Biopsychology

Overview of Biopsychology: Neurons and Synaptic Transmission

Introduction to Neurons

  • Specification Highlights:

    • Study of the structure and function of sensory, relay, and motor neurons.
    • Exploration of synaptic transmission, including neurotransmitters, excitation, and inhibition.

  • Nervous System Signals:

    • The nervous system communicates through billions of nerve cells known as neurons.
    • Neurons transmit signals through electrical and chemical messages within the body and the brain.

Key Terms in Neurons

  • Neuron:

    • The basic building blocks of the nervous system, neurons are nerve cells that process and transmit messages via electrical and chemical signals.

  • Sensory Neurons:

    • Carry messages from the Peripheral Nervous System (PNS) to the Central Nervous System (CNS).
    • Characterized by long dendrites and short axons.

  • Relay Neurons:

    • Connect sensory neurons to motor neurons or to other relay neurons.
    • Typically have short dendrites and short axons.

  • Motor Neurons:

    • Connect the CNS to effectors such as muscles and glands.
    • They feature short dendrites and long axons.

Structure and Function of Neurons

  • Neurons in the Human Nervous System:
    • Total of approximately 200 billion neurons, with 80% located in the brain.
    • Neurons provide primary communication means for the nervous system through electrical and chemical signals.
Types of Neurons
  1. Sensory Neurons
    • Transmit signals from sensory receptors to the CNS.
  2. Relay Neurons
    • Transmit signals between sensory and motor neurons within the CNS.
  3. Motor Neurons
    • Transmit signals from the CNS to effectors.
Structure of a Neuron
  • Neurons vary from under a millimetre to about a metre long but maintain a consistent basic structure.
  • Key Components:
    • Cell Body (Soma): Contains the nucleus with the cell’s genetic material.
    • Dendrites: Branchlike structures that carry nerve impulses from neighboring neurons towards the cell body.
    • Axon: Carries impulses away from the cell body.
    • Myelin Sheath: A fatty layer protecting the axon and accelerating electrical impulse transmission, segmented by gaps called nodes of Ranvier that help maintain high transmission speed.
    • Terminal Buttons: End of the axon communicates with the next neuron across the synapse.
Location of Neurons
  • Motor Neurons: Cell bodies found in the CNS; long axons extending into the PNS.
  • Sensory Neurons: Positioned in the PNS, often in clusters called ganglia.
  • Relay Neurons: Comprise 97% of all neurons, predominantly in the brain and visual system.

Electrical Transmission: Firing of a Neuron

  • At rest, the interior of the neuron is negatively charged relative to the outside.
  • Upon activation by a stimulus, a rapid influx of positive charge occurs, leading to an action potential and creating an electrical impulse that travels down the axon.

Synaptic Transmission

Chemical Transmission
  • Neural Networks: Neurons communicate as part of networks, separated by a tiny gap known as the synapse.
  • Signal Transmission:
    • Electrical impulses travel within neurons, but chemical messages transmit between them via neurotransmitters.
    • When an electrical impulse reaches the presynaptic terminal, it prompts neurotransmitter release from synaptic vesicles.
Neurotransmitters
  • Definition: Chemical messengers that diffuse across the synapse to the subsequent neuron.
  • Upon crossing the synapse, neurotransmitters are received by postsynaptic receptor sites on dendrites.
  • This triggers a conversion back into an electrical impulse, restarting the communication cycle.
  • Directionality: Signal transmission is unidirectional; neurotransmitters are released from the presynaptic neuron's terminal and picked up by the postsynaptic neuron.
Types of Neurotransmitters
  • Identification: Several dozen types identified with specific molecular structures that fit receptor sites like lock and key.
  • Functions of Neurotransmitters:
    • Example: Acetylcholine (ACh) facilitates muscle contraction at the neuromuscular junction.
Effects of Neurotransmitters
  • Excitatory Effect: Increases the positive charge in postsynaptic neurons, making them more likely to fire.
    • Example: Adrenaline, which is part of the stress response.
  • Inhibitory Effect: Increases the negative charge in postsynaptic neurons, reducing their likelihood of firing.
    • Example: Serotonin, which helps stabilize mood and is involved in depression.
Summation in Neural Communication

  • Determining Action: The firing of a postsynaptic neuron is influenced by two types of signals: excitatory and inhibitory.

  • Summation Process:

    • If the net effect is inhibitory, the neuron is less likely to fire; if excitatory, it is more likely to fire, momentarily raising its positive charge and triggering an impulse.

  • Threshold Requirement: The action potential in a postsynaptic neuron is only initiated if the summation of signals reaches a particular threshold.

Applications in Biopsychology

  • Psychoactive Drugs: Improved understanding of neurotransmitter modes has informed the development of treatments for mental disorders.
    • Example: Depression often linked to low serotonin levels; some drugs, like SSRIs (Selective Serotonin Reuptake Inhibitors), such as Prozac, inhibit the reuptake of serotonin, prolonging its action in the synapse.

Key Concepts Articulated

  • Synaptic Transmission: The process by which neurons communicate through chemical messages across the synapse.

  • Neurotransmitter: Brain chemicals released from presynaptic vesicles, relaying signals across the synapse.

  • Excitation: An increase in the positive charge of the postsynaptic neuron, which raises the likelihood of impulse transmission.

  • Inhibition: An increase in the negative charge of the postsynaptic neuron, which diminishes the likelihood of impulse transmission.

Concept Application: The Reflex Arc

  • Example of Reflex Arc:

    • Stimulus: Hammer strikes knee, detected by sensory organs.
    • Pathway: Message is sent via a sensory neuron, reaches the CNS (spinal cord), connects with a relay neuron, and transfers to a motor neuron.
    • Result: Signal carried to an effector (muscle), causing a contraction and movement at the knee.

  • Key Terms:

    • Effector: The muscle or gland performing the action.
    • Sensory Neuron: Conveying messages from sensory receptors to the CNS.
    • Motor Neuron: Carrying commands from the CNS to muscles/glands.
    • CNS: Central Nervous System, processing the information.
    • PNS: Peripheral Nervous System, conducting signals outside the CNS.
    • Relay Neuron: Connecting sensory and motor neurons within the CNS.