Nervous System Organization: Part II

Nervous System Organization: Part II

• The nervous system is organized into the central nervous system (CNS) and peripheral nervous system (PNS)
• Part I covered organization of the nervous system, classification of sensory receptors, somatic nervous system, autonomic nervous system, and cell types in the nervous system.

Neuronal Classification

• Neurons are categorized structurally into three types: unipolar, bipolar, and multipolar.
• Unipolar neurons: Possess a single process that branches close to the cell body into a trunk to supply dendrites for incoming signals and an axon for outgoing signals. Typically sensory neurons with receptors located in skin, joints, muscles, and internal organs. Dendritic and axon lengths vary.
• Bipolar neurons: Two processes emerge from the cell body; one forms a dendrite, the other forms an axon. Typically sensory neurons involved in visual and auditory systems. Axons are short.
• Multipolar neurons: Multiple branched extensions (dendrites) originate from the neuron cell body. Axons are typically long and allow for integration of information from other neurons. These are the majority of neurons in the body.

Functional Types of Neurons

• Sensory neurons: Long axons; transmit nerve impulses from sensory receptors to the central nervous system.
• Motor neurons: Long axons; transmit nerve impulses from the CNS to effectors (muscles and glands).
• Interneurons: Usually smaller, with many interconnections. Relay messages between other neurons.

Neuroglia

• Neuroglia are non-neuronal cells that support and protect neurons. Provide support, form myelin, and maintain homeostasis (e.g. proper levels of ions like Calcium and Potassium).
• Four main functions:
  • Surround neurons and hold them in place
  • Supply nutrients and oxygen to neurons
  • Insulate one neuron from another
  • Destroy pathogens and remove dead neurons.

Types of Neuroglia

• Astrocytes: Star-shaped glial cells throughout CNS tissue (brain and spinal cord). Maintain appropriate levels of ions and nutrients for neurons, help with migration of neurons during development, and aids in blood brain barrier formation.
• Microglial cells: Small glial cells responding to injury or disease in the CNS. Protect neurons from disease by clearing away debris and dead cells. Some texts describe microglia having a "mainly scavenging" function.
• Ependymal cells: Form lining of ventricles in brain and central canal of spinal cord. These cells produce and circulate cerebrospinal fluid.
• Oligodendrocytes: Found in CNS. Form supporting network around CNS neurons. Produces myelin sheath around several adjacent axons.
• Schwann cells: Found in PNS. Forms myelin sheath around a portion of a single axon. Aids in regeneration of PNS axons.

Nuclei vs. Ganglia

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

Tracts vs. Nerves

• Tracts: Collections of nerve fibers (axons) in the CNS.
• Nerves: Collections of nerve fibers (axons) in the PNS.

Myelinated vs. Unmyelinated Nerves

• Myelinated fibers conduct nerve impulses more rapidly than unmyelinated fibers.
• Myelin is formed by oligodendrocytes in the CNS, and Schwann cells in the PNS.
• Nodes of Ranvier are gaps between myelinated sections of the nerve fibers where depolarization occurs. This accelerates impulse propagation.

Brain Regions

• Cerebrum: Largest part of the brain; divided into right and left hemispheres. Involved in processing sensory and motor information and higher cognitive functions such as reasoning and planning.
• Cerebellum: Second largest part of the brain, responsible for coordinating movement, balance, and posture. Contains more neurons than other parts of the nervous system.
• Brainstem: Connects the cerebrum and cerebellum to the spinal cord. Composed of midbrain, pons, and medulla oblongata. Responsible for regulating many essential functions and reflexes.
• Diencephalon: Includes the thalamus (involved in relaying sensory information), hypothalamus (controls many autonomic functions like temperature and appetite) and pineal gland (produces melatonin).

Protective Coverings of CNS

• Outermost covering is bone (cranial bones and vertebrae).
• Innermost covering is the meninges (dura mater, arachnoid mater and pia mater). Dura mater is the tough outer layer, arachnoid mater resembles a web and lines the dura, and the innermost pia mater adheres to the spinal cord.

Brain Lobes

• Frontal lobe: Reasoning, planning, speech, movement, emotions, problem-solving
• Parietal lobe: Movement, orientation, recognition, perception
• Occipital lobe: Visual processing
• Temporal lobe: Auditory stimuli, memory, speech perception