Anatomy: Brain Structure and Lobes

Anatomy and Physiology Review: Brain Structure

• The brain is divided into three main parts: the forebrain, midbrain, and hindbrain (or brainstem).

Forebrain

• The forebrain consists of the cerebrum and the diencephalon.
• The cerebrum has right and left hemispheres, which are connected by the corpus callosum.
• The cerebral cortex is often referred to as the "gray matter" of the brain.

Lobes of the Cerebrum

• The frontal lobe is the executive decision maker and regulates behavior, including planning and adaptation.
  • It relies on data from other parts of the brain.
  • The prefrontal area relates to personality, motivation, and goal setting.
  • The motor and premotor areas control voluntary movement and the Parietal lobe handles perception, interpretation, touch, and body perception.
• The temporal lobe processes hearing.
• The occipital lobe is responsible for vision.

Limbic System

• The limbic system is known as "the emotional brain."
• It consists of many structures and is involved in memory, pleasure, emotions, and motivation.
• The limbic system handles olfactory and feeding functions, relating emotions, pleasure, smell, and food intake.

Key Structures Connecting to the Limbic System

• The hypothalamus regulates many body functions, including endocrine activity, sleep, appetite, and body temperature.
• The pineal gland plays a role in sleep by secreting melatonin and influencing consciousness.
• The thalamus associates sensory inputs with pleasant and unpleasant feelings.
• The hippocampus processes memories and forms new ones.
• The amygdala stores feelings related to memories.

Ventral Tegmental Area (VTA)

• The VTA, located in the midbrain, is also known as the "pleasure pathway" or reward center.
• It contains dopaminergic neurons that connect to limbic system sites involved in craving and addiction.

Basal Ganglia and Diencephalon

• The basal ganglia, or extrapyramidal system, regulates motor behavior, muscle tone, and posture and balances excitatory and inhibitory neurotransmitters.
• The diencephalon connects the cerebrum to the lower brain structures, relays sensory information to the cerebral cortex via the thalamus, and releases and inhibits hormones through the hypothalamus. The hypothalamus also regulates fluid balance, temperature, and gastrointestinal and cardiovascular functions.

Midbrain

• The midbrain, also called the mesencephalon, is responsible for visual, auditory, and balance reflexes, as well as posture.
• The substantia nigra in the midbrain produces dopamine.

Hindbrain and Brainstem

• The pons regulates respiration and muscle tone.
• The medulla regulates heart rate and blood pressure.
• The cerebellum controls muscle tone and coordination.

Reticular Formation

• The reticular formation integrates sensory input.
• The reticular activating system regulates alertness, the ability to pay attention by tuning out stimuli, and the sleep-wake cycle.

Neurons and Neurotransmission

• The central nervous system (CNS) is composed of neurons, with the cell body containing the nucleus and genetic material.
• Axons transmit messages to the next cell, and dendrites receive messages.
• There are three classes of neurons in the CNS: afferent (sensory), efferent (motor), and interneurons.
• The synapse is the contact point between two neurons, with the synaptic cleft being the space between them.
• The pre-synaptic neuron is the area of the axon where neurotransmitters are stored, and the postsynaptic neuron is the area of the dendrite where receptor sites are located.
• Neurotransmission is a chemical signaling process where neurotransmitters are released from the axon into the synaptic cleft and bind with receptors on the postsynaptic neuron, activating it.
• Calcium ions facilitate neurotransmitter release.
• After binding, the neurotransmitter substance is either deactivated or returned to the axon through reuptake.
• Nerve cells can release more than one type of neurotransmitter.

Role of Neurotransmitters

• Neurotransmitters play an important role in human emotions and behavior and influence motor activity and hormone levels.
• Neurotransmitters are a key target for psychotropic medications.

Categories of Neurotransmitters

• Acetylcholine (ACh) is involved in sleep, arousal, movement, and memory.
• Norepinephrine affects mood, cognition, perception, locomotion, cardiovascular functioning, sleep, and arousal.
• Dopamine affects movement, coordination, emotions, voluntary judgment, and the release of prolactin.
• Serotonin affects sleep, arousal, libido, appetite, mood, aggression, pain perception, coordination, and judgment.
• Histamine is involved in the immune/inflammatory response and appears to be involved in mood.
• Gamma Aminobutyric Acid (GABA) functions as an inhibitory neurotransmitter.
• Glutamate is an excitatory neurotransmitter.
• Neuropeptides such as endorphins and enkephalins modulate pain and reduce peristalsis, and they are also called natural or endogenous opiates.

Neuroendocrinology

• The hypothalamus controls the pituitary gland.
• Hormones, including TSH, melatonin, and ACTH, affect health.
• Circadian rhythms affect sleep cycles and regulate hormones, influencing mood and emotions, and play a role in mental illnesses.

Other Factors in Neurobiology

• Genetics plays a role, with research suggesting that major mental illnesses have genetic markers that increase the risk for these conditions.
• Psychoimmunology shows that normal immune responses are decreased with acute and chronic stress, grief, bereavement, and depression.

Neurobiology and Mental Illness

• In mental illnesses, neuroanatomical changes occur such as the enlargement of ventricles in schizophrenia and alcoholism, and atrophy of the hippocampus in cases of trauma or severe stress.
• Decreased cerebral blood flow in the prefrontal cortex in schizophrenia, and in the frontal lobe in attention-deficit hyperactivity disorder (ADHD).
• Neurotransmitter dysfunction involves a lack of availability or secretion such as too much dopamine in schizophrenia, insufficient serotonin or norepinephrine in depression, insufficient GABA in anxiety disorders, and too much norepinephrine in anxiety disorders.
• Neuroendocrine dysfunction examples include altered hypothalamic regulatory mechanisms in anorexia nervosa, increased hypothalamic-pituitary-adrenal axis (HPA) activity causing elevated cortisol levels due to early life deprivation, trauma, or stress, and thyroid dysfunction in anxiety or depression.