Nervous System Organization Part II

Nervous System Organization: Part II

• The nervous system is divided into the central nervous system (CNS) and peripheral nervous system (PNS).
• Previous sections covered the organisation of the nervous system and classification of sensory receptors, somatic and autonomic nervous systems, and types of cells in the nervous system.

Structural Classification of Neurons

• Neurons are categorized into three structural types:
  • Unipolar: A single process from the cell body, branching into a trunk for incoming signals (dendrites) and an axon for outgoing signals. Typically sensory neurons with receptors in the skin, joints, muscles, and internal organs. Axons usually extend to the spinal cord, with varying dendritic trunk lengths.
  • Bipolar: Two processes (one dendrite and one axon). Usually short and primarily sensory neurons associated with sensory receptor organs like the eyes and ears.
  • Multipolar: Several dendrites and a single axon extending from the cell body. The many dendrites of the multipolar neuron allow extensive information integration from other neurons. The axons of these neurons are typically long, transmitting information to distant parts of the nervous system. This is the most common type of neuron in the body.

Functional Types of Neurons

• Sensory neurons: Transmitting nerve impulses from sensory receptors to the CNS.
• Motor neurons: Transmitting nerve impulses from the CNS to effectors (muscles and glands).
• Interneurons: Smaller cells with many interconnections, acting as connectors or relay neurons within the CNS.

Neuroglia

• Non-neuronal cells in the CNS and PNS.
• Maintain homeostasis, form myelin, and support/protect neurons.
  • Astrocytes: Surround, support, and hold neurons in place; supply nutrients and oxygen to neurons; insulate neurons from each other; destroy pathogens and remove dead neurons; play a role in maintaining the blood-brain barrier.
  • Microglial: Protect neurons from disease; clear away debris/dead cells; function as scavengers in the CNS.
  • Ependymal: Line the ventricles of the brain and central canal of the spinal cord; form cerebrospinal fluid (CSF); aid in CSF circulation.
  • Oligodendrocytes: Form the supporting network around CNS neurons; equivalent to Schwann cells in PNS, producing myelin sheaths around multiple adjacent axons within the CNS.
  • Schwann cells: Form myelin sheaths around PNS axons, enabling faster impulse conduction.

Nuclei vs Ganglia

• Nuclei: Clusters of neuron cell bodies in the CNS.
• Ganglia: Clusters of neuron cell bodies in the PNS.

Tracts vs Nerves

• Tracts: Bundles of nerve fibers (axons) in the CNS.
• Nerves: Bundles of nerve fibers (axons) in the PNS.
• Nerves are surrounded and bundled together by three layers of connective tissue: endoneurium, perineurium, and epineurium.

Myelinated vs Unmyelinated Nerves

• Myelinated fibers conduct nerve impulses faster than unmyelinated fibers.
• Myelin is produced by oligodendrocytes in the CNS and Schwann cells in the PNS.
• Nodes of Ranvier are gaps in the myelin sheath along the axon that allow for faster saltatory conduction.

Brain Divisions

• The brain can be divided into:
  • Cerebrum: Divided into right and left hemispheres and connected by the corpus callosum; responsible for complex information processing, sensory and motor integration.
  • Cerebellum: Second largest part of the brain; coordinates movement, posture, and balance.
  • Brainstem: Includes the midbrain, pons, and medulla oblongata; regulates autonomic functions like breathing, heart rate.
  • Diencephalon: Includes thalamus, hypothalamus, and pineal gland; coordinating sensory information, integrating autonomic functions, regulating hormones.

Brain Lobes

• Frontal lobe: Associated with reasoning, planning, speech production, movement, and emotions.
• Parietal lobe: Associated with movement, orientation, recognition, and perception of stimuli.
• Temporal lobe: Associated with auditory processing and perception and memory.
• Occipital lobe: Associated with visual processing and perception.

Protective Coverings of the CNS

• Bone: Cranial bones encase the brain, vertebrae encase the spinal cord.
• Meninges: Inner membrane covering the CNS (dura mater, arachnoid mater, pia mater).