Introduction to the Nervous System and Nervous Tissue – Study Notes

Introduction to the Nervous System and Nervous Tissue

Overview of the Nervous System

The nervous system is a complex network responsible for coordinating the body’s activities by transmitting signals to and from different parts of the body. It is essential for sensation, integration, and response to internal and external stimuli.

• Major Functions: Sensory input, integration of data, and motor output.

• Central Nervous System (CNS): Consists of the brain and spinal cord; responsible for processing and integrating information.

• Peripheral Nervous System (PNS): Composed of nerves and ganglia outside the CNS; transmits signals between the CNS and the rest of the body.

• Functional Divisions of the PNS:

  • Somatic Nervous System: Controls voluntary movements of skeletal muscles.

  • Autonomic Nervous System: Regulates involuntary functions (e.g., heart rate, digestion).

Nervous System Structure and Function

The nervous system is composed of specialized cells called neurons and supporting cells known as neuroglia. Each component has a unique structure that supports its specific function.

• Neurons: The primary signaling cells; consist of a cell body (soma), dendrites (receive signals), and an axon (transmits signals).

• Types of Neurons:

  • Multipolar: Many dendrites, one axon; most common in CNS.

  • Bipolar: One dendrite, one axon; found in sensory organs.

  • Unipolar: Single process splits into two branches; common in sensory neurons of PNS.

• Neuroglial Cells: Support, protect, and nourish neurons.

  • CNS Neuroglia: Astrocytes, oligodendrocytes, microglia, ependymal cells.

  • PNS Neuroglia: Schwann cells, satellite cells.

Example: Oligodendrocytes in the CNS and Schwann cells in the PNS both form myelin sheaths, which insulate axons and increase the speed of nerve impulse conduction.

Electrophysiology of Neurons

Neurons communicate via electrical signals known as action potentials, which depend on the movement of ions across the cell membrane through voltage-gated ion channels.

• Voltage-Gated Ion Channels: Essential for initiating and propagating action potentials.

• Refractory Periods:

  • Absolute Refractory Period: No new action potential can be generated.

  • Relative Refractory Period: A stronger stimulus is required to initiate another action potential.

• Conduction Types:

  • Continuous Conduction: Occurs in unmyelinated axons; slower.

  • Saltatory Conduction: Occurs in myelinated axons; action potentials jump between nodes of Ranvier, increasing speed.

• Factors Affecting Conduction Velocity: Axon diameter (larger = faster) and myelination (myelinated = faster).

Equation: The Nernst equation describes the equilibrium potential for a particular ion: $E_{ion} = \frac{RT}{zF} \ln \left( \frac{[ion]_{outside}}{[ion]_{inside}} \right)$

Neuronal Synapses

Synapses are specialized junctions where neurons communicate with other neurons, muscles, or glands. They can be electrical or chemical.

• Electrical Synapses: Allow direct passage of ions and electrical signals through gap junctions; rapid communication.

• Chemical Synapses: Use neurotransmitters to transmit signals across a synaptic cleft; more common in the nervous system.

• Neurotransmitter and Receptor Relationship: The effect of a neurotransmitter depends on the receptor it binds to.

• Events of Chemical Synaptic Transmission:

  1. Action potential arrives at axon terminal.

  2. Voltage-gated Ca2+ channels open; Ca2+ enters the terminal.

  3. Neurotransmitter vesicles fuse with the membrane and release contents into the synaptic cleft.

  4. Neurotransmitter binds to postsynaptic receptors, causing a response.

• Postsynaptic Potentials:

  • Excitatory Postsynaptic Potential (EPSP): Depolarizes the postsynaptic membrane, increasing the likelihood of an action potential.

  • Inhibitory Postsynaptic Potential (IPSP): Hyperpolarizes the postsynaptic membrane, decreasing the likelihood of an action potential.

Neurotransmitters

Neurotransmitters are chemical messengers that transmit signals across synapses. Their effect depends on the type of receptor present on the postsynaptic cell.

• Excitatory vs. Inhibitory: A single neurotransmitter can be excitatory at one synapse and inhibitory at another, depending on the receptor subtype.

• Major Classes of Neurotransmitters:

  • Amino Acids: e.g., glutamate (excitatory), GABA (inhibitory).

  • Monoamines: e.g., dopamine, serotonin, norepinephrine.

  • Peptides: e.g., substance P, endorphins.

  • Others: e.g., acetylcholine (ACh).

• Common Neurotransmitters in the CNS:

  • Excitatory: Glutamate, acetylcholine.

  • Inhibitory: GABA, glycine.

Example: Acetylcholine is excitatory at neuromuscular junctions but can be inhibitory in the heart due to different receptor types.