Nervous System Functions and Organization

• The nervous and endocrine systems collaborate to maintain homeostasis
• The nervous system is fast-acting with short-lived effects, while the endocrine system has a slower response with longer-lasting effects

Functions of the Nervous System

• Perception involves sensory input, gathering internal and external stimuli
• Integration processes sensory input, interpreting data to determine necessary actions
• Motor output responds to stimuli by activating muscles or glands
• The nervous system function pathway is: sensory receptor, sensory input, integration (brain/spinal cord), motor output, effector (muscles/glands)

Universal Properties of Neurons

• Excitability allows neurons to respond to stimuli
• Conductivity enables neurons to transmit signals quickly
• Secretion enables neurons to release neurotransmitters

Organization of the Nervous System

• The central nervous system (CNS) includes the brain and spinal cord (interneurons)
• The peripheral nervous system (PNS) consists of nerves outside the CNS
• Spinal nerves connect to the spinal cord
• Cranial nerves connect to the brain, facilitating communication between the CNS and body parts
• The sensory (afferent) division carries information to the CNS
• Somatic sensory input comes from skin, muscles, and joints
• Visceral sensory input comes from organs
• The motor (efferent) division carries commands from the CNS
• The somatic nervous system is voluntary and controls skeletal muscles
• The autonomic nervous system (ANS) is involuntary, controlling smooth/cardiac muscles and glands
• The sympathetic division increases heart rate and inhibits digestion
• The parasympathetic division stimulates digestion and slows heart rate

Structure of a Neuron

• A neuron (nerve cell) is the basic unit of the nervous system
• The neurosoma (cell body) is the control center, containing the nucleus, mitochondria, and ribosomes
• Dendrites receive signals
• The axon transmits action potentials
• Terminal arborization is branched axon endings
• The presynaptic terminal releases neurotransmitters
• Neuron regeneration is limited due to the lack of centrioles
• Multipolar neurons: Are the most common in the CNS, have many dendrites and one axon
• Bipolar neurons: Have one dendrite and one axon
• Unipolar neurons: Have no dendrites and one axon, common in sensory neurons
• Anaxonic neurons: Have many dendrites but no axon, these are rare

Neuroglial (Glial) Cells

• Glial cells are support cells
• Astrocytes are the most abundant in the CNS and form the blood-brain barrier
• Microglia act as phagocytes, removing debris in the CNS
• Ependymal cells line brain cavities and help circulate cerebrospinal fluid (CSF)
• Oligodendrocytes form myelin sheaths in the CNS
• Satellite cells support neuron cell bodies, similar to astrocytes in the PNS
• Schwann cells form myelin in the PNS

Myelin & Nerve Transmission

• Myelin in the CNS is produced by oligodendrocytes
• Myelin in the PNS is produced by Schwann cells
• The neurilemma is the outermost layer of a Schwann cell
• Internodes are myelinated segments
• Nodes of Ranvier are gaps in myelin for faster signal transmission
• Myelin increases the speed of nerve impulses and prevents signal leakage
• Multiple Sclerosis (MS) is a disease caused by myelin degeneration

Synapses

• A synapse is the meeting point between a neuron and other cells
• Axodendritic synapses: Axon to dendrite
• Axosomatic synapses: Axon to soma
• Axoaxonic synapses: Axon to axon

Chemical Synapses

• Neurotransmitters (e.g., Acetylcholine (ACh)) are released
• Presynaptic neuron releases neurotransmitter
• The synaptic cleft is the gap between neurons
• The postsynaptic neuron has receptors for neurotransmitters

Neural Circuits

• A diverging circuit: One neuron spreads signals to many
• A converging circuit: Multiple neurons funnel signals into one
• A reverberating circuit: Neurons stimulate each other in a loop
• A parallel after-discharge circuit: One neuron stimulates several chains before reconverging