Introduction to Nervous and Endocrine Systems

The Nervous System

• The nervous system consists of two parts: the central nervous system (CNS) and the peripheral nervous system (PNS)
• The PNS includes nerves outside the brain and spinal cord, which serve as communication lines that link all parts of the body

Functional Classification of the PNS

• Two subdivisions: sensory (afferent) division and motor (efferent) division
• Sensory division: carries information from sensory receptors to the CNS
• Motor division: carries impulses away from the CNS to effector organs, muscles, and glands

Motor Division Subdivisions

• Somatic nervous system: voluntary, controls skeletal muscles (reflexes)
• Autonomic nervous system: involuntary, controls smooth and cardiac muscles and glands

Nervous Tissue Structure and Function

• Supporting cells (neuroglia/glial cells): provide support, insulation, and protection for delicate neurons
• The endocrine system works with the nervous system to regulate and maintain body homeostasis

Organization of the Nervous System

• CNS: brain and spinal cord, acts as the integrating and command center
• PNS: nerves outside the brain and spinal cord
• Ganglia: small collections of cell bodies outside the CNS
• Tracts: bundles of nerve fibers running through the CNS
• Nerves: bundles of nerve fibers running through the PNS

Functional Classification of Neurons

• Sensory (afferent) neurons: carry impulses from sensory receptors to the CNS
• Motor (efferent) neurons: carry impulses from the CNS to muscles and glands
• Interneurons (association neurons): connect sensory and motor neurons in the CNS

Structural Classification of Neurons

• Multipolar neurons: many processes extending from the cell body
• Bipolar neurons: two processes, one axon and one dendrite
• Unipolar neurons: a short single process leaving the cell body

Neuron Anatomy

• Cell body: contains the nucleus and metabolic center of the cell
• Processes: fibers that extend from the cell body
• Dendrites: conduct impulses toward the cell body
• Axons: conduct impulses away from the cell body
• Axonal terminals: contain vesicles with neurotransmitters

Axons and Nerve Impulses

• Axonal terminals release neurotransmitters into the synaptic cleft
• Synapse: junction between nerves
• Myelin sheaths: produced by Schwann cells, insulate and facilitate nerve impulse transmission

Nerve Fiber Coverings

• Schwann cells: produce myelin sheaths around nerve fibers
• Nodes of Ranvier: gaps in myelin sheath at regular intervals along the axon

Neuron Cell Body Location

• Most cell bodies are found in the CNS
• Nuclei: clusters of cell bodies within the white matter of the CNS
• White matter: contains dense collections of myelinated fibers (tracts)
• Gray matter: contains unmyelinated fibers and cell bodies

Neuron Function

• Conductivity: ability to transmit an impulse to other neurons, muscles, or glands
• Irritability: ability to respond to stimuli and convert it into a nerve impulse

Nerve Impulse Propagation

• Depolarization: a stimulus depolarizes the neuron's membrane
• Action potential: the exchange of ions initiates an action potential in the neuron
• Repolarization: potassium ions rush out of the neuron after sodium ions rush in
• Sodium-potassium pump: restores the original configuration, requires ATP