Nervous Tissue: Structure, Function, and Physiology

Nervous Tissue and the Nervous System

Introduction to the Nervous System

The nervous system is essential for feeling, thinking, moving, awareness, coordination of body functions, homeostasis, and responsiveness to changes. The main cell type is the neuron, which communicates via electrical impulses. Neurons transmit nerve impulses along nerve fibers to other neurons or cells outside the nervous system at a synapse, where neurotransmitters act as chemical messengers.

• Neurons typically have a cell body, an axon, and dendrites.

• Nerves are bundles of nerve fibers (axons).

• Synapse: Small space between communicating cells.

• Neurotransmitters: Chemicals that convey impulses across synapses.

Organization of the Nervous System

The nervous system is divided into two main groups:

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

• Peripheral Nervous System (PNS): Nerves connecting CNS to the rest of the body.

General functions include sensory, integrative, and motor activities. Neuroglia provide nutrients, insulation, and support for neurons.

Functional Organization of the Nervous System

General Functions

• Sensory function: Sensory receptors gather information and convert it into nerve impulses.

• Integrative function: Impulses are processed in the CNS to create sensations, memories, or thoughts.

• Motor function: Impulses are sent to effectors (muscles or glands) to perform actions.

Motor functions are divided into:

• Somatic nervous system: Voluntary control (skeletal muscles).

• Autonomic nervous system: Involuntary control (cardiac & smooth muscle, glands).

Neuron Structure and Classification

Neuron Structure

Neurons consist of a cell body, dendrites, and an axon.

• Dendrites: Conduct impulses toward the cell body; short and branching.

• Axon: Conducts impulses away from the cell body; arises from the axon hillock.

• Cell body: Contains organelles such as mitochondria, lysosomes, Golgi apparatus, Nissl bodies, neurofilaments, and a nucleus.

Structural Classification of Neurons

Neurons are classified based on their structure:

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

• Bipolar neurons: One dendrite, one axon; found in special senses (eyes, nose, ears).

• Unipolar neurons: One process splits into two; cell bodies in ganglia outside CNS; sensory neurons.

Examples of Dendritic Branching

Dendritic branching varies among neuron types, affecting their connectivity and function.

• Purkinje cells: Extensive dendritic trees, found in cerebellum.

• Pyramidal cells: Found in cerebral cortex.

Functional Classification of Neurons

• Sensory (afferent) neurons: Conduct impulses from peripheral receptors to CNS; usually unipolar or bipolar.

• Motor (efferent) neurons: Conduct impulses from CNS to effectors; multipolar.

• Interneurons: Lie within CNS; form links between other neurons; multipolar.

Examples of Sensory Receptors

Sensory receptors are specialized to detect various stimuli, such as touch, pressure, and pain.

• Cutaneous mechanoreceptors: Detect touch and pressure.

• Nociceptors: Detect pain.

Myelination and Neuroglia

Myelination of Axons

• Myelinated fibers: Larger axons are wrapped in myelin sheaths produced by Schwann cells (PNS) or oligodendrocytes (CNS).

• Neurilemma: Outer layer of myelin sheath in PNS.

• Nodes of Ranvier: Gaps between Schwann cells where ion exchange occurs.

Myelinated and Unmyelinated Axons

• White matter: Regions with many myelinated axons; appears white in CNS.

• Gray matter: Regions with neuron cell bodies and unmyelinated axons.

Neuroglia of the CNS

Neuroglia are non-excitable cells that support neurons. Types in CNS:

• Microglia: Phagocytize bacteria and debris; produce scar tissue.

• Oligodendrocytes: Form myelin sheath in CNS.

• Astrocytes: Structural support, regulate nutrients/ions, form blood-brain barrier, scar tissue.

• Ependymal cells: Line ventricles and cover choroid plexuses.

Neuroglia of the PNS

• Schwann cells: Produce myelin in PNS.

• Satellite cells: Structural support and regulation of nutrients/ions.

Regeneration and Repair of Neurons

Regeneration in the PNS

Damaged PNS neurons can regenerate if the cell body is intact and Schwann cells are functional. Steps:

1. Chromatolysis

2. Wallerian degeneration

3. Formation of a regeneration tube

Electrical Signals in Neurons

Resting Membrane Potential

Neurons are polarized due to unequal ion distribution.

• Inside is more negative than outside.

• Resting potential is typically -70 mV.

• Maintained by Na+/K+ pump.

Potential Changes and Action Potentials

• Depolarization: Membrane potential becomes less negative.

• Threshold potential: -55 mV; triggers action potential.

• Repolarization: Return to resting potential.

• Hyperpolarization: Membrane becomes overly negative.

Sequence of Events in an Action Potential

• Sodium channels open, Na+ enters cell (depolarization).

• Potassium channels open, K+ leaves cell (repolarization).

• Resting potential is restored.

Impulse Conduction

Continuous vs. Saltatory Conduction

• Continuous conduction: Unmyelinated fibers; impulse travels along entire membrane.

• Saltatory conduction: Myelinated fibers; impulse jumps from node to node, much faster.

• Speed is proportional to axon diameter and myelination.

Synapses and Neurotransmitters

The Synapse

• Presynaptic neuron: Sends impulse.

• Postsynaptic neuron: Receives impulse.

• Synaptic knob: Expansion at distal end of presynaptic neuron.

• Neurotransmitters released in response to impulse, bind to postsynaptic receptors.

Synaptic Transmission

• Neurotransmitters can be excitatory (increase Na+ permeability) or inhibitory (decrease Na+ permeability).

• Postsynaptic neuron sums excitatory and inhibitory inputs.

Removal of Neurotransmitter

• Diffusion

• Enzymatic degradation

• Reuptake into cells

• Uptake by neuroglial cells

Types of Synapses

• Electrical synapse: Gap junctions allow direct transfer of information.

• Chemical synapse: One-way transfer via neurotransmitter.

Neurotransmitters and Their Actions

Major Neurotransmitters

• Acetylcholine: Excitatory; neuromuscular junctions.

• Epinephrine: Excitatory.

• GABA, glycine: Inhibitory.

• Dopamine, norepinephrine, glutamate, aspartate: Various functions.

Neurotransmitter	Location	Major Actions
Acetylcholine	CNS/PNS	Stimulates skeletal muscle actions
Dopamine	CNS	Creates sense of feeling good; deficiency linked to Parkinson's disease
GABA	CNS	Generally inhibitory
Glutamate	CNS	Excitatory; important for learning and memory
Norepinephrine	PNS	Creates sense of well-being; low levels may lead to depression
Serotonin	CNS	Inhibitory; leads to sleep
Glycine	CNS	Generally inhibitory
Endorphins	CNS	Inhibitory; reduce pain

Impulse Processing and Neural Circuits

Neuronal Pools and Summation

Neurons in the CNS are organized into pools that process information.

• Facilitation: Increased neurotransmitter release upon repeated stimulation.

• Convergence: Multiple inputs to a single neuron.

• Divergence: Single neuron sends impulses to multiple outputs.

Neural Circuits

• Reverberating circuit: Later cells repeatedly stimulate early cells (short-term memory).

• Parallel after-discharge circuit: Single cell stimulates a group, which all stimulate a common postsynaptic cell (complex processing).

Types of Nerves and Neural Pathways

Types of Nerves

• Sensory (afferent) nerves: Bring information to CNS.

• Motor (efferent) nerves: Carry impulses from CNS to effectors.

• Mixed nerves: Contain both sensory and motor fibers.

Reflex Arcs

Reflex arcs are the simplest neural pathways, providing the basis for involuntary actions. Components:

1. Sensory receptor

2. Sensory neuron

3. Interneuron (reflex center)

4. Motor neuron

5. Effector

Regeneration and Plasticity

Regeneration in CNS and PNS

• CNS axons rarely regenerate due to inhibitory proteins and scar tissue.

• PNS axons can regenerate if conditions are favorable.

Neural Plasticity

• Neuroplasticity: Ongoing changes in neurons, glia, and vascular cells.

• Synaptic pruning: Elimination of unused or weak connections.

• Learning and memory: Short-term memory depends on electrical/chemical events; long-term memory involves structural changes.

Comparison of Graded and Action Potentials

Characteristic	Graded Potentials	Action Potentials
Origin	Arise mainly in dendrites and cell body	Arise at trigger zones and propagate along axon
Types of channels	Ligand-gated and mechanically gated	Voltage-gated for Na+ and K+
Conduction	Decremental; not propagated	Propagate; communication over longer distances
Amplitude	Varies with stimulus strength	All-or-none; typically about 100 mV
Duration	Longer; milliseconds to minutes	Shorter; milliseconds
Polarity	May be depolarizing or hyperpolarizing	Always consist of depolarizing phase followed by repolarizing phase
Refractory period	None	Present; summation cannot occur

Summary

Nervous tissue is fundamental to the function of the nervous system, enabling communication, integration, and response to stimuli. Understanding neuron structure, neuroglia, myelination, synaptic transmission, and neural plasticity is essential for comprehending how the nervous system maintains homeostasis and adapts to changes. Key equations:

• Resting membrane potential: $V_{m} = V_{inside} - V_{outside}$

• Typical resting potential: $-70\ \text{mV}$

• Threshold potential: $-55\ \text{mV}$

Additional info: Academic context was added to clarify structural and functional classifications, synaptic mechanisms, and neural plasticity.