Nervous System Organization

Nervous System Subdivisions

• The central nervous system (CNS) comprises the brain and spinal cord
• The peripheral nervous system (PNS) includes cranial nerves, spinal nerves, their branches, ganglia, and sensory receptors

Organization of the Nervous System

• The CNS is composed of the brain and spinal cord
• The PNS is divided into the somatic, autonomic, and enteric nervous systems
• The somatic nervous system (SNS) controls voluntary movements via skeletal muscle innervation
• The autonomic nervous system (ANS) regulates involuntary functions and has sympathetic and parasympathetic divisions
• The enteric nervous system (ENS), sometimes called the "brain of the gut," independently controls digestive functions separate from the CNS

Histology of Nervous Tissue

• Neurons are the nervous system's functional units that generate and transmit electrical signals
• Neuroglia support, protect, and nourish neurons but do not conduct nerve impulses

Neuronal Structure

• The cell body (soma) contains the nucleus and organelles
• Dendrites are short, branched structures that receive signals from other neurons
• An axon is a long projection transmitting electrical impulses away from the cell body

Structural Classification of Neurons

• Multipolar neurons, the most common type, have one axon and multiple dendrites
• Bipolar neurons have one axon and one dendrite, found in sensory organs, such as the retina, inner ear, and olfactory system
• Unipolar neurons have a single process splitting into two branches and are mainly found in sensory neurons of the PNS

Histological Classification of Neurons

• Purkinje cells are found in the cerebellum and play a crucial role in motor coordination
• Renshaw cells are located in the spinal cord, providing inhibitory feedback to motor neurons
• Pyramidal cells are found in the brain, primarily in the cerebral cortex, and are involved in motor control and cognitive functions

Functional Classification of Neurons

• Sensory (afferent) neurons transmit sensory information from receptors to the CNS
• Motor (efferent) neurons carry commands from the CNS to muscles and glands
• Interneurons (association neurons) connect sensory and motor neurons, facilitating processing and integration

Neuroglia Properties

• Neuroglia, also called glia, means "glue" and helps hold nervous tissue together
• Neuroglia do not generate or transmit nerve impulses
• Unlike neurons, neuroglia are capable of mitosis after maturation
• Neuroglia provide structural support, protection, and nutrition to neurons

Types of Neuroglia

• Astrocytes maintain the blood-brain barrier, provide structural support, and regulate ion balance
• Oligodendrocytes form myelin sheaths in the CNS to insulate neurons and enhance signal conduction
• Microglia act as immune cells in the CNS, removing debris and pathogens
• Ependymal cells line ventricles of the brain and the central canal of the spinal cord and are involved in cerebrospinal fluid (CSF) production and circulation
• Schwann cells form myelin sheaths in the PNS and aid in nerve regeneration
• Satellite cells surround neuron cell bodies in the PNS and regulate their environment

Myelin Sheath-Producing Neuroglia

• Schwann cells in the PNS myelinate axons in the peripheral nervous system
• Oligodendrocytes in the CNS myelinate axons in the central nervous system

Myelination and the Myelin Sheath

• The myelin sheath is a lipid-rich insulating layer that speeds up nerve impulse transmission

Types of Axons

• Unmyelinated axons lack a myelin sheath, which results in slower signal conduction
• Myelinated axons are wrapped in a myelin sheath, increasing the speed of electrical impulses

Gray and White Matter

• Gray matter contains neuronal cell bodies, dendrites, and unmyelinated axons and is involved in processing and integration
• White matter is composed of myelinated axons that transmit signals rapidly over long distances

Electrical Signals in Neurons

• Neurons use action potentials and graded potentials as electrical signals

Action Potential

• Action potential is a large, rapid change in membrane potential that propagates along the axon
• Resting membrane potential is maintained by specific conditions like unequal ion distribution, plasma membrane permeability to Na+ and K+, and the presence of an electrochemical gradient

Graded Potentials

• Graded potentials are small, localized changes in membrane potential
• Hyperpolarizing graded potential increases the membrane potential, making it more negative
• Depolarizing graded potential decreases the membrane potential, making it less negative

Action Potential Generation

• Action potentials follow an "all-or-none principle" and occur in three phases
• Depolarizing Phase: Na+ channels open, Na+ enters, and the cell becomes more positive inside
• Repolarizing Phase: K+ channels open, K+ exits, restoring the negative membrane potential
• Refractory Phase: Another action potential cannot be immediately generated

Ion Channels

• There are four types of ion channels: leakage, voltage-gated, ligand-gated, and mechanically-gated

Propagation of Nerve Impulses

• Continuous conduction
• Saltatory conduction
• Effect of axon diameter relates to nerve impulse propagation

Signal Transmission at Synapses

• Electrical synapses use gap junctions
• Chemical synapses use voltage-gated Ca+ channels
• Excitatory and Inhibitory Postsynaptic Potentials (EPSP & IPSP) are involved
• Removal of neurotransmitter is essential
• Spatial and temporal summation of postsynaptic potentials occurs

Neurotransmitters

• Small-molecule neurotransmitters include acetylcholine (ACh), amino acids, biogenic amines (norepinephrine, epinephrine, dopamine, catecholamines, serotonin), ATP, other purines, and nitric oxide

Neural Circuits

• Neural circuits can be simple series, diverging, converging, reverberating, or parallel after-discharge circuits