Nervous System: CNS & PNS

• The nervous system has two principal parts: the Central Nervous System (CNS) and the Peripheral Nervous System (PNS).

Central Nervous System (CNS)

• Composed of the brain and spinal cord.
• Interprets sensory information and creates appropriate responses

Peripheral Nervous System (PNS)

• Composed of nerves that carry impulses.
• Spinal nerves extend from the spine.
• Cranial nerves extend from the brain.

Sensory (Afferent) Division

• In the PNS it conveys impulses to the CNS from sensory receptors throughout the body (skin, muscles, joints).
• Visceral afferents carry impulses from visceral organs.

Motor (Efferent) Division

• In the PNS it conveys impulses FROM the CNS to effector organs, muscles, and glands.
• This division has two sub-divisions: the Somatic Nervous System and the Autonomic Nervous System

Somatic Nervous System

• Conducts voluntary responses

Autonomic Nervous System

• Conducts involuntary responses

Autonomic Nervous System further divisions:

• Sympathetic Nervous System focuses on maintenance of homeostasis when the body is in a state of stress, called "Fight or Flight".
  • Examples include pupil dilation, blood pressure increase, and heart rate increase.
• Parasympathetic Nervous System focuses on activities the body completes when the body is in a state of rest, called "Rest and Digest".
  • Examples include salivation, lacrimation, defecation, urination, and digestion.

Histology - Supporting Cells (Neuroglial Cells)

• Astrocytes anchor neurons to their nutrient supplies.
• Microglial cells protect the CNS from invading organisms and dead tissues.
• Ependymal cells line the cranial and spinal cavities creating a semipermeable membrane between cerebrospinal fluid and tissue fluids.
• Myelin sheaths protect nerve fibers.
• Oligodendrocytes are myelin sheaths in the CNS.

Histology: Neurons

• The cell body, also known as the perikaryon or soma, is the center of a neuron containing the organelles.
• Neurons have processes, including dendrites and axons
• Dendrites are short, branching extensions which receive input signals from nearby cells, sending them TO the cell body
• Axons are long, single extensions that generate impulses and transmit them AWAY from cell bodies
• Myelin Sheaths protect, insulate nerve fibers, and conduct impulses
• Myelinated axons send impulses rapidly.
• In the PNS, myelin sheaths composed of Schwann cells wrap around the axon holding the nucleus and cytoplasm in place; it's external to the myelin sheath
• The outer portion of the Myelin Sheath is called the neurilemma or Sheath of Schwann
• Gaps between Schwann cells are called Nodes of Ranvier

Structural Classification of Neurons

• Multipolar neurons have many processes extending from the cell body; are most abundant in the body, and primarily motor neurons.
• Bipolar neurons have two processes extending from the cell body; are the rarest in the body, and found in some sensory organs (olfaction, sight).
• Unipolar neurons have one process extending from the cell body; and are mostly sensory.

Functional Classification of Neurons

• Sensory (Afferent) neurons transmit impulses from receptors in the skin or internal organs toward the CNS; all are unipolar EXCEPT for some in special sensory organs
• Motor (Efferent) carry impulses away from the CNS to muscles or glands, are multipolar with cell bodies in the CNS.
• Interneurons lie between motor and sensory neurons, shuttling signals; and are multipolar and mostly contained in the CNS.

Synapse

• A unique junction that mediates transfer of information from one neuron to the next.
• Most occur between the axonal endings of one neuron and the dendrites or soma of other neurons.
  • These are called Axodendritic or Axosomatic synapses.
• Presynaptic neurons conduct information TOWARD synapses
  • They are the information senders.
• Postsynaptic neurons conduct information AWAY from synapses.

Chemical Synapse

• Specialized synapses use neurotransmitters to open or close ion channels, triggering a response.
• Has two components
  • Presynaptic axonal terminal with synaptic vesicles holding neurotransmitter molecules
  • Postsynaptic receptor region having binding sites for the neurotransmitter molecules
• Components are separated by a fluid-filled area known as the synaptic cleft