Forebrain and its Components

Forebrain

  • Most modern portion of the brain.
  • Largest portion of the brain in humans by weight and volume.
  • Contains:
    • Regions derived from the diencephalon
    • Derivatives of the telencephalon

Diencephalon

  • Thalamus
  • Hypothalamus
  • Posterior Pituitary
  • Pineal Gland

Telencephalon

  • Cerebral Cortex
  • Basal Ganglia
  • Limbic System

Thalamus

  • Relay station for incoming sensory information (except smell).
  • Sorts and transmits sensory impulses to the appropriate areas of the cerebral cortex.
  • Sensory way station.

Hypothalamus

  • Subdivided into:
    • Lateral Hypothalamus (LH)
    • Ventromedial Hypothalamus (VMH)
    • Anterior Hypothalamus
  • Homeostatic functions.
  • Key player in emotional experiences during high arousal states, aggressive behavior, and sexual behavior.
  • Controls some endocrine functions and the autonomic nervous system.
  • Regulates metabolism, temperature, and water balance.
  • Osmoreceptors trigger the release of antidiuretic hormone (ADH) to increase water reabsorption.
  • Primary regulator of the autonomic nervous system.
  • Important in drive behaviors (hunger, thirst, sexual behavior).

Lateral Hypothalamus (LH)

  • Hunger center.
  • Detects when the body needs more food or fluids.
  • Triggers eating and drinking.
  • Destruction leads to refusal to eat and drink (starvation if not force-fed).

Ventromedial Hypothalamus (VMH)

  • Satiety center.
  • Provides signals to stop eating.
  • Brain lesions lead to obesity.

Anterior Hypothalamus

  • Controls sexual behavior.
  • Stimulation leads to mounting behavior.
  • Damage leads to permanent inhibition of sexual activity in many species.
  • Regulates sleep and body temperature.

Other Parts of the Diencephalon

  • Posterior Pituitary Gland
    • Comprised of axonal projections from the hypothalamus
    • Site of release for hypothalamic hormones: antidiuretic hormone (ADH/vasopressin) and oxytocin.
  • Pineal Gland
    • Key player in several biological rhythms.
    • Secretes melatonin, which regulates circadian rhythms.
    • Receives direct signals from the retina for coordination with sunlight.

Basal Ganglia

  • Coordinate muscle movement.
  • Receive information from the cortex and relay it via the extrapyramidal motor system to the brain and spinal cord.
  • Extrapyramidal system gathers information about body position and carries this information to the central nervous system but does not function directly through motor neurons.
  • Help make our movement smooth and posture steady.
  • Destruction of portions associated with Parkinson's disease (jerky movements and uncontrolled rest tremors).
  • May play a role in schizophrenia and obsessive-compulsive disorder.

Limbic System

  • Comprises interconnected structures looping around the central portion of the brain.
  • Associated with emotion and memory.

Primary Components:

  • Septal Nuclei
  • Amygdala
  • Hippocampus
  • Anterior Cingulate Cortex
Other Components:
  • Thalamus
  • Hypothalamus
  • Cortex

Septal Nuclei

  • Primary pleasure centers in the brain.
  • Mild stimulation is intensely pleasurable.
  • Association between these nuclei and addictive behavior.

Amygdala

  • Important role in defensive and aggressive behaviors (fear and rage).
  • Damage reduces aggression and fear reactions, resulting in docility and hypersexual states.

Hippocampus

  • Vital role in learning and memory processes.
  • Helps consolidate information to form long-term memories.
  • Can redistribute remote memories to the cerebral cortex.
  • Communicates with other portions of the limbic system through the fornix.
  • Henry Molaison (H.M.)
    • Parts of temporal lobes, including the amygdala and hippocampus, were removed
    • Suffered drastic and irreversible loss of memory for any new information (anterograde amnesia).
    • Memory for events before brain injury remained intact.
    • Retrograde amnesia refers to memory loss of events before brain surgery.
  • Anterograde Amnesia
    • Not being able to establish new long term memories
  • Retrograde Amnesia
    • Memory loss of events that transpired before brain surgery

Anterior Cingulate Cortex

  • Connection with the frontal and parietal lobes.
  • Functions in higher-order cognitive processes (regulation of impulse control and decision-making).
  • Maintains connections to other parts of the limbic system (role in emotion and motivation).

Cerebral Cortex

  • Outer surface of the brain.
  • Neocortex (most recent brain region to evolve).
  • Numerous bumps and folds (gyri and sulci).
  • Convoluted structure provides increased surface area.
  • Divided into two halves (cerebral hemispheres).
  • Divided into four lobes:
    • Frontal Lobe
    • Parietal Lobe
    • Occipital Lobe
    • Temporal Lobe

Frontal Lobe

  • Prefrontal Cortex
  • Motor Cortex
Prefrontal Cortex
  • Manages executive functions.
  • Supervises and directs operations of other brain regions.
  • Communicates with the reticular formation in the brainstem to regulate attention and alertness.
  • Supervises processes associated with perception, memory, emotion, impulse control, and long-term planning.
  • Does not store memory traces but reminds the individual to remember.
  • Lesions impair supervisory functions, leading to impulsivity, decreased control of behavior, angry outbursts, crying, vulgar remarks, and apathy to emotional responses of others.
  • Good example of an association area (integrates input from diverse regions of the brain).
Association Area
  • Area that integrates input from diverse regions of the brain
Motor Cortex
  • Located on the precentral gyrus.
  • Initiates voluntary motor movements by sending neural impulses down the spinal cord toward the muscles.
  • Considered a projection area in the brain.
Projection Area

  • Perform more rudimentary, perceptual, and motor tasks.

  • Neurons arranged systematically according to the parts of the body to which they are connected (motor homunculus).

  • Muscles requiring fine motor control take up additional space in the cortex.

Broca's Area
  • Important for speech production.
  • Usually found in only one hemisphere (dominant hemisphere, usually the left).

Parietal Lobe

  • Located to the rear of the frontal lobe.
Somatosensory Cortex
  • Located on the postcentral gyrus.
  • Involved in somatosensory information processing (touch, pressure, temperature, and pain).
  • Projection area.
  • Closely related to motor cortex (sensorimotor cortex).
Central Region
  • Associated with spatial processing and manipulation.
  • Enables orientation in three-dimensional space, spatial manipulation of objects, and spatial orientation skills (map reading).

Occipital Lobe

  • Located at the very rear of the brain.
Visual Cortex
  • Also called the striate cortex.
  • Visual cortex appears furrowed or striped
  • Areas also implicated in learning and motor control.

Temporal Lobe

  • Associated with a number of functions.
Auditory Cortex
  • Located in the temporal lobe.
  • Primary site of most sound processing (speech, music, and other sound information).
Wernicke's Area

  • Located in the temporal lobe.

  • Associated with language reception and comprehension.

  • Functions in memory processing, emotion, and language.

  • Electrical stimulation can evoke memories for past events.

  • Hippocampus is located deep inside the temporal lobe.

  • Lobes are not truly independent of one another.

  • Sensory modality may be represented in more than one area.

Cerebral Hemispheres and Laterality

  • In most cases, one side of the brain communicates with the opposite side of the body (contralaterally).
  • In other cases, hemispheres communicate with the same side of the body (ipsilaterally).

Dominant Hemisphere

  • More heavily stimulated during language reception and production.
  • Hand dominance was used as a proxy for hemispheric dominance (not always accurate).
  • 95%95\% of right-handed individuals are left-brain dominant.
  • 18%18\% of left-handed individuals are right-brain dominant.
  • Primarily analytic in function (language, logic, and math skills).
  • Language production (Broca's area) and language comprehension (Wernicke's area) are primarily driven by the dominant hemisphere.

Nondominant Hemisphere

  • Associated with intuition, creativity, music cognition, and spatial processing.
  • Simultaneously processes the pieces of a stimulus and assembles them into a holistic image.
  • Less prominent role in language.
  • More sensitive to the emotional tone of spoken language.
  • Permits us to recognize others' moods based on visual and auditory cues.
  • Dominant hemisphere screens incoming language to analyze its content, and the nondominant hemisphere interprets it according to its emotional tone.