Nervous Tissue

Soma Nervous Tissue

  • Neurite Growth Cone

    • Red = Microtubules

    • Green = Actin Cytoskeleton

    • Role in neuronal growth and pathways development

Structures of the Nervous System

  • Brain: Contains approximately 100 billion neurons.

  • Cranial Nerves: Nerves I through XII.

  • Spinal Cord: Connects to spinal nerves.

  • Ganglia: Clusters of neuronal cell bodies.

What Is A Neuron?

  • Basic structural and functional unit of the nervous system.

Parts of a Neuron

Cell Body (Perikaryon or Soma)

  • Contains cell organelles including:

    • Ribosomes: Nissl bodies (free ribosomes + rough endoplasmic reticulum).

    • Neurofibrils: Provide shape and support.

    • Microtubules: Involved in transport and structure.

Dendrites

  • Extensions from the cell body that receive signals.

Axon

  • Axon Hillock: Connects axon to cell body.

  • Axon Terminals: Release neurotransmitters.

  • Myelin Sheath: Insulates the axon to speed up conduction.

  • Nodes of Ranvier: Gaps in myelin that facilitate rapid conduction.

Neurogenesis

  • The formation of new neurons; limited to the hippocampus and olfactory bulb in adult mammals.

    • Elizabeth Gould: Notable researcher in neurogenesis.

Classification of Neurons

Structural Classification

  1. Multipolar Neurons: Common in the brain and spinal cord.

  2. Bipolar Neurons: Found in retina and inner ear.

  3. Unipolar Neurons: Typically sensory neurons.

Functional Classification

  1. Sensory (Afferent): Carry signals to the central nervous system (CNS).

  2. Motor (Efferent): Transmit signals from CNS to effectors.

  3. Interneurons: Connect sensory and motor neurons within the CNS.

Neuroglia of Central Nervous System (CNS)

  • Astrocytes: Provide physical and metabolic support; regulate ion transport; maintain blood-brain barrier.

  • Oligodendrocytes: Form and maintain myelin sheath, increase speed of nerve impulses.

  • Microglia: Act as phagocytes; remove damaged neurons and pathogens.

  • Ependymal Cells: Line ventricles and produce cerebrospinal fluid (CSF).

Functions of Neuroglia

Astrocytes

  • Isolate neurons from harmful substances.

  • Regulate growth, migration, and interconnections of neurons.

Oligodendrocytes

  • Provide myelination, enhancing signal speed.

Microglia

  • Act as immune defense in the CNS; remove dead cells and debris.

Ependymal Cells

  • Maintain CSF barrier and regulate flow of CSF.

Neuroglia of Peripheral Nervous System (PNS)

  • Schwann Cells: Form myelin sheaths, insulated axons to speed up conduction.

  • Satellite Cells: Support and protect neuron cell bodies in ganglia.

Definitions

  • Ganglion: Cluster of neuronal cell bodies in the PNS.

  • Nucleus: Cluster of neuronal cell bodies in the CNS.

  • Nerve: Bundle of axons in the PNS.

  • Tract: Bundle of axons in the CNS.

  • White Matter: Consists of myelinated axons.

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

Electrical Signals in Neurons

Key Definitions

  • Voltage: Potential energy due to separated charges;

  • Current: Flow of electrical charge.

Resting Membrane Potential

  • Typical value: -70mV.

  • Measured by voltmeter in various cellular environments.

Synaptic Potentials

Excitatory Potentials

  • Depolarize postsynaptic membrane; facilitate action potential generation.

  • Occurs primarily at dendrites.

Inhibitory Potentials

  • Hyperpolarize postsynaptic membrane; inhibit action potential generation.

  • Occurs mainly at the cell body.

Summation of Postsynaptic Potentials

  • Spatial Summation: Multiple synaptic signals received simultaneously.

  • Temporal Summation: Multiple signals received in rapid succession.

Hebbian Plasticity

  • Principle: Neurons that fire together wire together; increases in efficiency with frequent co-activation.

Action Potential

  • Rapid depolarization followed by repolarization; involves the opening of ion channels.

  • Results in the transmission of signals along the axon.

Propagation of Action Potentials

Unmyelinated vs. Myelinated Axons

  • Continuous Conduction: Occurs in unmyelinated axons.

  • Saltatory Conduction: Occurs in myelinated axons via Nodes of Ranvier, speeding up signal transmission.

Factors Affecting Speed of Propagation

  1. Myelination: Increases speed.

  2. Axon Diameter: Larger diameters increase speed.

  3. Temperature: Higher temperatures increase speed.

Electrical vs Chemical Synapses

Electrical Synapses

  • Fast communication through gap junctions; synchronization typical in heart and smooth muscle.

Chemical Synapses

  • Use neurotransmitters across a synaptic cleft; slow but allows for complex signaling.

Neurotransmitters vs Neuropeptides

  • Neurotransmitter: Released into synaptic cleft; alters postsynaptic membrane potential.

  • Neuropeptide: Larger neurotransmitter with up to ~40 amino acids; often modulatory.

Examples of Neurotransmitter Classes

  • Amino Acids: Glutamate (excitatory), GABA (inhibitory).

  • Monoamines: Dopamine, norepinephrine, serotonin.

Disorders Related to Neurotransmitters

Glutamate Disorders

  • Excess glutamate linked with neurodegenerative diseases such as Alzheimer's and Parkinson's disease.

GABA Disorders

  • Imbalances linked to anxiety, mood disorders, and epilepsy.

Dopamine and Parkinson's Disease

  • Low dopamine levels associated with disorders like Parkinson's disease; MPTP linked to induced PD symptoms in research.

Tetrodotoxin and Action Potentials

  • TTX inhibits voltage-gated sodium channels, blocking action potential generation.