Nervous Tissue and Neural Physiology: Study Notes

Nervous Tissue

Cell Types in Nervous Tissue

Nervous tissue is composed of two main types of cells: neurons and neuroglia (glial cells). Each plays a distinct role in the function and maintenance of the nervous system.

• Neurons: Specialized for intercellular communication, transmitting electrical and chemical signals throughout the body.

• Neuroglia (Glial Cells): Support, protect, and nourish neurons. They outnumber neurons and help maintain the structure of nervous tissue.

Anatomical Divisions of the Nervous System

• Central Nervous System (CNS): Consists of the brain and spinal cord. Responsible for integrating, processing, and coordinating sensory data and motor commands.

• Peripheral Nervous System (PNS): Includes all neural tissue outside the CNS. Divided into sensory (afferent) and motor (efferent) divisions.

Structure of a Neuron

Major Parts of a Neuron

• Cell Body (Soma): Contains the nucleus and organelles. The perikaryon is the cytoplasm around the nucleus, containing neurotubules, neurofibrils, and neurofilaments.

• Dendrites: Branch-like extensions that carry information toward the soma.

• Axon: Long projection that carries electrical impulses away from the soma. Key features include:

  • Axolemma: The plasma membrane of the axon.

  • Axoplasm: The cytoplasm within the axon.

  • Axon Hillock: The region where the axon attaches to the soma.

  • Collaterals: Branches of the axon.

  • Axon Terminals: The endpoints where the neuron communicates with other cells.

• Nissl Bodies: Clusters of rough endoplasmic reticulum and ribosomes involved in protein synthesis.

Neuroglia (Glial Cells)

Types of Glial Cells

• Ependymal Cells: Line the central canal of the spinal cord and ventricles of the brain; involved in producing and circulating cerebrospinal fluid (CSF).

• Astrocytes: Maintain the blood-brain barrier, provide structural support, and regulate ion, nutrient, and dissolved gas concentrations.

• Oligodendrocytes: Provide myelin in the CNS, insulating axons to increase the speed and strength of electrical impulses.

• Microglia: Smallest and least numerous; act as phagocytes, removing debris and pathogens.

• Satellite Cells: Regulate the environment around neurons in ganglia (PNS).

• Schwann Cells (Neurolemmocytes): Myelinate axons in the PNS; the outer layer is called the neurolemma.

Myelination and Nervous Tissue Color

• Myelin: A protein-lipid sheath that insulates axons, increasing the speed of impulse conduction.

• Nodes (Nodes of Ranvier): Gaps in the myelin sheath where action potentials are regenerated.

• Internodes: Myelinated segments of the axon.

• White Matter: Regions with many myelinated axons.

• Gray Matter: Regions with unmyelinated axons, cell bodies, and dendrites.

Types of Neurons

Structural Classification

• Anaxonic Neurons: Found in the brain and special sense organs; cannot distinguish axons from dendrites.

• Bipolar Neurons: Have one dendrite and one axon with the soma in between; found in special sense organs (e.g., retina).

• Unipolar Neurons: Dendrites and axon are continuous; cell body lies off to the side. Common in sensory neurons of the PNS.

• Multipolar Neurons: Most common type; many dendrites and one axon. Found in the CNS and control skeletal muscles.

Functional Classification

• Sensory Neurons: Monitor external (somatic) and internal (visceral) environments.

• Motor Neurons: Carry instructions from the CNS to effectors (muscles and glands).

• Interneurons: Connect sensory and motor neurons; most abundant type, involved in higher functions like learning and memory.

Sensory Receptors

• Interoceptors: Monitor internal systems (e.g., digestive, respiratory).

• Exteroceptors: Monitor external environment (touch, temperature, smell, sight, etc.).

• Proprioceptors: Monitor position and movement of skeletal muscles and joints.

Neural Physiology

Membrane Potentials

Neurons communicate via electrical signals generated by changes in membrane potential.

• Resting Membrane Potential (RMP): The baseline electrical charge across the membrane, typically $-70\ \mathrm{mV}$.

• Graded Potential: A temporary, localized change in membrane potential. Decreases with distance from the stimulus.

• Action Potential: A large, rapid depolarization that propagates along the axon without diminishing in strength.

Ion Channels and Membrane Permeability

• Leak Channels: Always open; allow passive movement of ions.

• Active (Gated) Channels: Open or close in response to stimuli; require energy.

• Types of Gated Channels:

  • Chemically Gated: Open when bound to specific chemicals (e.g., neurotransmitters like ACh); found on dendrites and soma.

  • Voltage Gated: Open or close in response to changes in membrane potential; found on axons.

Phases of Action Potential

1. Depolarization: Membrane potential becomes more positive due to sodium influx.

2. Repolarization: Potassium channels open, potassium exits, restoring negative potential.

3. Hyperpolarization: Membrane potential becomes more negative than resting due to slow closing of potassium channels.

Threshold: The minimum membrane potential required to trigger an action potential, typically between $-60$ to $-55\ \mathrm{mV}$.

All-or-None Principle: An action potential either occurs fully or not at all.

Refractory Periods

• Absolute Refractory Period: No action potential can be generated, regardless of stimulus strength.

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

Propagation of Action Potentials

• Continuous Propagation: Occurs in unmyelinated axons; action potential moves stepwise along the entire membrane.

• Saltatory Propagation: Occurs in myelinated axons; action potential jumps from node to node, increasing speed.

Types of Nerve Fibers

Type	Myelination	Diameter	Speed	Function
Type A	Myelinated	Large	Up to 120 m/s	Rapid sensory and motor signals
Type B	Myelinated	Medium	Up to 18 m/s	Autonomic functions
Type C	Unmyelinated	Small	Up to 1 m/s	Slow pain, autonomic functions

Synapses and Neurotransmitters

Types of Synapses

• Electrical Synapses: Direct physical contact between cells; allow rapid signal transmission. Mostly found in the brain.

• Chemical Synapses: Use neurotransmitters to transmit signals across a synaptic cleft. Most common type.

Synaptic Transmission

• Presynaptic Cell: The neuron sending the message.

• Postsynaptic Cell: The cell receiving the message.

• Synaptic Delay: The time required for neurotransmitter release and binding.

• Synaptic Fatigue: Occurs when neurotransmitter synthesis cannot keep up with demand during intense stimulation.

Neurotransmitters

• Excitatory Neurotransmitters: Cause depolarization and promote action potential generation.

• Inhibitory Neurotransmitters: Cause hyperpolarization and suppress action potential generation.

• Acetylcholine (ACh): A common neurotransmitter; can be excitatory or inhibitory depending on the receptor.

• Biogenic Amines: Includes norepinephrine (NE), which is released at adrenergic synapses and has an excitatory, depolarizing effect.

Key Equations and Concepts

• Resting Membrane Potential: $V_{rest} = -70\ \mathrm{mV}$

• Threshold Potential: $V_{threshold} = -60$ to $-55\ \mathrm{mV}$

• Potassium Equilibrium Potential: $E_K \approx -90\ \mathrm{mV}$

• Sodium Equilibrium Potential: $E_{Na} \approx +66\ \mathrm{mV}$

• Electrochemical Gradient: The sum of chemical and electrical forces acting on an ion across the membrane.

Example: Action Potential Sequence

1. Stimulus causes graded potential in dendrites and soma.

2. If threshold is reached at the axon hillock, voltage-gated sodium channels open, causing rapid depolarization.

3. At peak depolarization ($\approx +30\ \mathrm{mV}$), sodium channels close and potassium channels open, leading to repolarization.

4. Potassium channels close slowly, causing hyperpolarization before returning to resting potential.

Additional info: These notes expand on the original fragmented content to provide a comprehensive overview of nervous tissue structure and function, suitable for exam preparation in an anatomy and physiology course.