass through the short
ciliary ne rves to the eyeball and the constrictor pupillae muscle of the iris.
- Both pupils constrict in the consensual light reflex because the pretectal
nucleus sends fibers to the parasympathetic nuclei on both sides of the
midbrain
- The fibe rs that cross the median plane do so close to the cerebral
aqueduct in the posterior commissure.
Accommodation reflex:
- When the eyes are directed from a distant to a near object, contraction of
the medial recti brings about convergence of the ocular axes; the lens
thickens to increase its refractive powe r by contraction of the ciliary
muscle; and the pupils constrict to restrict the light waves to the thickest
central part of the lens.
- The afferent impulses travel through the optic nerve, the optic chiasma,
the optic tract, the lateral geniculate body, and the optic radiation to the
visual cortex.
- The visual cortex is connected to the eye field of the frontal cortex
- From here, cortical fibers descend through the internal capsule to the
oculomotor nuclei in the midbrain.
- The oculomotor nerve travels to the medial recti muscles.
- Some of the descending cortical fibers synapse with the parasympathetic
nuclei (Edinger-Westphal nuclei) of the third cranial nerve on both sides .
- Here, the fibers synapse, and the parasympathetic ne rves travel through
the third cranial nerve to the ciliary ganglion in the orbit
- Finally, postganglionic parasympathetic fibers pass through the short
ciliary ne rves to the ciliary muscle and the constrictor pupillae muscle of
the iris
Corneal reflex:
- Light touching of the cornea or conjunctiva results in blinking of the
eyelids.
- Afferent impulses from the cornea or conjunctiva travel through the
ophthalmic division of the trigeminal ne rve to the sensory nucleus of the
trigeminal nerve
- Inte rnuncial neurons connect with the motor nucleus of the facial nerve
on both sides through the medial longitudinal fasciculus.
- The facial nerve and its branches supply the orbicularis oculi muscle,
which causes closure of the eyelids.
Visual body reflexes:
- The automatic scanning movements of the eyes and head that are made
when reading, the automatic movement of the eyes, head, and neck
toward the source of the visual stimulus, and the protective closing of the
eyes and even the raising of the arm for protection are reflex actions that
involve the following reflex arcs
- The visual impulses follow the optic nerves, optic chiasma, and optic
tracts to the superior colliculi.
- Here, the impulses are relayed to the tectospinal and tectobulbar
(tectonuclear) tracts and to the neurons of the anterior gray columns of
the spinal cord and cranial motor nuclei.
Pupillary skin reflex:
- The pupil will dilate if the skin is painfully stimulated by pinching.
- The afferent sensory fibers are believed to have connections with the
efferent preganglionic sympathetic neurons in the lateral gray columns of
the first and second thoracic segments of the spinal cord.
- The white rami communicantes of these segments pass to the sympathetic
trunk, and the preganglionic fibe rs ascend to the superior cervical
sympathetic ganglion .
- The postganglionic fibers pass through the internal carotid plexus and the
long ciliary nerves to the dilator pupillae muscle of the iris.
ciliary ne rves to the ciliary muscle and the constrictor pupillae muscle of
the iris
Corneal reflex:
- Light touching of the cornea or conjunctiva results in blinking of the
eyelids.
- Afferent impulses from the cornea or conjunctiva travel through the
ophthalmic division of the trigeminal ne rve to the sensory nucleus of the
trigeminal nerve
- Inte rnuncial neurons connect with the motor nucleus of the facial nerve
on both sides through the medial longitudinal fasciculus.
- The facial nerve and its branches supply the orbicularis oculi muscle,
which causes closure of the eyelids.
Visual body reflexes:
- The automatic scanning movements of the eyes and head that are made
when reading, the automatic movement of the eyes, head, and neck
toward the source of the visual stimulus, and the protective closing of the
eyes and even the raising of the arm for protection are reflex actions that
involve the following reflex arcs
- The visual impulses follow the optic nerves, optic chiasma, and optic
tracts to the superior colliculi.
- Here, the impulses are relayed to the tectospinal and tectobulbar
(tectonuclear) tracts and to the neurons of the anterior gray columns of
the spinal cord and cranial motor nuclei.
Pupillary skin reflex:
- The pupil will dilate if the skin is painfully stimulated by pinching.
- The afferent sensory fibers are believed to have connections with the
efferent preganglionic sympathetic neurons in the lateral gray columns of
the first and second thoracic segments of the spinal cord.
- The white rami communicantes of these segments pass to the sympathetic
trunk, and the preganglionic fibe rs ascend to the superior cervical
sympathetic ganglion .
- The postganglionic fibers pass through the internal carotid plexus and the
long ciliary nerves to the dilator pupillae muscle of the iris.
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