Neural Tissue and the Nervous System: Structure, Function, and Physiology

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Neural Tissue and the Nervous System

Overview of Neural Tissue

Neural tissue is specialized for the conduction of electrical impulses and is fundamental to the function of the nervous system. It consists of two main types of cells: neurons and neuroglia (glial cells).

Neurons: Cells that send and receive signals, forming the basic functional unit of the nervous system.
Neuroglia (Glial Cells): Support and protect neurons, maintaining the environment around them.

Organs of the Nervous System

Central Nervous System (CNS): Brain and spinal cord.
Sensory Receptors: Specialized structures in sense organs (e.g., eyes, ears).
Nerves: Connect the nervous system to other body systems.

Anatomical and Functional Divisions of the Nervous System

Anatomical Divisions

Central Nervous System (CNS): Consists of the brain and spinal cord. Contains neural tissue, connective tissues, and blood vessels.
Peripheral Nervous System (PNS): Includes all neural tissue outside the CNS. Connects the CNS to limbs and organs.

Functions of the CNS

Processing and coordination of sensory data from inside and outside the body.
Issuing motor commands to control activities of peripheral organs (e.g., skeletal muscles).
Higher brain functions: intelligence, memory, learning, and emotion.

Functions of the PNS

Delivers sensory information to the CNS.
Carries motor commands to peripheral tissues and systems.
Composed of cranial nerves (connect to brain) and spinal nerves (attach to spinal cord).

Functional Divisions of the PNS

Afferent Division: Carries sensory information from PNS sensory receptors to CNS.
Efferent Division: Carries motor commands from CNS to PNS muscles and glands.
- Somatic Nervous System (SNS): Controls voluntary and involuntary (reflex) skeletal muscle contractions.
- Autonomic Nervous System (ANS): Controls subconscious actions, such as contractions of smooth and cardiac muscle, and glandular secretions.
  - Sympathetic Division: Stimulating effect.
  - Parasympathetic Division: Relaxing effect.

Receptors and Effectors

Receptors: Detect changes or respond to stimuli (e.g., neurons, specialized cells, sensory organs).
Effectors: Respond to efferent signals (e.g., muscles, glands).

Structure of Neurons

General Structure

Cell Body (Soma): Contains nucleus, nucleolus, cytoplasm (perikaryon), mitochondria, rough endoplasmic reticulum (RER), ribosomes, and cytoskeleton (neurofilaments, neurotubules, neurofibrils).
Nissl Bodies: Dense areas of RER and ribosomes; give gray matter its color.
Dendrites: Highly branched processes that receive information from other neurons; dendritic spines increase surface area.
Axon: Long process that carries electrical signals (action potentials) to target cells.
- Axoplasm: Cytoplasm of the axon.
- Axolemma: Specialized cell membrane covering the axon.
- Axon Hillock: Thick section of cell body attaching to the initial segment of the axon.
- Collaterals: Branches of a single axon.
- Telodendria: Fine extensions at the distal end of the axon.
- Synaptic Terminals: Tips of axon where communication with other cells occurs.

Synapse

Presynaptic Cell: Neuron sending the message.
Postsynaptic Cell: Cell receiving the message (neuron, muscle, or gland).
Synaptic Cleft: Small gap separating presynaptic and postsynaptic membranes.
Synaptic Knob: Expanded area of axon containing synaptic vesicles with neurotransmitters.
Neurotransmitters: Chemical messengers released at the presynaptic membrane, affecting receptors on the postsynaptic membrane. They are broken down by enzymes and reassembled at the synaptic knob.

Types of Synapses

Neuromuscular Junction: Synapse between a neuron and a muscle cell.
Neuroglandular Junction: Synapse between a neuron and a gland cell.

Structural Classifications of Neurons

Type	Location	Features
Anaxonic	Brain, sense organs	No obvious axon; all processes look alike
Bipolar	Special sensory organs (sight, smell, hearing)	One dendrite, one axon; small
Unipolar	Sensory neurons of PNS	Fused dendrites and axon; cell body to one side
Multipolar	CNS, skeletal muscle motor neurons	Multiple dendrites, one axon; most common

Functional Classifications of Neurons

Sensory Neurons (Afferent): Carry sensory information from receptors to CNS. Usually unipolar, cell bodies in sensory ganglia.
Motor Neurons (Efferent): Carry instructions from CNS to effectors (muscles, glands). Axons called efferent fibers. In the ANS, signals pass through autonomic ganglia (preganglionic and postganglionic fibers).
Interneurons (Association Neurons): Located in CNS and autonomic ganglia; connect sensory and motor neurons. Involved in distribution of sensory information, coordination of motor activity, and higher functions (memory, planning, learning).

Sensory Receptors

Interoceptors: Monitor internal systems (digestive, respiratory, cardiovascular, urinary, reproductive) and internal senses (taste, deep pressure, pain).
Exteroceptors: Monitor external senses (touch, temperature, pressure) and distance senses (sight, smell, hearing).
Proprioceptors: Monitor position and movement of skeletal muscles and joints.

Neuroglia (Glial Cells)

Neuroglia of the Central Nervous System (CNS)

Type	Function
Ependymal Cells	Line central canal of spinal cord and ventricles of brain; secrete, circulate, and monitor cerebrospinal fluid (CSF)
Astrocytes	Maintain blood-brain barrier, repair damaged neural tissue, guide neuron development
Oligodendrocytes	Form myelin sheaths around CNS axons, increasing speed of action potentials
Microglia	Migrate through neural tissue, clean up debris, waste, and pathogens

White Matter: Regions with many myelinated axons.
Gray Matter: Regions with unmyelinated axons and cell bodies.
Nodes of Ranvier: Gaps between myelinated segments (internodes) where axons may branch.

Neuroglia of the Peripheral Nervous System (PNS)

Satellite Cells (Amphicytes): Surround neuron cell bodies in ganglia; regulate environment around neurons.
Schwann Cells (Neurilemmacytes): Form myelin sheath around peripheral axons; each cell sheaths one segment of an axon.

Ganglia

Masses of neuron cell bodies in the PNS, surrounded by neuroglia.

Neural Responses to Injury

PNS Regeneration

Wallerian Degeneration: Axon distal to injury degenerates.
Schwann Cells: Form a path for new growth and wrap new axon in myelin.

CNS Regeneration

Limited by chemicals released by astrocytes that block growth and produce scar tissue.

Membrane Potential and Nerve Impulse

Membrane Potential

The membrane potential is the voltage difference across a cell's plasma membrane, resulting from the uneven distribution of ions.

Maintained by the sodium-potassium pump (Na+/K+ ATPase), which pumps 3 Na+ out and 2 K+ in, creating a net negative charge inside the cell.
Resting membrane potential in neurons is approximately -70 mV.
Membrane is more permeable to K+ than Na+.
Fixed anions inside the cell contribute to the negative charge.

Equation for Resting Membrane Potential (Nernst Equation):

Additional info: The Nernst equation calculates the equilibrium potential for a particular ion based on its concentration gradient across the membrane.

The Nerve Impulse (Action Potential)

Resting Potential: The state of the neuron before sending a nerve impulse; inside is negative relative to outside.
Depolarization: Decrease in membrane potential (towards zero).
Hyperpolarization: Increase in membrane potential (more negative).
Threshold of Excitement: The critical level (about -55 mV) that must be reached to trigger an action potential.
Action Potential: Rapid depolarization and repolarization of the membrane; follows the all-or-none law.

Sequence of Action Potential:

Depolarization to threshold opens voltage-gated Na+ channels; Na+ rushes in.
Membrane potential rises to about +60 mV.
Na+ channels close, K+ channels open; K+ exits the cell, repolarizing the membrane.
Resting potential is restored by the sodium-potassium pump.

Refractory Periods

Absolute Refractory Period: No new action potential can be generated.
Relative Refractory Period: A stronger-than-normal stimulus is required to initiate another action potential.

Propagation of Action Potentials

Action potentials are regenerated along each segment of the axon in unmyelinated fibers.
Saltatory Conduction: In myelinated axons, action potentials "jump" from node to node (nodes of Ranvier), increasing speed and conserving energy.
Larger axon diameter increases conduction speed.
Local anesthetics (e.g., Novocain) block sodium channels, preventing action potentials.

Local Neurons and Graded Potentials

Local neurons have short axons and communicate with nearby cells.
Produce graded potentials—membrane potentials that vary in magnitude and do not follow the all-or-none law.

The Synapse and Neurotransmission

Structure and Function of Synapses

Synapse: Specialized junction where a neuron communicates with another cell (neuron, muscle, or gland).
Presynaptic Cell: Releases neurotransmitter into the synaptic cleft.
Postsynaptic Cell: Has receptors for neurotransmitter; responds to the signal.
Synaptic Vesicles: Store neurotransmitters in the presynaptic terminal.
Calcium (Ca2+): Influx triggers exocytosis of neurotransmitter vesicles.

Sequence of Synaptic Transmission

Neurotransmitter is synthesized and stored in vesicles at the axon terminal.
Action potential arrives at axon terminal, causing vesicles to release neurotransmitter into the synaptic cleft.
Neurotransmitter diffuses across the cleft and binds to receptors on the postsynaptic cell.
Activated receptors cause changes in the postsynaptic cell's activity.
Neurotransmitter is released from receptors and diffuses back into the cleft.
Neurotransmitter is reabsorbed by the presynaptic neuron (reuptake), diffuses away, or is inactivated by enzymes.
Postsynaptic cell may send negative feedback to slow further neurotransmitter release.

Types of Synapses

Neuromuscular Junction: Neuron to muscle cell.
Neuroglandular Junction: Neuron to gland cell.

Example: Multiple Sclerosis

Destruction of the myelin sheath leads to impaired saltatory conduction and poor muscle coordination.

Summary Table: Key Features of Neurons and Neuroglia

Component	Function
Neuron	Transmit electrical impulses
Astrocyte	Support, blood-brain barrier, repair
Oligodendrocyte	Myelinate CNS axons
Schwann Cell	Myelinate PNS axons
Microglia	Phagocytosis of debris/pathogens
Ependymal Cell	Produce/circulate CSF

Additional info: This guide covers the structure and function of neural tissue, the organization of the nervous system, the physiology of nerve impulses, and the process of synaptic transmission, providing a comprehensive overview for exam preparation in anatomy and physiology.