Overview
This lecture introduces the nervous system, its cellular structure, how nerve impulses work, and an overview of the central and peripheral subdivisions.
Nervous Tissue & Cell Types
- Nervous tissue is the fourth tissue type, alongside epithelial, connective, and muscle tissues.
- Neurons are the primary cells in nervous tissue, specialized for transmitting electrical signals.
- Neurons have branching extensions—dendrites (receive signals) and an axon (transmits signals).
- Neurons are amitotic (do not divide) and can last a lifetime.
- Neuroglia (glial cells) support, protect, and insulate neurons.
Neuron Structure
- The cell body contains the nucleus, nucleolus, and typical organelles.
- Dendrites receive incoming signals, while the axon sends signals away from the cell body.
- Axons originate at the axon hillock and end at axon terminals, releasing neurotransmitters.
- Many axons are covered by a myelin sheath (Schwann cells in the PNS), increasing signal speed.
- Nodes of Ranvier are gaps between Schwann cells along the axon.
Types and Functions of Neurons
- Neurons by structure: unipolar (one process), bipolar (two processes), and multipolar (three or more, most common).
- Neuron functions: sensory (afferent) neurons carry input to CNS; motor (efferent) neurons send output to muscles/glands; interneurons connect sensory and motor neurons and are mostly in CNS.
Electrical Impulses & Synapses
- Neurons have a resting membrane potential due to ion concentration differences across the membrane.
- Ion channels in the membrane allow ions in/out; can be non-gated or gated (chemical, voltage, mechanical).
- Action potentials (nerve impulses) occur when depolarization exceeds threshold, propagating down the axon.
- Repolarization and hyperpolarization reset the neuron after firing.
- Synapses are junctions where neuron signals are transmitted using neurotransmitters.
Organization of the Nervous System
- The nervous system receives input, integrates it, and produces a motor output (response).
- Divided into: Central Nervous System (CNS: brain and spinal cord) and Peripheral Nervous System (PNS: nerves outside CNS).
- PNS splits into sensory/afferent (to CNS) and motor/efferent (from CNS).
- Motor division splits into: somatic (voluntary muscle control) and autonomic (involuntary, e.g., heart, digestion).
- Autonomic system divides into sympathetic (fight or flight) and parasympathetic (rest and digest) divisions.
Key Terms & Definitions
- Neuron — main nerve cell specialized for transmitting electrical signals.
- Neuroglia (Glial Cells) — supporting cells that protect and insulate neurons.
- Dendrite — neuron extension that receives signals.
- Axon — neuron extension that transmits signals.
- Myelin Sheath — insulating covering that increases impulse speed.
- Schwann Cell — glial cell forming myelin in the PNS.
- Node of Ranvier — gap between Schwann cells on an axon.
- Action Potential — electrical signal traveling down a neuron.
- Synapse — junction between two neurons.
- Resting Membrane Potential — voltage difference across a neuron's membrane at rest.
- Central Nervous System (CNS) — brain and spinal cord.
- Peripheral Nervous System (PNS) — nerves outside the CNS.
Action Items / Next Steps
- Review details of each nervous system tier, starting with the central nervous system.