Anatomy and Physiology of Thalamus, Hypothalamus and Basal Ganglia
Thalamus:
The thalamus, measuring about 3 cm in length and comprising 80% of the diencephalon, consists of paired oval masses of gray matter organized into nuclei with interspersed tracts of white matter. A bridge of gray matter known as the interthalamic adhesion joins the right and left halves of the thalamus in about 70% of human brains.
A vertical Y-shaped sheet of white matter called the internal medullary lamina divides the gray matter of the thalamus. It contains myelinated axons that enter and leave the thalamic nuclei. Axons connecting the thalamus and cerebral cortex pass through the internal capsule, a thick band of white matter lateral to the thalamus.
The thalamus acts as the major relay station for sensory impulses reaching the primary sensory areas of the cerebral cortex from the spinal cord and brainstem. It also contributes to motor functions by transmitting information from the cerebellum and basal nuclei to the primary motor cortex, and it plays a role in maintaining consciousness.
Major Nuclei of the Thalamus:
- Anterior nucleus: Receives input from the hypothalamus and sends output to the limbic system. Functions in emotions and memory.
- Medial nuclei: Receive input from the limbic system and basal nuclei and send output to the cerebral cortex. Functions in emotions, learning, memory, and cognition.
- Lateral group nuclei: Receive input from the limbic system, superior colliculi, and cerebral cortex. Includes:
- Lateral dorsal nucleus – emotion expression
- Lateral posterior & pulvinar nuclei – sensory integration
- Ventral group nuclei:
- Ventral anterior nucleus – movement control (from basal nuclei)
- Ventral lateral nucleus – movement control (from cerebellum and basal nuclei)
- Ventral posterior nucleus – relays somatic sensations like touch, pressure, vibration, pain, and proprioception
- Lateral geniculate nucleus – visual relay (retina to visual cortex)
- Medial geniculate nucleus – auditory relay (ear to auditory cortex)
- Intralaminar nuclei: Connect with reticular formation, cerebellum, and cortex. Functions in arousal and integration of sensory and motor information.
- Midline nucleus: Located adjacent to the third ventricle; involved in memory and olfaction.
- Reticular nucleus: Surrounds the thalamus; monitors and integrates thalamic activities.
Hypothalamus:
The hypothalamus, weighing about 7 grams, is located below and in front of the thalamus, above the pituitary gland. It plays a central role in maintaining homeostasis and consists of several nuclei grouped into four main regions.
Regions of the Hypothalamus:
- Mammillary region: Contains the mammillary bodies and posterior hypothalamic nuclei. Involved in reflexes related to smell.
- Tuberal region: The widest part; includes the dorsomedial, ventromedial, and arcuate nuclei, and the infundibulum connecting the pituitary gland.
- Supraoptic region: Located above the optic chiasm; includes paraventricular and supraoptic nuclei which form the hypothalamo-hypophyseal tract to the posterior pituitary.
- Preoptic region: Lies anterior to the supraoptic region and helps regulate autonomic functions.
The hypothalamus links the nervous and endocrine systems by controlling the pituitary gland. Through nerve fibers and blood vessels, it regulates the output of hormones from both lobes of the pituitary gland.
Functions of the Hypothalamus:
- Regulates the autonomic nervous system
- Controls appetite and satiety
- Maintains thirst and water balance
- Regulates body temperature
- Controls emotional behavior (pleasure, fear, rage)
- Influences sexual behavior and child-rearing
- Regulates sleep and circadian rhythms
Basal Ganglia:
The basal ganglia are a group of subcortical nuclei responsible for motor control, learning, emotion, and behavior regulation. Located at the base of the forebrain, they play a key role in movement coordination and are affected in disorders like Parkinson’s and Huntington’s disease.
Structure of Basal Ganglia:
The basal ganglia consist of the corpus striatum (caudate and lenticular nuclei), the subthalamic nucleus, and the substantia nigra. These structures are interconnected, forming pathways that either promote or inhibit movement.
Divisions of Basal Ganglia:
- Corpus Striatum: The largest component, divided into:
- Dorsal striatum – controls conscious motor movements (caudate nucleus and putamen).
- Ventral striatum – involved in reward and emotion (nucleus accumbens and olfactory tubercle).
- Globus Pallidus: Includes internal and external segments; involved in regulating voluntary movement.
- Subthalamic Nucleus (STN): Located in the subthalamus, involved in motor control.
- Substantia Nigra (SN): Located in the midbrain; divided into pars compacta and pars reticulata. It produces dopamine and connects with other basal ganglia nuclei.
Functions of Basal Ganglia:
- Participation in reward and reinforcement mechanisms
- Regulation of voluntary motor movements
- Control of posture and muscle tone
- Learning of motor skills and habits
- Emotional and cognitive processing
Detailed Notes
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