neurons

Neurons and Synaptic Transmission

Specification Overview

  • Focus on the structure and function of neurons:
    • Sensory neurons
    • Relay neurons
    • Motor neurons
  • Explore the process of synaptic transmission, including:
    • Neurotransmitters
    • Excitation
    • Inhibition

Introduction to Neurons

  • The nervous system transmits signals via billions of nerve cells (neurons).
  • Neurons communicate through electrical and chemical messages within the body and the brain.

Key Terms Definition

  • Neuron: The basic building blocks of the nervous system, neurons are nerve cells that process and transmit messages through electrical and chemical signals.
  • Sensory neurons: Carry messages from the Peripheral Nervous System (PNS) to the Central Nervous System (CNS).
    • Characteristic: Long dendrites and short axons.
  • Relay neurons: Connect sensory neurons to motor or other relay neurons.
    • Characteristic: Short dendrites and short axons.
  • Motor neurons: Connect the CNS to effectors, such as muscles and glands.
    • Characteristic: Short dendrites and long axons.

Types of Neurons

  • Total Neurons in Human Nervous System: 100 billion, with 80% located in the brain.
  • Function: Transmit signals electrically and chemically.
  • Three main types of neurons:
    • Sensory Neurons
    • Relay Neurons
    • Motor Neurons

Structure of a Neuron

  • Neurons vary in size from less than 1 mm to up to 1 m long but share a common structure:
    • Cell Body (Soma): Contains the nucleus with genetic material.
    • Dendrites: Branch-like structures carrying nerve impulses from neighboring neurons toward the cell body.
    • Axon: Carries impulses away from the cell body down the length of the neuron, covered by the myelin sheath.
    • The myelin sheath protects the axon and speeds up electrical transmission of the impulse.
    • Segmented by gaps called nodes of Ranvier; these gaps allow impulses to jump, enhancing speed of transmission.
    • Terminal Buttons: Communicate with the next neuron through the synapse.
Location of Neurons
  • Motor Neurons: Cell bodies in the CNS with long axons forming part of the PNS.
  • Sensory Neurons: Located outside the CNS, clustered in ganglia within the PNS.
  • Relay Neurons: Comprise around 97% of all neurons, primarily found in the brain and visual system.

Electrical Transmission - The Firing of a Neuron

  • Resting State: Inside of the neuron is negatively charged.
  • When activated by a stimulus:
    • Inside becomes positively charged, generating an action potential.
    • Creates an electrical impulse traveling down the axon.

Synaptic Transmission

Overview
  • Neurons communicate within groups called neural networks.
  • Neurons are separated by an extremely tiny gap termed the synapse.
Electrical vs. Chemical Transmission
  • Signals within neurons are transmitted electrically, while signals between neurons are transmitted chemically across the synapse.
  • When the electrical impulse reaches the presynaptic terminal:
    • Triggers the release of neurotransmitters from synaptic vesicles.
Neurotransmitters
  • Definition: Chemicals that diffuse across the synapse to the next neuron.
  • Upon crossing the gap, neurotransmitters bind to postsynaptic receptor sites on the dendrites of the next neuron.
    • Converts the chemical message back into an electrical impulse, restarting the transmission process.
  • One-way Directionality: Neurotransmitters are released from the presynaptic neuron and received only by the postsynaptic neuron.
Types of Neurotransmitters
  • Many types of neurotransmitters have been identified, each with specific functions:
    • Acetylcholine (ACh): Found at points where motor neurons meet muscles, causing muscle contraction upon release.

Excitation and Inhibition

  • Neurotransmitters can have either an excitatory or inhibitory effect:
    • Excitation: Example - Adrenaline increases the positive charge in the postsynaptic neuron, making it more likely to fire.
    • Inhibition: Example - Serotonin increases the negative charge in the receiving neuron, making it less likely to fire.
Summation
  • The decision for a postsynaptic neuron to fire is based on summation of excitatory and inhibitory influences:
    • If the net effect is inhibitory, the postsynaptic neuron is less likely to fire.
    • If the net effect is excitatory, it is more likely to fire.
    • The action potential is triggered only if the excitatory signals exceed a certain threshold.

Practical Applications

Psychoactive Drugs
  • Increased understanding of neurotransmitter action has led to the development of psychoactive drugs for treating mental disorders.
  • Example: SSRIs (Selective Serotonin Reuptake Inhibitors) like Prozac slow the reuptake of serotonin, improving mood stability by ensuring serotonin remains active longer in the synapse.

Key Terms Recap

  • Synaptic Transmission: Process through which neighboring neurons communicate by sending chemical messages across the synaptic gap.
  • Neurotransmitter: Brain chemicals released from synaptic vesicles that relay signals across the synapse from one neuron to another.
    • Divided into excitatory and inhibitory functions.
  • Excitation: When a neurotransmitter, such as adrenaline, increases the positive charge of the postsynaptic neuron, enhancing the likelihood of electrical impulse passage.
  • Inhibition: When a neurotransmitter, such as serotonin, increases the negative charge of the postsynaptic neuron, reducing the likelihood of electrical impulse passage.

Example of a Reflex Arc

  • Knee-jerk reflex: Example of a reflex arc:
    • A stimulus, such as a hammer, hits the knee.
    • Detected by sense organs in the PNS.
    • Signals conveyed along a sensory neuron.
    • Message reaches the CNS, connecting with a relay neuron.
    • Relay neuron transfers the message to a motor neuron.
    • Motor neuron carries the message to an effector (e.g., muscle), causing it to contract and knee to jerk.

Review Questions

  1. Explain the process of synaptic transmission.
  2. With reference to neurotransmitters, explain what is meant by both 'excitation' and 'inhibition'.
  3. Outline one difference between a sensory neuron and a relay neuron.