The Nervous System and Neural Tissue

Basic Structure and Function of the Nervous System

The nervous system is a complex network responsible for sensing environmental changes, integrating information, and coordinating responses. It is divided into two main anatomical divisions: the Central Nervous System (CNS) and the Peripheral Nervous System (PNS).

• Central Nervous System (CNS): Composed of the brain (in the cranial cavity) and spinal cord (in the vertebral column). The CNS is the seat of all mental activity, interpreting sensory input and dictating motor responses.

• Peripheral Nervous System (PNS): Consists of cranial and spinal nerves, serving as communication lines between the CNS and the rest of the body.

• Functional Divisions of the PNS:

  • Sensory Division (Afferent): Conducts impulses from receptors to the CNS. Sensory receptors in somatic (skin, muscle, joints) and visceral (organs) systems detect stimuli and transmit them for interpretation. This is the "input" region.

  • Motor Division (Efferent): Conducts impulses from the CNS to effector organs (muscles and glands) to produce responses. This is the "output" region, further subdivided into:

    • Somatic Nervous System (SNS): Voluntary control of skeletal muscles.

    • Autonomic Nervous System (ANS): Involuntary control of cardiac muscle, smooth muscle, and glands.

      • Parasympathetic: Conserves energy, maintains homeostasis ("rest and digest").

      • Sympathetic: Mobilizes body for emergencies ("fight or flight").

    • Enteric Nervous System (ENS): Controls smooth muscle and glands of the digestive tract.

• Basic Functions:

  • Sensation: Detects stimuli and transmits them to the CNS.

  • Response: Sends impulses from CNS to effectors for appropriate responses.

  • Integration: Processes and interprets sensory stimuli, triggering motor responses.

  • Homeostasis: Maintains internal balance through regulatory functions.

• Nervous Tissue: Composed of neurons (functional cells) and glial cells (supportive cells).

Neural Tissue

Neural tissue consists of neurons and glial cells, each with distinct roles in nervous system function.

• Neurons: Basis of nervous tissue, responsible for electrical signaling, sensation, movement, and thought processes.

  • Unique Characteristics:

    • Excitable: Polarized membrane allows conduction of nerve impulses.

    • Longevity: Can function for over 100 years.

    • High Metabolic Rate: Require constant oxygen and glucose; contain many mitochondria.

    • Large Size: Among the largest cells in the body.

    • Amitotic: Most CNS neurons do not divide after maturity; PNS neurons may regenerate.

  • Parts of a Typical Neuron:

    • Cell Body (Soma): Contains nucleus and organelles; clusters called nuclei (CNS) or ganglia (PNS); contains Nissl bodies (rough ER).

    • Cell Processes: Extensions called tracts (CNS) or nerves (PNS).

      • Dendrites: Branched processes receiving signals toward the cell body; conduct graded potentials.

      • Axons: Generate and transmit action potentials away from the cell body; axolemma (membrane), axoplasm (cytoplasm), axon hillock (trigger zone), axon terminals (synaptic terminals).

    • Myelin Sheath: Whitish, fatty covering insulating axons; formed by Schwann cells (PNS) or oligodendrocytes (CNS); myelinated fibers (white matter) conduct rapidly, unmyelinated (gray matter) conduct slowly; nodes of Ranvier are gaps aiding transmission.

  • Classification of Neurons:

    • Structural:

      • Anaxonic: Processes indistinguishable; found in brain and special sense organs.

      • Bipolar: One dendrite, one axon; rare, found in retina and nasal mucosa.

      • Unipolar: Dendrites and axon fuse; most sensory neurons of PNS.

      • Multipolar: One axon, two or more dendrites; most common in CNS and all motor neurons.

    • Functional:

      • Sensory (Afferent): Transmit impulses to CNS; unipolar (skin/internal organs), bipolar (special senses).

      • Motor (Efferent): Transmit impulses from CNS to effectors; mostly multipolar.

      • Association (Interneurons): Transmit impulses between sensory and motor neurons; found in CNS; mostly multipolar or anaxonic.

    • Other Types: Pyramidal cells (multipolar, pyramid-shaped), Purkinje cells (cerebellum), olfactory neurons (named for function).

• Glial Cells (Neuroglia): Support, feed, protect, and insulate neurons; 700-900 glial cells per neuron.

  • CNS Glial Cells:

    • Astrocytes: Star-shaped, connect neurons to blood supply (blood-brain barrier), regulate chemical environment, absorb/recycle neurotransmitters, form scar tissue.

    • Microglia: Act as macrophages, engulf microbes and debris, provide protection.

    • Oligodendrocytes: Insulate axons in myelin, wrap around several neurons.

    • Ependymal Cells: Line brain ventricles and spinal cord, produce and circulate cerebrospinal fluid (CSF).

  • PNS Glial Cells:

    • Schwann Cells: Form myelin sheath around axons, associate with single neuron, outer layer is neurilemma, nodes of Ranvier are gaps.

    • Satellite Cells: Surround cell bodies, regulate chemical environment.

Function of Nervous Tissue

Nervous tissue maintains homeostasis through a three-step process: sensory input, integration, and motor output. This pathway is exemplified by the response to environmental stimuli, such as temperature changes.

• Input-Integration-Output Pathway:

  1. Sensation: Activation of sensory receptors (e.g., thermoreceptors in skin) initiates electrical signals sent to the CNS.

  2. Integration: Signals are processed in the CNS (e.g., thalamus and cerebral cortex) for conscious perception and decision-making.

  3. Response: CNS sends commands via motor neurons to effectors (e.g., skeletal muscle) for appropriate action.

• Nerve Impulse Transmission: Similar to muscle contraction, involves resting membrane potential, depolarization, action potential propagation, and repolarization.

Action Potential

Action potentials are rapid changes in membrane potential that allow neurons to transmit signals. Ion channels regulate the flow of ions across the membrane, establishing and altering membrane potentials.

• Types of Ion Channels:

  • Passive (Leakage) Channels: Always open, allow ions to pass freely; regulate by electrochemical exclusion, size exclusion, or non-specificity.

  • Active (Gated) Channels:

    • Ligand-Gated: Open when a chemical ligand binds; important for graded potentials (dendrites/cell body).

    • Voltage-Gated: Open in response to changes in membrane potential; crucial for action potential generation (axon).

    • Mechanical-Gated: Open in response to physical deformation (touch, pressure, vibration, temperature).

  • Leakage Channels: Randomly open/close; maintain resting membrane potential.

• Resting Membrane Potential:

  • Exists only across the membrane; bulk solutions are neutral.

  • Typical value: .

  • Inside is negative, outside is positive.

  • Na+ high outside, K+ high inside.

  • Voltage-gated channels closed; passive gates open but minimal movement.

• Depolarization and Graded Potential:

  • Neurotransmitter opens ligand-gated channels; Na+ rushes in.

  • Charge shifts toward positive inside; depolarization from toward .

  • Graded potential spreads; if reaches at axon hillock, triggers action potential.

• Propagation of Action Potential:

  • Voltage-gated channels open at axon hillock; Na+ flows in.

  • Action potential travels down axon; charge changes from to .

  • All-or-None Principle: Once generated, cannot be stopped.

  • Myelinated nerves: Saltatory conduction (jumps node to node, rapid).

  • Unmyelinated nerves: Continuous conduction (slow).

  • Refractory period: Cell insensitive to new stimulus during action potential.

• Repolarization:

  • Neurotransmitter removed; ligand-gated channels close.

  • K+ flows out, restoring negative inside.

  • Membrane hyperpolarizes to .

  • Sodium-potassium pump restores ion locations: pumps out for every in.

Equation for Sodium-Potassium Pump:

Communication Between Neurons

Neurons communicate via graded potentials, synapses, and neurotransmitters, allowing for complex integration and response.

• Graded Potentials: Occur in dendrites; can be depolarizing or hyperpolarizing; summate to determine if threshold is reached.

  • Generator Potential: Stimulus in unipolar neurons generates action potential.

  • Receptor Potential: Specialized sensory receptors (taste, vision, hearing) release neurotransmitters.

  • Postsynaptic Potentials: Excitatory (EPSP) or inhibitory (IPSP); EPSP moves toward threshold, IPSP away.

  • Summation:

    • Spatial: Multiple presynaptic neurons stimulate one neuron.

    • Temporal: Multiple action potentials from one presynaptic neuron.

• Synapses: Junctions between neurons or between neuron and effector.

  • Presynaptic Neuron: Conducts impulse toward synapse.

  • Postsynaptic Neuron: Conducts impulse away from synapse.

  • Types:

    • Electrical Synapse: Direct connection via gap junctions; rare.

    • Chemical Synapse: Neurotransmitter released into synaptic cleft; common.

• Neurotransmitters: Chemical ligands released at synapse; effect depends on receptor type.

  • Types:

    • Cholinergic System: Uses acetylcholine (ACh); found at neuromuscular junction.

    • Amino Acids: Glutamate, glycine, GABA.

    • Biogenic Amines: Serotonin, dopamine, epinephrine, norepinephrine.

    • Neuropeptides: GIP, VIP.

  • Receptor Types:

    • Ionotropic: Ligand-gated ion channels (e.g., nicotinic receptor for ACh, glycine receptor).

    • Metabotropic: G protein-coupled receptors; second messenger systems.

  • Effect: Depends on receptor, not neurotransmitter; can be excitatory or inhibitory.

Table: Structural and Functional Classification of Neurons

Table: Glial Cells of CNS and PNS

Example: The sensation of touching hot water activates thermoreceptors in the skin, which send signals to the CNS for integration and then trigger a motor response to withdraw the hand.

Additional info: Loss of myelination (e.g., in Multiple Sclerosis or Guillain-Barre Syndrome) impairs rapid nerve impulse transmission, leading to neurological deficits.