Anatomy of the Brain - Motor Cortex

Central Nervous System

• The brain receives and processes sensory information, initiates responses, stores memories, generates thoughts and emotions.
• Cerebrum has two hemispheres: left and right, which are extensively folded, with gyri (outward folds) and sulci (inward folds).
• Functions of the cerebrum include conscious experience of sensation and initiation of voluntary movement.
• Medulla oblongata regulates heart rate, blood pressure, breathing, walking, sleeping, and swallowing.
• Pons influences cortex to maintain consciousness and alertness, and regulates posture, locomotion, and visceral function.

Brainstem

• Midbrain is the location of the brainstem UMN pathways and subconscious posture and voluntary skilled/learned movements.

Cerebellum

• Cerebellum is responsible for premotor frontal cortex, which plans and organizes the sequence of movement.
• Supplementary motor cortex is responsible for preparatory orientation of the body to execute a particular motor task.
• Cerebellum integrates and interprets information, producing a specific learned output.

Spinal Nerves

• There are 36 pairs of spinal nerves that communicate the spinal cord with sensory receptors, muscles, viscera, and vessels.
• Each segment gives rise to paired spinal nerves that exit the vertebral canal via the lateral vertebral foramen of the atlas or via intervertebral foramen.
• Neurons innervating the limbs are confined to cervical and lumbar intumescences.

Peripheral Nervous System

• Sensory/Afferent system brings information to the CNS from receptors in peripheral tissues and organs.
• Motor/Efferent system carries motor commands from CNS to peripheral tissue and systems.
• Visceral Sensory provides information about internal organs, while Somatic Sensory provides information about position, touch, pressure, pain, and temperature.
• Somatic Motor controls skeletal muscle contraction, while Visceral Motor provides autonomic regulation of smooth muscle, cardiac muscle, glands, and adipose tissue.

Receptors

• Receptors are of different types, including free nerve endings, complex receptors, and special senses receptors.

Neurons

• Primary (first-order) neurons receive signals and send information to CNS.
• Secondary (second-order) neurons conduct impulses from spinal cord to brainstem to thalamus.
• Tertiary (third-order) neurons conduct impulses from thalamus to primary somatosensory cortex.
• Quaternary (fourth-order) neurons are located in sensory area of cerebral cortex.

Cranial Nerves

• There are 12 pairs of cranial nerves that arrive in the brainstem.
• Individual nerves have specific sensory and/or motor, somatic and/or autonomic functions.

Cerebrum or Telencephalon

• The cerebrum is the largest part of the brain and is divided into two hemispheres.
• It is composed of the cerebral cortex, white matter, and subcortical structures.

Cerebral Cortex

• The cerebral cortex is a layer of gray matter at the surface of the cerebrum.
• It is divided into 6 separate horizontal layers parallel to the surface of the cortex.
• The layers are:
  • I: Molecular
  • II: Outer granular (external granular)
  • III: Outer pyramidal (external pyramidal)
  • IV: Inner granular (internal granular)
  • V: Inner pyramidal (internal pyramidal)
  • VI: Multiform
• Different distribution of layers according to functional areas.
• Most incoming signals are received in layer IV.
• Most output signals leave the cortex through layers V and VI.
• Layer V sends signals to the brainstem and spinal cord.
• Layer VI sends signals to the thalamus.
• Layers I, II, and III are involved in intracortical association functions.

White Matter

• The white matter is composed of myelinated axons that connect the cerebral cortex with other brain regions.
• It is divided into:
  • Projection fibers
  • Association fibers
  • Commissural fibers
• Projection fibers leave the white matter and terminate in the basal nuclei, brainstem, or spinal cord.
• Association fibers connect regions of the cerebral cortex within one hemisphere.
• Commissural fibers connect the cortices from the right and left cerebral hemispheres.

Subcortical Structures

• Basal nuclei (Basal Ganglia) are gray matter nuclei located deep within the white matter of the cerebral hemisphere.
• They include the Caudate nucleus, nucleus accumbens, putamen, globus pallidum, and claustrum.
• The subthalamic nucleus and substancia nigra are also called Basal nuclei.
• Basal nuclei project output via the thalamus into the supplementary and premotor cortices.
• They are involved in planning and preparation for movement.
• They also send output directly to the brainstem.

Brain Regions

• The CNS can be anatomically subdivided into 7 major regions:
  • 1. Telencephalon or Cerebrum
  • 2. Diencephalon (thalamus and hypothalamus)
  • 3. Cerebellum
  • 4. Mesencephalon or Midbrain
  • 5. Pons
  • 6. Medulla oblongata
  • 7. Spinal cord

Functional Areas

• The brain can be divided into 5 major functional areas:
  • 1. Cerebrum
  • 2. Thalamus
  • 3. Hypothalamus
  • 4. Cerebellum
  • 5. Brainstem (Midbrain, Pons, and Medulla)

Commissures and Decussations

• A commissure is a place where fibers cross and connect the two cerebral hemispheres.
• Example: Corpus callosum
• A decussation is an intersection of pathways in the form of an X.
• This accounts for why each side of our brain has control over the opposite side of our body.

Basal Nuclei

• Basal nuclei are an accessory motor system that helps execute the initiation and control of movement.
• They are responsible for the cognitive control of motor activity.
• Studies in monkeys show that changes in neuronal activity of the basal nuclei occur prior to the firing of the neuronal activity of the motor cortex and the movement of body parts.