Nervous System Organization

Nervous System Organization

• Composed of nervous tissues, divided into Central Nervous System (CNS) and Peripheral Nervous System (PNS)

Central Nervous System (CNS)

• Includes brain and spinal cord
• Processes sensory information
• Source of thoughts, emotions, memories
• Initiates muscle contractions and gland secretions

Spinal Cord

• Attached to the brain
• Enclosed by the vertebral column

Peripheral Nervous System (PNS)

• Nervous tissues outside the CNS
• Includes nerves and sensory receptors
• Divided into afferent and efferent divisions

Nerves

• Bundles of axons
• Located outside the brain and spinal cord

Sensory Receptors

• Structures that detect changes in the environment (external or internal)

Afferent Division

• Carries sensory information from sensory receptors to the CNS
• Includes somatic senses (touch, temperature, pain, body position) and special senses (smell, taste, vision, hearing, balance)

Efferent Division

• Carries motor commands from the CNS to effectors (muscles and glands)
• Divided into somatic and autonomic nervous systems

Somatic Nervous System

• Part of efferent division
• Controls skeletal muscles
• Voluntary responses

Autonomic Nervous System

• Part of efferent division
• Controls smooth, cardiac muscles, and glands
• Involuntary responses

ANS Subdivisions

• Parasympathetic: "rest and digest," calming the body
• Sympathetic: "fight or flight," preparing the body for stressful situations
• Enteric: controls the GI tract; can function independently but communicates with other ANS branches

Parasympathetic vs. Sympathetic

• Opposite actions (e.g., sympathetic speeds heart rate, parasympathetic slows it)
• Parasympathetic promotes relaxation and digestion; sympathetic prepares for emergencies

Enteric Nervous System

• Subdivision of the autonomic nervous system
• Regulates activity of smooth muscles and glands in the gastrointestinal tract
• Can function independently, but communicates with the other branches of the autonomic nervous system

Nervous System Functions

• Sensory function: detects stimuli, transmits to CNS
• Integrative function: processes information, determines appropriate response
• Motor function: elicits response, transmits to effectors

Neurons (Nerve Cells)

• Basic functional unit of the nervous system
• Responsible for sensing, thinking, remembering, muscle control, and gland secretion

Neuron Components

• Dendrites: receive signals, input portion
• Cell body (soma): contains organelles, control center, input portion
• Axon: generates action potentials, output portion
  • Axon hillock: trigger zone for action potential generation
  • Axon collaterals: branches of the axon
  • Axon terminals: end of axon branches, form synapses

Ganglia and Nuclei

• Ganglia: clusters of neuron cell bodies in the PNS
• Nuclei: clusters of neuron cell bodies in the CNS

Axon Terminals/Synaptic End Bulbs

• Contain synaptic vesicles with neurotransmitters
• Release neurotransmitters into synapses (spaces between neurons or other cells)

Neuronal Communication

• Action potential arrives at synaptic end bulb
• Neurotransmitters released, influencing target cells

Axonal Transport

• Moves materials between cell body and axon terminals
• Anterograde: moves material forward (cell body to terminals, using kinesin)
• Retrograde: moves material backward (terminals to cell body, using dynein)

Sensory Neurons

• Part of afferent division
• One axon extends from cell body with dendrites at the periphery
• Sensory receptors are either dendrites or adjacent structures

Motor Neurons

• Part of efferent division
• Carry signals from CNS to effectors
• Most have one axon and dendrites
• Divided into somatic and autonomic motor neurons

Interneurons

• Located within the CNS
• Connect sensory and motor neurons
• Process information and mediate responses

Neuroglia

• Supporting cells in the nervous system
• Nourish, protect, maintain homeostasis
• Do not generate action potentials, but can divide/grow

Neuroglia Types

• Astrocytes: most abundant, surround blood vessels, help maintain blood-brain barrier
• Oligodendrocytes: form myelin sheaths in CNS
• Microglia: remove cellular debris
• Ependymal cells: line ventricles, produce and circulate cerebrospinal fluid
• Schwann cells: form myelin sheaths in PNS; important in axonal regeneration

Myelin Sheath

• Multilayered lipid/protein covering axons
• Insulates axons, speeds action potential conduction

Myelination (CNS vs. PNS)

• CNS: one oligodendrocyte can myelinate multiple axon segments
• PNS: one Schwann cell myelinates one axon segment

White and Gray Matter

• White matter: mainly myelinated axons
• Gray matter: cell bodies, dendrites, unmyelinated axons, axon terminals, neuroglia

Electrical Signals in Neurons

• Graded potentials: short-distance signals
• Action potentials: long-distance signals; combination of depolarization and repolarization

Action Potential Mechanism

• Depolarization phase: sodium channels open, influx of Na+
• Repolarization phase: potassium channels open, outflow of K+
• After-hyperpolarization: potassium channels remain open for a time
• Threshold: minimum depolarization required to initiate action potential

Factors Affecting Action Potential Velocity

• Axon diameter: larger diameter, faster conduction
• Myelination: myelin speeds conduction (saltatory conduction)

Classification of Neurons

• A fibers: myelinated, fast; sensory and motor
• C fibers: unmyelated, slow; sensory and motor

Synaptic Communication

• Chemical synapse: neurotransmitters released; slower but more complex
  • Steps: AP arrival, Ca2+ influx, neurotransmitter release, binding to receptors, postsynaptic potential, signal termination
• Electrical synapse: direct ion flow; faster ( gap junctions)

Postsynaptic Potentials

• EPSP (excitatory): depolarization, increase chance of action potential
• IPSP (inhibitory): hyperpolarization, decrease chance of action potential

Summation of Postsynaptic Potentials

• Temporal summation: multiple EPSPs or IPSPs occur in rapid succession
• Spatial summation: multiple EPSPs or IPSPs from different synapses occur at the same time

Neurotransmitter Effects

• Can be excitatory or inhibitory
• Can affect target cell in various ways (e.g., synthesizing proteins, changing ion concentrations)

Neurotransmitter Receptors

• Ionotropic receptors: ligand-gated channels
• Metabotropic receptors: involve G proteins

Explore the complex organization of the nervous system, including the Central Nervous System (CNS) and Peripheral Nervous System (PNS). Understand the roles of the brain, spinal cord, sensory receptors, and the divisions of nerves that facilitate sensory and motor functions.