The Nervous System Overview

The Nervous System

• The nervous system is divided into two major divisions: the central nervous system (CNS) and the peripheral nervous system (PNS).
• The CNS consists of the brain and spinal cord.
• The PNS contains nerves that connect the brain and spinal cord to muscles, glands, and sense organs.

Nervous System Overview

• The brain (100 billion neurons) is involved in information processing.
• The spinal cord (100 million neurons) transmits information to and from the brain and performs some information processing (reflexes).
• Cranial nerves provide sensory and motor functions for the face and vital organs (e.g., heart, respiratory muscles).
• Spinal nerves provide sensory and motor functions to specific body regions.
• Enteric plexuses are networks of neurons within digestive organs that regulate digestion.
• Sensory receptors monitor internal and/or external environmental changes.

Peripheral Nervous System (PNS)

• Somatic nervous system: Voluntary control of movements
  • Afferent neurons bring signals from receptors to the CNS.
  • Efferent neurons carry signals from the CNS to skeletal muscles.
• Autonomic nervous system: Involuntary control of smooth and cardiac muscle, and glands
  • Contains sensory neurons from visceral organs and motor neurons conveying impulses from the CNS to smooth muscle tissue, cardiac muscle tissue, and glands.
  • Works with the endocrine system to maintain homeostasis.
• Enteric nervous system: Regulates the gastrointestinal system(part of the autonomic nervous system.)

Classification of Neurons

• Structural: Multipolar, bipolar, unipolar
• Functional:
  • Sensory (afferent) neurons: Bring sensory information to the brain
  • Motor (efferent) neurons: Carry signals from the brain to effectors (muscles and glands)
  • Interneurons (association neurons): Connect sensory and motor neurons within the CNS; integrate and process information.

Histology of Nervous Tissue

• Neurons:
  • Possess electrical excitability.
  • Three parts: cell body, dendrites, axon.
• Neuroglia:
  • Smaller than neurons, more numerous (5–25 times);
  • Support, nourish, and protect neurons.
  • Types: astrocytes, oligodendrocytes, microglia, ependymal cells (in CNS).
  • Types: Schwann cells, satellite cells (in PNS).

Neuroglia

• Two types of neuroglia produce myelin sheaths:
  • Oligodendrocytes myelinate axons in the CNS.
  • Schwann cells myelinate axons in the PNS.

Myelination

• The myelin sheath is a fatty substance that insulates axons, increasing the speed of nerve impulses.
• Nodes of Ranvier are gaps in the myelin sheath that allow for faster signal propagation.

Collections of Nervous Tissue

• Ganglia: Clusters of neuronal cell bodies in the PNS, associated with cranial and spinal nerves.
• Nuclei: Clusters of neuronal cell bodies in the CNS.
• Nerves: Bundles of axons in the PNS(cranial or spinal).
• Tracts: Bundles of axons in the CNS, connecting neurons in the brain and spinal cord.

Gray and White Matter

• White matter: Primarily myelinated neurons; white color from myelin.
• Gray matter: Neuronal cell bodies, dendrites, unmyelinated axons, axon terminals, and neuroglia; grayish color from cellular organelles.

Action Potentials

• Resting membrane potential (~-70mV): The inside of the cell is more negative than the outside.
• Threshold potential (~-55mV): The stimulus needed to initiate an action potential.
• Depolarization (+30mV): Sodium (Na+) ions rush into the cell, making the inside more positive.
• Repolarization: Potassium (K+) ions rush out of the cell, returning the inside to a negative charge.
• Hyperpolarization: The membrane potential temporarily becomes more negative than the resting potential. This prevents further immediate action potentials.

Synaptic Transmission

• A synapse is a junction between two neurons or a neuron and an effector cell (e.g., a muscle or gland).
• The presynaptic neuron sends the signal, and the postsynaptic neuron receives it.
• Neurotransmitters are released at the synapse to transmit the signal across the synaptic cleft.

Explore the structure and function of the nervous system, including its two main divisions: the central nervous system (CNS) and peripheral nervous system (PNS). Understand the roles of the brain, spinal cord, cranial nerves, and sensory receptors in processing information and controlling bodily functions.