Central Nervous System Overview

Central Nervous System (CNS)

• The nervous system is a single unit, anatomically divided into two parts:
  • Central Nervous System (CNS):
    • Consists of the brain and spinal cord
    • Acts as the integrating and command center
  • Peripheral Nervous System (PNS):
    • Includes cranial and spinal nerves
    • Facilitates communication between the CNS and the body

Sensory and Motor Divisions

• Sensory division:
  • Somatic and visceral fibers carry signals from receptors to the CNS.
• Motor division:
  • Motor nerve fibers carry signals from the CNS to effectors.
  • Somatic nervous system: controls voluntary movements, targeting skeletal muscles.
  • Autonomic nervous system: regulates involuntary functions, affecting cardiac muscle, smooth muscle, and glands.
    • Sympathetic division: "fight or flight" response
    • Parasympathetic division: conserves energy during rest

Brain Structure

• Complexity of wiring is more important than size
• Subdivisions:
  • Cerebral hemispheres
  • Diencephalon (thalamus, hypothalamus, epithalamus)
  • Brain stem (midbrain, pons, medulla)
  • Cerebellum

Gray and White Matter

• Gray matter:
  • Consists of short, nonmyelinated neurons and neuron cell bodies
  • Some gray matter is organized into nuclei
  • Some is distributed as cortical areas
• White matter:
  • Primarily myelinated axons
  • Some nonmyelinated axons may also be present

Spinal Cord Structure

• Central cavity surrounded by gray matter (butterfly shape).
• White matter surrounds the gray matter.
• Brain displays similar design, but with additional gray matter regions, particularly in the cerebral hemispheres and cerebellum.

Ventricles of the Brain

• Continuous with each other and the central cavity of the spinal cord.
• Filled with cerebrospinal fluid (CSF).
• Lined with ependymal cells.
• Paired lateral ventricles are separated by the septum pellucidum.
• Each communicates with a narrow third ventricle.
• The third ventricle connects to the fourth ventricle via the cerebral aqueduct.
• The fourth ventricle continues with the central canal.
• Three apertures connect the ventricles to the subarachnoid space, which surrounds the brain.

Cerebral Hemispheres

• Superior part (~83% of brain mass)
• Gyri are separated by sulci.
• Anatomical landmarks include the longitudinal fissure and a transverse fissure
• Lobes (frontal, parietal, occipital, temporal, insular)
  • Central sulcus separates precentral/postcentral gyrus.
  • Other important sulci include the parieto-occipital and lateral sulci.

Cerebral Cortex (Gray Matter)

• Enables perception, communication, memory, understanding, appreciation, and voluntary movements (conscious behavior)
• Cell bodies, dendrites, and unmyelinated axons comprise a thin layer (2-4 mm), although convolutions increase the surface area.
• Brodmann areas (BA) are numbered based on thickness and histology.
• Functional areas include motor, sensory, and association areas.
• Each hemisphere handles sensory and motor functions of the opposite (contralateral) side of the body.

Functional Areas

• Motor areas:
  • Primary motor cortex (BA4): precentral gyrus: controls skeletal muscles (axons project to spinal cord)
  • Premotor cortex (BA6): plans and sequences motor movements, sometimes referred to as muscle memory.
  • Broca's area (BA44): organizes the sounds that become words.
• Frontal eye field (close to BA 8): controls voluntary eye movements.
• Sensory areas:
  • Primary somatosensory cortex (PSC) (BA1-3) in the postcentral gyrus: receives info from somatic sensory receptors (skin and proprioceptors).
  • Includes spatial discrimination.
  • Somatosensory association cortex (BA 5-7): integrates sensory input.
• Visual areas:
  • Primary visual cortex (PVC) in the posterior tip of the occipital lobe.
  • Contains map of visual space on retina (contralateral)
  • Visual association: surrounds PVC, interprets visual images, recognizes faces, letters.

Auditory areas

• Primary auditory cortex processes sound based on pitch, rhythm, and loudness
• Auditory association area interprets sound in relation to memory (speech, words, music.

Vestibular cortex

• Awareness of balance - posterior part of insula and adjacent parietal cortex (in the lateral sulcus).

Olfactory cortex

• Medial aspects of temporal lobes (the uncus)
• Important part of the limbic system
• Responsible for conscious awareness of odors

Gustatory cortex

• Located in insula
• Responsible for taste perception

Visceral sensory area

• Posterior to gustatory cortex
• Processes visceral information

Multimodal Association Areas

• Receive input from multiple senses and send output to multiple areas.
• Important areas include the anterior association (prefrontal) cortex and the posterior association area.

Anterior Association Area (Prefrontal Cortex):

• Complex learning, abstract thinking, personality, working memory, judgment, reasoning, persistence, planning, concern, and conscience.
• Dependent on feedback from the social environment for maturation.
• Closely linked to the limbic system.

Basal Nuclei

• Include the caudate nucleus, putamen, and globus pallidus.
• Receive input from the entire cerebral cortex, other subcortical nuclei, and one other to project to the premotor and prefrontal cortices and have influence on muscle movements.
• Involved in starting, stopping, and regulating the intensity of movement.
• Involve complex cognitive functions through filtering actions.

Disorders of Basal Nuclei

• Huntington's disease: Too much movement.
• Parkinson's disease: Too little movement.

Diencephalon

• Consists of the thalamus and hypothalamus.
• Encloses the third ventricle.
• Thalamus: two masses of gray matter connected by the interthalamic adhesion.
  • Sensory input is organized and routed to appropriate cortical regions.
• Hypothalamus: involved in autonomic control, emotional responses, body temperature regulation, hunger, thirst, sleep-wake cycles, endocrine system control via releasing factors and nuclei
  • Regulation of the endocrine system.

Epithalamus

• Most dorsal part of the diencephalon.
• Forms the roof of the third ventricle.
• Includes the pineal gland.
• The choroid plexus is also part of the epithalamus.