Nervous System: CNS and PNS

• Nervous tissue comprises neurons and neuroglia.
• The nervous system has two main divisions: the central nervous system and the peripheral nervous system.
• Neurons transmit impulses in response to stimuli.
• Neuroglia supports and protects neurons.

Central Nervous System (CNS)

• Consists of the brain and spinal cord.
• Acts as the integrating command center of the nervous system.
• Interprets incoming sensory information.

Peripheral Nervous System (PNS)

• Consists of nervous structures outside the CNS such as cranial, spinal, ganglia, and peripheral nerves.
• Functions as a communication line, carrying impulses from the CNS to target organs.
• Links all parts of the body by carrying impulses from the CNS to target organs.

Functional Classification of the Nervous System

• Sensory (Afferent) division conveys impulses to the CNS from sensory receptors.
• Motor (Efferent) division conveys impulses from the CNS to effector organs.
• Somatic nervous system is controlled voluntarily, except for reflexes.
• Autonomic nervous system controls involuntary activities.
• Sympathetic nervous system triggers the "fight-or-flight" response.
• Parasympathetic nervous system inhibits the sympathetic response.

Neuron Structure and Function

• Neuron is the functional unit of the CNS and PNS.
• Neuron has three main parts: a cell body, dendrites, and an axon.
  • Cell body is the synthetic and tropic center of the neuron.
  • Dendrites receive stimuli from other neurons.
  • Axon conducts impulses to other cells.

Neuron Classification

• Multipolar neurons have one axon and two or more dendrites and are the most common type.
• Bipolar neurons have one dendrite and one axon and are found in the retina, olfactory area, and inner ear.
• Unipolar neurons include all other sensory neurons and have a single process that bifurcates close to the perikaryon.
• Anaxonic neurons have many dendrites but no true axon.

Cell Body Details

• Also known as soma or perikaryon.
• Contains the nucleus and surrounding cytoplasm, excluding cell processes.
• Has an unusually large, euchromatic nucleus with a prominent nucleolus.
• Regions with concentrated RER and polysomes are basophilic, known as Nissl bodies.
• Contains mitochondria.
• Contains microtubules, actin filaments, and intermediate filaments (neurofilaments) that form the cytoskeleton.

Dendrite Characteristics

• Short, small processes emerging and branching off the soma that receives signals from other cells.
• Becomes thinner as they branch.
• CNS dendrites have dendritic spines for synaptic signal processing.

Axon Features

• Long process originating from axon hillock.
• Unlike dendrites, axon has a relatively uniform size up to its axon terminal.
• Transmits stimulus to a gland, muscle, organ, or other neuron.
• Myelin sheath is a protective, fatty wrapping around axons for insulation.
• Schwann cells myelinate axons in the PNS.
• Oligodendrocytes myelinate axons in the CNS.
• Nodes of Ranvier are gaps in the myelin sheath where action potential (nerve impulse) develops.

Action Potential

• Nerve impulse (action potential) results from ion movement across a neuron's cell membrane (axon).
• During the resting state, Na+ and K+ gated channels are normally closed, but leak channels allow K+ and Na+ to move in a balanced manner.
• In the resting state, the cell membrane is polarized with a negative charge inside (approx. -70mV) and positive outside.
• An action potential starts when a stimulus disturbs the cell membrane and the Na+ channels open temporarily as the cells reach a threshold of -55mV.
• Influx of Na+ causes depolarization that changes the charge difference from negative to positive (+40mV).
• Depolarization causes K+ channels to open, allowing K+ to flow out of the cell, returning the axon to its negative state (repolarization) and causing the voltage to fall.
• Sometimes there is hyperpolarization, when K+ channels remains open, causing an efflux of more K+ outside the axon. In this state, the membrane voltage is more negative and cannot transmit impulses (refractory period).

Synapse Function

• At an axon terminal a space known as the synaptic cleft allows for transmission of electrical impulses and release of neurotransmitters.
• Action potential causes voltage-regulated Ca+ channels to open, allowing calcium to enter and cause exocytosis of synaptic vesicles containing neurotransmitters.
• Neurotransmitters released into the synaptic cleft bind with receptors on adjacent gated Na+ channels of post-synaptic neurons (dendrites), allowing sodium to enter and facilitating action potential in the next cell.

Neuroglia Functions

• They are also known as nerve glue.
• Set of cells that support, insulate, and protect the delicate neurons
  • Astrocytes are star-shaped cells that anchor neuron to blood supply for nutrition.
  • Microglia are phagocytic cells that defend neurons.
  • Oligodendrocytes wrap around CNS nerve fibers.
  • Ependymal cells produce CSF
  • Satellite cells cushion PNS neurons
  • Schwann cells produce the myelin sheath within the PNS