Definition: The nervous system is the master controlling and communicating system of the body.
Major Functions:
Sensory Input: Monitoring stimuli from the environment.
Integration: Interpretation of sensory input.
Motor Output: Response to stimuli.
Central Nervous System (CNS):
Composed of the brain and spinal cord.
Acts as the integration and command center.
Peripheral Nervous System (PNS):
Includes paired spinal (spinal nerves) and cranial nerves (cranial nerves).
Carries messages to and from the spinal cord and brain.
Links all parts of the body to the CNS.
Central Nervous System (CNS):
Brain
Spinal Cord
Peripheral Nervous System (PNS):
Cranial nerves
Spinal nerves
Ganglia
Sensory (Afferent) Division:
Sensory Afferent Fibers: Transmit impulses from skin, skeletal muscle, and joints to the brain.
Visceral Afferent Fibers: Transmit impulses from visceral organs to the brain.
Motor (Efferent) Division:
Transmits impulses from the CNS to target organs (muscles and glands).
Somatic Nervous System:
Known as the “voluntary nervous system.”
Composed of somatic motor nerve fibers that conduct impulses from CNS to skeletal muscles.
Allows conscious control of skeletal muscles.
Autonomic Nervous System (ANS):
Known as the “involuntary nervous system.”
Consists of visceral motor nerve fibers.
Regulates smooth muscle, cardiac muscle, and glands.
Subdivisions: Sympathetic and parasympathetic branches.
Cell Types:
Neurons: Excitable cells that transmit electrical signals.
Neuroglia: Support cells that surround and wrap neurons.
Neuroglia Functions:
Provide support for neurons.
Insulate neurons to speed up action potential conduction.
Promote health and growth of neurons.
Astrocytes (CNS):
Most abundant and versatile glial cells.
Functions:
Support and brace neurons.
Anchor neurons to nutrient supplies.
Control the chemical environment.
Microglia (CNS):
Small, spiny cells acting as macrophages that monitor the health of neurons and phagocytize debris.
Ependymal Cells (CNS):
Line the central cavities of the brain and spinal column.
Contain cilia that help circulate cerebrospinal fluid (CSF).
Oligodendrocytes (CNS):
Form the myelin sheath around CNS nerve fibers.
Schwann Cells (PNS):
Form myelin sheaths around larger PNS nerve fibers.
Vital for regeneration of damaged peripheral nerve fibers.
Satellite Cells (PNS):
Surround neuron cell bodies; function not fully understood.
Structure:
Composed of a body, dendrites, and axon.
Long-lived, amitotic, with a high metabolic rate.
Plasma Membrane Functions:
Electrical signaling.
Cell-to-cell signaling during development.
Dendrites:
Receptive regions carrying impulses towards the soma.
Axons:
Carry impulses away from the soma.
Only one axon, but can branch to connect with multiple cells.
Formation:
Formed by Schwann cells in PNS and oligodendrocytes in CNS.
Functions:
Protect and electrically insulate axons.
Increase the speed of action potentials.
White Matter:
Dense collections of myelinated fibers; appears white in the CNS.
Gray Matter:
Mostly somas and unmyelinated fibers; appears gray in the CNS.
Structural:
Unipolar Neuron: Single, short process, exclusively sensory.
Bipolar Neuron: Two processes; found in olfactory epithelium and retina.
Multipolar Neuron: One axon and multiple dendrites; most common type.
Functional:
Sensory (Afferent) Neurons: Transmit impulses toward the CNS.
Motor (Efferent) Neurons: Carry impulses away from the CNS.
Interneurons: Shuttle signals within the CNS pathways.
Action Potentials:
Electrical impulses that travel along the axons.
Principal means of neural communication; consistent in strength.
Generated by muscle cells and neurons; do not decrease over distance.
Four Main Steps:
Resting State: All Na⁺ and K⁺ channels closed.
Depolarization: Na⁺ channels open.
Repolarization: K⁺ channels open.
Hyperpolarization: Some K⁺ channels remain open; Na⁺ channels reset.
Threshold:
Membrane depolarizes by 15-20 mV for an action potential to occur.
All-or-None Phenomenon: Action potentials happen completely or not at all.
Impulse propagation rate determined by:
Axon Diameter: Larger diameter leads to faster impulses.
Myelin Sheath Presence: Myelination greatly increases impulse speed.
Occurs in myelinated nerves.
Action potentials jump between nodes of Ranvier.
Faster than unmyelinated axon conduction (30x faster).
Classified by:
Diameter, degree of myelination, and speed of conduction.
Group A Fibers: Large diameter, thick myelin - 150 m/s.
Group B Fibers: Intermediate diameter, light myelination - 15 m/s.
Group C Fibers: Small diameter, no myelination - 1 m/s.
Definition: Axon terminal releases neurotransmitter to communicate with the target cell.
Composed of:
Presynaptic axon terminal with synaptic vesicles.
Receptor region on the target cell.
Description: Fluid-filled space separating presynaptic neuron and target.
Transmission Process:
Involves release of neurotransmitter, ensuring unidirectional communication.
Depends on neurotransmitter removal:
Diffusion from the synaptic cleft
Reabsorption by presynaptic axon terminals
Degraded by enzymes (e.g., acetylcholine degraded by acetylcholinesterase).
Types:
EPSP: Excitatory postsynaptic potential; helps generate action potentials.
IPSP: Inhibitory postsynaptic potential; reduces ability to produce action potentials.
A single EPSP cannot initiate an action potential.
Summation Types:
Temporal Summation: Rapid-fire impulses from presynaptic neurons.
Spatial Summation: Simultaneous stimulation from multiple terminals.
Chemical Classifications: Acetylcholine, biogenic amines, amino acids, peptides, and dissolved gases.
Acts as chemical signals for communication.
Excitatory vs. Inhibitory:
Excitatory neurotransmitters cause depolarization.
Inhibitory neurotransmitters cause hyperpolarization.
Excitatory and inhibitory based on their effects on the postsynaptic neuron.
Example: Acetylcholine is excitatory at neuromuscular junctions but inhibitory in cardiac muscle.
Definition: Groups of neurons that integrate incoming information and forward to appropriate destinations.
Diverging Circuit: One input leading to many outputs; amplifies signal.
Converging Circuit: Many inputs leading to one output; integrates multiple signals.
Reverberating Circuit: A feedback circuit; controls rhythmic activity.
Parallel After-Discharge Circuit: Impulses reach a single output cell at different times, involved in complex processing tasks.
Serial Processing: Direct, single pathway to destination; all-or-none response.
Parallel Processing: Multiple pathways; integrates in different CNS systems; evokes various responses to a single stimulus.
Originates from the neural tube and neural crest.
Three phases of differentiation involve cell proliferation, migration, and differentiation into neuroblasts.