Neuroscience Chapter on Brain Structure

Nervous System Functions

• The nervous system gathers sensory input, processes and interprets sensory input, and decides whether action is needed.
• Sensory input is when sensory receptors monitor changes inside and outside the body.
• Integration is where the nervous system processes and interprets sensory input to determine if an action is needed.
• Motor output is the response or effect that activates muscles or glands.

Nervous System Organization

• Nervous system classification is based on structures (structural classification) and activities (functional classification).
• The central nervous system (CNS) consists of the brain and spinal cord, which is the command center.
  • The brain interprets incoming sensory information and sends outgoing instructions.
• The peripheral nervous system (PNS) consists of nerves that extend from the brain and spinal cord.
  • Spinal nerves carry impulses to and from the spinal cord.
  • Cranial nerves carry impulses to and from the brain.
  • These nerves work as communication lines between sensory organs, the brains, and spinal cord, and glands or muscles.
• The PNS has two functional subdivisions: sensory (afferent) and motor (efferent).
  • Sensory (afferent) division carries information to the CNS from the skin, skeletal muscles, and joints. It also carries information from visceral organs.
  • Motor (eff)erent division carries impulses away from the CNS to effector organs (muscles and glands).
    • Somatic nervous system (voluntary) controls skeletal muscles.
    • Autonomic nervous system (involuntary) controls smooth and cardiac muscles, and glands.
      • Further divided into sympathetic and parasympathetic nervous systems.

Nervous Tissue: Supporting Cells

• Neuroglia, or glial cells, support, insulate, and protect neurons in the CNS.
  • Astrocytes are abundant, star-shaped cells, that brace and anchor neurons to blood capillaries and determine permeability and exchanges between blood and neurons. They protect neurons from harmful substances in the blood and control the chemical environment of the brain.
  • Microglia are spiderlike phagocytes that monitor health of nearby neurons and dispose of debris.
  • Ependymal cells line cavities of the brain and spinal cord and assist in circulation of cerebrospinal fluid.
  • Oligodendrocytes wrap around nerve fibers in the central nervous system to form myelin sheaths.
• Schwann cells produce myelin sheaths around nerve fibers in the PNS.
• Satellite cells protect and cushion neuron cell bodies in the PNS.

Nervous Tissue: Neurons

• Neurons are cells specialized to transmit messages (nerve impulses).
• Neurons have a cell body, containing nucleus and the metabolic center of the cell, and processes extending from the cell body.
  • Dendrites conduct impulses toward the cell body.
  • Axons conduct impulses away from the cell body.
  • Axons end in axon terminals, which contain neurotransmitter vesicles and are separated from the next neuron by a gap.
  • The synaptic cleft is the gap between axon terminals and the next neuron.
  • A synapse is the functional junction between nerves where a nerve impulse is transmitted.
• Myelin is a fatty material that covers axons and protects and insulates fibers, speeding nerve impulse transmission.
  • Myelin sheaths are formed by Schwann cells (PNS), containing a neurilemma, and Nodes of Ranvier, gaps in the sheath. While oligodendrocytes form myelin sheaths in the CNS, they lack a neurilemma.
• Terminology for neuron groups includes nuclei in the CNS and ganglia in the PNS. White matter consists of bundles of myelinated fibers(tracts). Gray matter consists of mostly unmyelinated fibers and cell bodies.
• Functional classification of neurons:
  • Sensory (afferent) neurons carry impulses from sensory receptors to the CNS.
    • Include receptors like cutaneous sense organs and proprioceptors in muscles and tendons.
  • Motor (efferent) neurons carry impulses from the CNS to effector organs (muscles and glands).
  • Interneurons (association neurons) are located in the CNS and connect sensory and motor neurons.

Nervous Tissue: Reflexes

• Reflexes are rapid, predictable, and involuntary responses to stimuli.
• Reflexes occur over neural pathways called reflex arcs.
• There are two types of reflexes:
  • Somatic reflexes stimulate skeletal muscles.
    • Example: Pulling your hand away from a hot object.
  • Autonomic reflexes regulate the activity of smooth muscles, the heart, and glands.
    • Example: regulation of smooth muscles, heart and blood pressure, glands, digestive system.
• A reflex arc has five elements:
  1. Sensory receptor
  2. Sensory Neuron
  3. Integration Center
  4. Motor Neuron
  5. Effector

Central Nervous System (CNS)

• The brain consists of several regions:
  • Cerebral hemispheres
  • Diencephalon
  • Brain stem
  • Cerebellum
  • Each region has specific functions.
• The cerebral cortex has three main regions:
  1. Superficial gray matter
  2. White matter
  3. Basal nuclei (deep pockets of gray matter)
• The cerebral cortex is involved in special senses, including visual, auditory and olfactory areas.
  • The primary motor area is located in the frontal lobe, involved in voluntary movement of skeletal muscles via axons descending to the spinal cord.
  • Broca's area (motor speech area) is in the frontal lobe involved in speaking. Anterior and posterior association areas are involved in higher functions.

Protection of the CNS

• Meninges protect the brain and spinal cord. The dura mater is the outermost layer, the arachnoid mater is the middle layer, and the pia mater is the inner layer.
• Cerebrospinal fluid (CSF) acts as a cushion to protect the brain and spinal cord and circulate through the arachnoid space, ventricles, and central canal of the spinal cord.
• The blood-brain barrier prevents harmful substances from entering the brain.

Brain and Spinal Cord

• Dysfunction can happen due to trauma or disease.

Peripheral Nervous System (PNS)

• The PNS includes nerves, which are bundles of neurons.
  • Nerves have coverings: endoneurium, perineurium, and epineurium.
  • Mixed nerves contain both sensory and motor fibers.
  • Sensory nerves carry impulses toward the CNS
  • Motor nerves carry impulses away from the CNS
• Cranial nerves provide sensory and motor functions for the head and neck region, with the exception of the vagus nerve which extends to thoracic and abdominal cavities.
• 31 pairs of spinal nerves exit the spinal cord, made of a combination of ventral and dorsal roots. Spinal nerves divide into dorsal and ventral rami.
  • Ventral rami form nerve plexuses to serve the motor and sensory needs of the limbs.
• The autonomic nervous system is a motor system outside the spinal cord that controls visceral (internal) activities, such as heart rate, blood pressure, and digestion.
  • Has two arms
    • Sympathetic division (thoracolumbar): fight-or-flight response.
    • Parasympathetic division (craniosacral): rest-and-digest response.

Note: The following concepts are also present, but could be elaborated further based on the specific material covered in each section:

• Specific nervous pathways and their functions: For example, specific tracts in the brain stem and spinal cord or sensory pathways to the cerebral cortex.
• Neurotransmitters: Delineating specific neurotransmitters and associated effects.