Brain Structure and Function

Brain Organization and Function

Spinal Cord

The spinal cord connects the brain to the external world, enabling the brain to act. It functions as a relay station, routing messages to and from the brain, and also operates as an automatic processing center for reflexes.

  • The top of the spinal cord merges with the brainstem, controlling basic life processes like breathing and digestion.
  • It extends from the brainstem to just below the ribs and is organized into 30 segments, each corresponding to a vertebra.
  • Each segment connects to a specific body part via the peripheral nervous system.
  • Sensory nerves transmit messages to the brain while motor nerves send messages from the brain to muscles and organs.
  • Spinal reflexes allow for immediate reactions to sensory messages without brain input, such as withdrawing from a hot object or the knee-jerk reflex. This saves crucial seconds in survival situations.
  • Damage to a spinal cord segment disrupts communication between the brain and lower segments, leading to paralysis. The lower the damage, the fewer functions are lost.

Neuroplasticity

Neuroplasticity is the nervous system's ability to change and adapt due to experiences, development, or injury.

  • It involves creating new synapses, pruning unused synapses, altering glial cells, and even generating new neurons.
  • Neuroplasticity enables continuous brain adaptation throughout life, although it is most prominent in youth.
  • Example: Bob Woodruff's recovery from traumatic brain injury through intensive therapy, showcasing the brain's capacity to regain lost functions.

Two Hemispheres

The cerebral cortex, the brain's surface, features gyri (folds) and sulci (grooves) that delineate functional areas.

  • The longitudinal fissure separates the brain into the left and right hemispheres.
  • Lateralization refers to the specialization of function in each hemisphere, particularly in language processing.
  • The left hemisphere controls the right side of the body and excels in forming memory associations, selective attention, and positive emotions.
  • The right hemisphere controls the left side of the body and is superior in pitch perception, arousal, and negative emotions.
  • The interaction between both hemispheres is crucial for behavior, rather than attributing behaviors to one hemisphere.
  • The corpus callosum, a band of approximately 200 million axons, facilitates communication between the hemispheres.
  • Split-brain patients, who have had their corpus callosum severed to control epilepsy, demonstrate the independent functions of each hemisphere.
  • For example, a split-brain patient may not be able to name an object presented to the left visual field (processed by the right hemisphere) but can draw it with the left hand (also controlled by the right hemisphere). The verbal left hemisphere can then name the object once it sees the drawing.

Lobes of the Brain

The forebrain, the largest brain region, includes the cerebral cortex and subcortical structures such as the thalamus, hypothalamus, pituitary gland, and limbic system.

  • The cerebral cortex is associated with higher-level processes like consciousness, thought, emotion, reasoning, language, and memory.
  • Each hemisphere divides into four lobes: frontal, parietal, temporal, and occipital lobes.
Frontal Lobe

Located at the front of the brain, extending back to the central sulcus, the frontal lobe is involved in reasoning, motor control, emotion, and language.

  • It contains the motor cortex for planning and coordinating movement, the prefrontal cortex for higher-level cognitive functions, and Broca’s area for language production.
  • Damage to Broca’s area impairs the ability to produce language.
  • Phineas Gage’s case illustrates the frontal lobe’s role in personality and impulse control. An iron rod pierced his frontal lobe, leading to personality changes and loss of impulse control.
  • Subsequent investigations suggest damage to pathways between the frontal lobe and limbic system, affecting emotional regulation.
Parietal Lobe

Located behind the frontal lobe, the parietal lobe processes sensory information from the body.

  • It contains the somatosensory cortex, which processes touch, temperature, and pain.
  • Spatial relationships in the body are mirrored in the organization of the somatosensory cortex, with larger areas dedicated to body parts with more surface area and nerves, such as fingers compared to toes.
Temporal Lobe

Located on the side of the head, the temporal lobe is associated with hearing, memory, emotion, and aspects of language.

  • The auditory cortex, responsible for processing auditory information, and Wernicke’s area, important for speech comprehension, are located here.
  • Damage to Wernicke’s area results in the ability to produce sensible language but an inability to understand it.
Occipital Lobe

Located at the back of the brain, the occipital lobe contains the primary visual cortex, responsible for interpreting incoming visual information.

  • The occipital cortex is organized retinotopically, closely relating the position of an object in a person’s visual field to its representation on the cortex.

Other Forebrain Areas

Thalamus

The thalamus is a sensory relay center for the brain. All senses except smell are routed through the thalamus before being directed to other areas for processing.

Limbic System

The limbic system processes emotion and memory.

  • Smell projects directly to the limbic system, evoking emotional responses more strongly than other senses.
  • Key structures include the hippocampus, essential for learning and memory; the amygdala, involved in emotion and tying emotional meaning to memories; and the hypothalamus, which regulates homeostatic processes, interfaces between the nervous and endocrine systems, and regulates sexual motivation and behavior.
  • The case of Henry Molaison (H.M.), who had his hippocampus and amygdala removed to control seizures, illustrates the hippocampus's role in forming new explicit memories. After the surgery H.M. lost the ability to form many types of new memories.

Midbrain and Hindbrain Structures

Midbrain

The midbrain is located between the forebrain and hindbrain and includes the reticular formation, substantia nigra, and ventral tegmental area (VTA).

  • The reticular formation regulates the sleep/wake cycle, arousal, alertness, and motor activity.
  • The substantia nigra and VTA produce dopamine and are critical for movement, mood, reward, and addiction. Degeneration of these regions is involved in Parkinson’s disease.
Hindbrain

The hindbrain, located at the back of the head, contains the medulla, pons, and cerebellum.

  • The medulla controls autonomic processes like breathing, blood pressure, and heart rate.
  • The pons connects the hindbrain to the rest of the brain and regulates brain activity during sleep.
  • The brainstem includes the medulla, pons, and other structures spanning the midbrain and hindbrain.
  • The cerebellum controls balance, coordination, movement, and motor skills. It also processes procedural memory. H.M.'s ability to learn new tasks despite not forming new explicit memories indicates the cerebellum's role in procedural memory.

Brain Death and Life Support

Cases like Terri Schiavo's raise complex ethical questions about brain death and life support.

  • Schiavo, in a vegetative state with no cerebral cortex activity but an intact brainstem, could breathe and move her eyes involuntarily. The decision to remove her feeding tube led to legal battles between her husband and parents.
  • Such cases highlight the challenges in determining when to discontinue medical support for patients declared brain dead.

Brain Imaging Techniques

Brain imaging techniques provide information about brain function in living individuals, complementing data from brain injury studies.

Techniques Involving Radiation
Computerized Tomography (CT) Scan
  • CT scans use multiple X-rays to create an image of the brain.
  • They are used to identify tumors or significant atrophy.
Positron Emission Tomography (PET) Scan
  • PET scans create pictures of the living, active brain.
  • The subject drinks or is injected with a mildly radioactive substance.
  • A computer monitors the movement of the tracer and creates a rough map of active and inactive areas of the brain during a given behavior.
  • CT/PET scans allow better imaging of the activity of neurotransmitter receptors and open new avenues in schizophrenia research.
Techniques Involving Magnetic Fields
Magnetic Resonance Imaging (MRI)
  • MRI uses a strong magnetic field to cause hydrogen atoms in the body’s cells to move.
  • When the magnetic field is turned off, the hydrogen atoms emit electromagnetic signals as they return to their original positions.
  • Tissues of different densities give off different signals, which a computer interprets and displays on a monitor.
Functional Magnetic Resonance Imaging (fMRI)
  • fMRI operates on the same principles as MRI, but it shows changes in brain activity over time by tracking blood flow and oxygen levels.
  • MRI and fMRI are often used to compare the brains of healthy individuals to the brains of individuals diagnosed with psychological disorders. This comparison helps determine what structural and functional differences exist between these populations.
Techniques Involving Electrical Activity
Electroencephalography (EEG)
  • EEG measures a brain’s electrical activity.
  • An array of electrodes is placed around a person’s head.
  • The signals received by the electrodes result in a printout of the electrical activity of their brain, or brainwaves, showing both the frequency (number of waves per second) and amplitude (height) of the recorded brainwaves, with an accuracy within milliseconds.
  • EEG is helpful to researchers studying sleep patterns among individuals with sleep disorders.