Neuropsychology of Handedness and Lateralization

Nervous System

• The nervous system is composed of two primary divisions: the Central Nervous System (CNS) and the Peripheral Nervous System (PNS).
• The CNS is composed of the brain and spinal cord.
• The PNS is composed of the somatic and autonomic nervous systems.
• The somatic nervous system interacts with the external environment.
  • Afferent nerves carry sensory signals from skin, muscles, joints, eyes, and ears to the CNS.
  • Efferent nerves carry motor signals from the CNS to skeletal muscles.
• The autonomic nervous system regulates the body's internal environment.
  • Afferent nerves carry sensory signals from internal organs to the CNS.
  • Efferent nerves carry motor signals from the CNS to internal organs.
  • Two types of efferent nerves:
    • Sympathetic nerves project from the lumbar and thoracic regions of the spinal cord, stimulating, organizing, and mobilizing energy resources in threatening situations.
    • Parasympathetic nerves project from the brain and the sacral region of the spinal cord, working to conserve energy.
• The somatic and autonomic nervous systems work together to regulate bodily functions and control responses to stimuli.

Meninges

• The brain and spinal cord are protected by three protective membranes called meninges.
• The outermost tough membrane is called the dura mater.
• Beneath the dura mater is the arachnoid membrane, a spider web-like membrane containing blood vessels and cerebrospinal fluid (CSF).
• The innermost delicate membrane is called the pia mater and adheres to the surface of the CNS.
• The CNS is protected by cerebrospinal fluid (CSF).
  • CSF fills the subarachnoid space.
  • CSF fills the central canal of the spinal cord.
  • CSF fills the cerebral ventricles of the brain.
  • The cerebral ventricles are four large internal chambers of the brain.
    • Two lateral ventricles
    • Third ventricle
    • Fourth ventricle
• CSF supports and cushions the brain. Loss of CSF can cause severe headaches and pain with sudden head movements.

Hydrocephalus

• Occurs when CSF flow is blocked (often by a tumor) near a narrow channel linking the ventricles, leading to the expansion of the ventricle walls and the entire brain.

Neurons

• Specialized cells for the reception, conduction, and transmission of electrochemical signals.
• Come in various shapes and sizes but many share similar structures.
  • Neuron cell membrane
    • Composed of a lipid bilayer of fat molecules
    • Embedded with protiens
      • Channel proteins: Allow certain molecules to pass through.
      • Signal proteins: Allow transfer signals to the inside of the neuron when specific molecules bind to them on the outside of the membrane.
  • Classes of Neurons
    • Multipolar neurons– Have more than two processes extending from their cell body (most common).
    • Unipolar neurons– Have one process extending from their cell body.
    • Bipolar neurons– Have two processes extending from their cell body.
    • Interneurons– (within CNS) have short axons or no axons at all and integrate neural activity within a single brain structure.
  • Neurons and Neuroanatomical Structure
    • Gross neural structures are composed of either cell bodies or axons.
      • In the CNS:
        • Clusters of cell bodies are called nuclei.
        • Bundles of axons are called tracts.
      • In the PNS:
        • Clusters of cell bodies are called ganglia.
        • Bundles of axons are called nerves.
• Glial cells:
  • Oligodendrocytes: Located in the CNS and form myelin sheaths around axons (increasing axonal conduction) - one oligodendrocyte helps many neurons
  • Schwann Cells: Located in the PNS and form the myelin sheath; plays a role in guiding axonal regeneration after nerve damage (unlike oligodendrocytes).
  • Microglia: Smallest glial cells, play a role in responding to injury or disease (multiplying, engulfing cellular debris, triggering inflammatory responses, and synapse elimination).
  • Astrocytes: Largest glial cells shaped like stars, whose extensions cover blood vessels and neurons, regulating chemical passage between blood and CNS neurons.
    • Supporting essential functions: Exchange chemical signals with neurons and other astrocytes; modulate neutral activity and form functional networks.

Functional Brain Imaging (fMRI)

• Monitors brain activity while subjects perform language tasks.
• Shows greater activation in the left hemisphere during language processing.

Cerebral Lateralization

• Left Hemisphere: controls the right side of the body; is dominant for language production in 95% of right-handers.

• Right Hemisphere: controls the left side of the body; often plays a role in spatial reasoning, musical aptitude and aspects of nonverbal communication.

  • 80% of left-handers do NOT have left hemisphere dominance.
  • Some left-handers have right-hemisphere speech dominance.

• Processes sensory information from each sensory modality in both hemispheres, although slightly stronger input comes from the contralateral ear (each hemisphere receives input from both ears).

• Taste and smell are uncrossed--each hemisphere recieves signals from each side of the tongue and nasal passages.

• Language (speech) is usually more dominant in the left hemisphere in humans (specifically the left frontal lobe).

• The Corpus Callosum and interhemispheric commissures like the anterior commissure and hippocampal commissure allow communication between hemispheres coordinating movement and processing information.

• Anatomical differences between the hemispheres. The planum temporale is generally larger in the left hemisphere but is also larger in the left hemisphere of adults.