AFFERENT NERVOUS SYSTEM

AFFERENT NERVOUS SYSTEM

Overview

  • This section outlines the organization and functionality of the afferent nervous system as per Sherwood Chapter 6.

LECTURE OBJECTIVES

  • Understand the organization of the nervous system.

  • Identify the different types of neurons within the nervous system.

  • Distinguish between afferent neurons, efferent neurons, and interneurons.

  • Explain the functioning of sensory receptors located on afferent neurons.

  • Describe the two major kinds of sensory receptors.

  • Discuss how stimuli are transmitted from the peripheral nervous system (PNS) to the central nervous system (CNS).

ORGANIZATION OF THE NERVOUS SYSTEM

The two major kinds of sensory receptors are classified as either mechanoreceptors, which respond to mechanical pressure or distortion, and chemoreceptors, which react to chemical stimuli. Mechanoreceptors play a crucial role in our sense of touch, hearing, and balance, while Chemoreceptors are essential for taste and smell.
When sensory stimuli are detected by these receptors, they generate electrical signals that are transmitted via afferent neurons in the peripheral nervous system (PNS) to the central nervous system (CNS), where they are processed and interpreted. This transmission process involves several steps, including the conversion of the physical stimulus into an action potential, which travels along the axon of the afferent neuron and ultimately reaches the spinal cord and brain for further processing.

  • Input to CNS from periphery (AFFERENT): Represents sensory data that enters the central nervous system (CNS). This data is crucial for enabling the CNS to respond appropriately to environmental changes and is essential for coordinating motor functions and reflexes.

  • Central Nervous System (CNS): Comprises the brain and spinal cord.

  • Output from CNS to periphery (EFFERENT): Represents motor commands that leave the CNS to enact responses.The response after processing what the CNS receives from the afferent nervous system involves activating various effector organs, helping to facilitate appropriate reactions such as muscle movement or glandular secretion.

  • This process is important for homeostasis.

  • Afferent Division:

    • Responsible for sensory stimuli (input) to the CNS. Taking messages from the PNS to the CNS

    • Includes visceral stimuli from internal organs. Examples, hunger, thirst, oxygen, co2

    • They generate action potential at sensory receptors , which then travel along afferent fibers to the central nervous system for processing and interpretation.

  • Efferent Division:

    • Responsible for sending motor commands to effector organs (muscles and glands).

    • Comprises somatic nervous system (voluntary control), the motor neurons eg skeletal muscle and autonomic nervous system (involuntary control), sympathetic and parasympathetic. Cardiac muscles, smooth muscles etc.

    • The autonomic system further divides into sympathetic and parasympathetic divisions, as well as the enteric nervous system for digestive organs.

TYPES OF NEURONS

Neuron Classifications

Afferent Neuron

  • Function: Sends signals towards the CNS from the PNS, allowing the brain to receive sensory information from the external environment as well as from internal bodily processes.

  • Action Potential Generation: Generates action potentials from sensory receptors.

  • Axon Structure: Features long axons within the PNS. The cell body of the afferent neuron has no dendrites because the neuron has a sensory receptor. The cell body, sensory receptor and central axon are all in the PNS whereas the AXON TERMINALS are in the CNS.

Efferent Neuron

  • Function: Sends signals away from the CNS to effector organs (muscles/glands).

  • Axon Structure: Also has long axons within the PNS. The cell body of this neuron has dendrites, they Axon terminals are in the PNS while the cell body is in the CNS receiving information from the INTERNEURON that has been sent by the afferent neuron and sending it to its direct target.

Interneurons

  • Location: Stays or found entirely within the CNS.

  • Role: Acts as a relay between afferent and efferent neurons. It is the middle man which receives and sends the message being sent from afferent neurons to the efferent neuron.

AFFERENT NEURON: SENSORY RECEPTORS

Types of Sensory Receptors

  • Photoreceptors: Sensitive to light. Found in eyes

  • Mechanoreceptors: Detect mechanical changes (pressure, vibration, and stretch). Found on skin

  • Thermoreceptors: Respond to temperature changes. Found on skin

  • Nociceptors: Detect pain and harmful stimuli. Found on skin

  • Osmoreceptors: Respond to changes in osmotic pressure. Found in kidneys

  • Chemoreceptors: Sensitive to chemical stimuli. Found in nose, taste buds etc.

  • General Function: All sensory receptors are sensitive to different forms of energy and respond to stimuli by generating graded potentials.

MECHANISM OF SENSORY RECEPTION

Dual Mechanism of Receptor Potential Generation

  1. In Specialized Afferent Ending:

    • The stimulus opens stimulus-sensitive nonspecific cation channels, allowing Sodium (Na+) entry, thus generating a receptor potential.

    • This is when the stimulus sent opens up and Sodium ions which are found on the outside rush in to the intracellular space and generates concentration gradient which leads to DEPOLARIZATION. This then opens a Voltage-gated sodium channel, leading to an increase in sodium getting into the cell, further depolarizing the cell membrane and initiating an ACTION POTENTIAL.

    • DEPOLARIZATION MEANS BECOMING LESS NEGATIVE, the inside of the cell is negative and sodium is positive so more sodium going in less negative it becomes.

    • Local current flow from depolarized region to adjacent areas opens voltage-gated Na+ channels, initiating an action potential.

  2. In Separate Receptor Cell:

    • The stimulus similarly opens channels, leading to Na+ entry and receptor potential. Then the local depolarization that happens due to the influx of Na+ leads to the opening of a Voltage-gated Calcium Channels. Ca2+ then rushes into the cell, further contributing to the depolarization.

    • Local depolarization subsequently opens voltage-gated Ca2+ channels.

    • Ca2+ the entry of the calcium then causes exocytosis of neurotransmitters, which bind to chemically gated receptor-channels at the afferent ending, allowing Na+ entry and depolarization that leads to an action potential.

  3. THE SPECIALIZED AFFERENT ENDING IS FASTER THAN THE SEPARATE RECEPTOR CELL!!!

SENSORY RECEPTORS: ADAPTATION

Types of Adaptation

  • Phasic Adaptation: This type occurs rapidly/quickly, resulting in the sensory response to a stimulus diminishing/going down over time, allowing the organism to become less sensitive to constant stimulation. Such as when you wear your glasses and feel it when you put it on but then your brain forgets that it is there and once you take it off you feel stimulated again. This is why you are not constantly aware of your clothes, watch, glasses etc.

  • Tonic Adaptation: This type occurs slowly, where the sensory receptors continue to send signals to the nervous system, ensuring that the organism remains aware of an ongoing stimulus. We do not adapt because we do not want to stop sending message.

  • Phasic (Phase):

    • Adapts rapidly to stimuli, but generates an “off” response when the stimulus is removed.

    • Responses depend on changes in stimulus intensity.

  • Tonic (Tone):

    • Do not adapt or adapt slowly, providing continuous information about the stimulus.

    • Maintained information is crucial for ongoing awareness of a stimulus.

TRANSMISSION OF STIMULUS

Pathway of Sensory Signals

  • Afferent neurons transmit sensory information from peripheral receptors to the CNS.

  • Signals communicate with reflex networks in the spinal cord and travel to the thalamus.

  • The thalamus acts as a relay station to transmit information to the cerebral cortex for further processing.

  • Sensory Units and Ascending Pathways: Information travels in labeled lines which ensure specificity in sensory perception

  • Information travels in a certain order from the pns to the cns. The information goes from the afferent neurons peripheral receptors to the CNS. The THALAMUS receives this message.

  • First order - SENSORY UNITS. They transmit the sensory information from the periphery to the CNS. FROM RECEPTOR TO CNS

  • Second order- ASCENDING PATHWAYS. These are reflex integrated at this point. They transfer the information from the Spinal Cord to the thalamus.

  • Third order - PROCESSING CENTER. This is when the information is sent or relayed from the Thalamus to Cerebral Cortex. This allows for conscious perception and interpretation of sensory information, enabling appropriate responses to stimuli.

TYPES OF NERVE FIBERS

Classification of Nerve Fibers
Three Main Types

  • Type A Fibers:

    • Myelinated and transmit sensations of cutaneous pressure, touch, cold sensation, mechanical pain, and heat pain.

    • Subtypes include At{α}, A{β}, and A{δ}. Different in sizes hence facilitates how fast things are transmitted. FAST MESSENGERS.

  • Type B Fibers:

    • Myelinated fibers associated with cutaneous and subcutaneous mechanoreceptors.

  • Type C Fibers:

    • Unmyelinated fibers transmitting warmth, hot sensations, and pain induced by mechanical, chemical, and temperature changes.

  • SLOW MESSENGER DUE TO BEING UNMYELINATED

SUMMARY

Neuronal Structure Overview

  • Peripheral Nervous System (PNS):

    • Comprises afferent neurons with central and peripheral axons, and sensory receptors.

  • Efferent Neurons:

    • Communicate with effector organs (muscles or glands).

    • Efferent autonomic nerve pathways consist of a two-neuron chain between the CNS and the effector organ.

  • Overall Neuronal Circuitry:

    • Afferent neurons initiate sensory input, interneurons process that input, and efferent neurons execute responses.