Nervous Tissue

Nervous Tissue

Objectives of Nervous System Study

• Understand anatomical and functional divisions of the nervous system.

• Sketch and label typical neuron structures and functions.

• Classify neurons based on structure and function.

• Describe locations and functions of neuroglia, Schwann cells, and satellite cells.

• Describe action potential generation and its propagation types (continuous and saltatory).

• Discuss synapse structures in CNS & PNS; details of chemical synapses.

• Explain significance of excitatory and inhibitory postsynaptic potentials.

Nervous System Overview

• Functions: Detects changes impacting the body, collaborates with the endocrine system to respond to changes, responsible for behaviors, memories, movement due to electrical excitability.

• Components:

  • Central Nervous System (CNS): Brain, spinal cord.

  • Peripheral Nervous System (PNS): Cranial nerves, spinal nerves, ganglia, enteric plexuses, sensory receptors.

Information Processing in Nervous System

• Sensory Division (Afferent): Gather information from internal/external environment to CNS.

• Integration: Process information by CNS.

• Motor Division (Efferent): Commands go from CNS to effectors (muscles, glands).

  • Somatic Nervous System (SNS)

  • Autonomic Nervous System (ANS): Includes parasympathetic and sympathetic divisions.

Nervous Tissue Cells

• Neurons:

  • Unique, excitable cells, no mitotic division post-formation, and involved in nerve impulses.

• Neuroglia:

  • Support, nourish, and protect neurons with ongoing mitotic division.

Neuron Structure and Components

• Key Components:

  • Axon: Carries information towards other cells.

  • Dendrites: Receive stimuli; highly branched structure.

  • Cell Body (Soma): Contains nucleus and organelles (e.g., Nissl bodies).

  • Dendritic Spines: Increases receptive surface area.

Synaptic Communication and Types of Neurons

• Synapse: Connection point for communication between neurons using neurotransmitters.

• Types of Neurons:

  1. Anaxonic Neurons: No distinct axonal features, found in brain & special senses.

  2. Bipolar Neurons: One dendrite and one axon; rare, found in special senses.

  3. Unipolar Neurons: Continuous dendrite & axon; mainly sensory neurons in PNS.

  4. Multipolar Neurons: Multiple dendrites & one axon; includes most CNS neurons and all motor neurons to skeletal muscles.

Functional Classification of Neurons

• Sensory Neurons: Carry signals toward CNS.

• Interneurons: Connect sensory and motor neurons; coordinate signals and responsible for higher brain functions.

• Motor Neurons: Send commands from CNS to muscles/glands.

Glial Cells in CNS and PNS

• CNS Neuroglia: Ependymal cells, microglia, astrocytes, oligodendrocytes.

• PNS Neuroglia:

  • Schwann Cells: Myelinate PNS axons.

  • Satellite Cells: Regulate environment around neuron cell bodies.

Membrane Potential and Neural Activity

• Membrane Potential: Arises from charge distribution across neuron membranes impacted by ion movement.

• Resting Potential: Typical value around -70 mV; maintained by leak channels and Na+/K+ pump.

  • Leak Channels: Allow passive diffusion of ions; always open.

  • Gated Channels: Impede or allow ion movement; include chemically, voltage, and mechanically gated channels.

Graded Potentials and Action Potentials

• Graded Potentials: Local changes in membrane potential from stimuli; decreases with distance.

• Action Potentials: All-or-none events that allow long-range communication; involve voltage-gated Na+ and K+ channels.

• Generation Steps:

  1. Depolarization to Threshold: Graded potential triggers opening of Na+ channels.

  2. Rapid Depolarization: Na+ influx leads to sharp rise in membrane potential (+30 mV).

  3. Repolarization: Na+ channels inactivate, K+ channels open, leading to K+ efflux.

  4. Return to Resting Potential: Membrane stabilizes post-repolarization.

• Refractory Periods: Absolute (no response possible) and relative (only strong stimulus can elicit a response).

Action Potential Propagation

• Continuous Propagation: Occurs in unmyelinated axons; slower and stepwise.

• Saltatory Propagation: Faster via myelinated axons; electrical signal jumps across nodes of Ranvier.

Synapse Functioning and Types

• Chemical Synapses: Common; utilize neurotransmitters like acetylcholine (ACh).

• Electrical Synapses: Rare; involve gap junctions for direct electrical communication.

  • EPSP (Excitatory Post-Synaptic Potential): Plates membrane closer to threshold.

  • IPSP (Inhibitory Post-Synaptic Potential): Moves membrane potential further from threshold.

Summation of Postsynaptic Potentials

• Integration of multiple EPSPs and IPSPs; net change may involve no action based on balance of excitatory and inhibitory inputs.