Overview
This lecture covers the structure and function of nerves (neurons), the transmission of nerve impulses, types of neurons, the reflex arc, and the role of synapses.
Structure and Types of Neurons
- The human body has about seven trillion nerve cells called neurons.
- The longest neuron is the sciatic nerve, running from the lower back to the heel.
- Neurons communicate through electrochemical messages: electrical impulses and neurotransmitters.
- Sensory neurons (afferent) carry impulses from receptors to the central nervous system (CNS).
- Motor neurons (efferent) carry impulses from the CNS to effectors (muscles/glands).
- Sensory neurons are unipolar (one outgrowth); motor neurons are multipolar (multiple dendrites).
- Main neuron structures: dendrites (receive impulses), cell body (contains nucleus, controls functions), axon (carries impulses away), myelin sheath (insulates axon, speeds up transmission).
Transmission of Nerve Impulses
- Impulses enter neurons through dendrites, pass through the cell body, travel along the axon, and exit via terminal branches.
- Neurons are separated by synaptic gaps (synapses); impulses cross these using neurotransmitters.
The Reflex Arc and Reflex Actions
- A reflex arc is the path an impulse travels from receptor to effector to produce a reflex action.
- A reflex action is a rapid, automatic, protective response to a stimulus (e.g., touching something hot).
- Reflex actions minimize further injury by bypassing the brain and utilizing the spinal cord.
- Pathway: receptor → sensory neuron (via dorsal root) → interneuron (in spinal cord) → motor neuron (via ventral root) → effector (muscle/gland).
- Interneurons serve as bridges for impulse transfer in the spinal cord.
Synapses and Neurotransmitters
- A synapse is a small gap between neurons; impulses travel across via neurotransmitters (e.g., dopamine, serotonin).
- Synapses allow one-way transmission, joining/splitting of nerves, and prevent stimulus overload.
- Enzymes inactivate neurotransmitters after use, preventing reverse signaling; dysfunction can cause disorders like Parkinson's.
Key Terms & Definitions
- Neuron — A nerve cell that transmits impulses.
- Sensory Neuron (Afferent) — Carries impulses to the CNS from receptors.
- Motor Neuron (Efferent) — Carries impulses from the CNS to effectors.
- Effector — Organ or cell that acts in response to a stimulus.
- Dendrite — Branch-like structure receiving impulses.
- Axon — Long fiber transmitting impulses away from the cell body.
- Myelin Sheath — Fatty layer insulating the axon and speeding up impulse transmission.
- Reflex Arc — Pathway followed by a nerve impulse in a reflex action.
- Reflex Action — Rapid, automatic response to a stimulus.
- Synapse — Gap between neurons where impulses transfer via neurotransmitters.
- Interneuron — Neuron connecting sensory and motor neurons within the CNS.
- Neurotransmitter — Chemical messenger transmitting signals across synapses.
Action Items / Next Steps
- Review the neuron structures and their functions.
- Study the pathway and explanation of the reflex arc.
- Learn the definitions of all key terms.
- Prepare for questions on neuron structure, synapse function, and reflex actions in exams.