35.4 The Peripheral Nervous System - Biology 2e _ OpenStax

Learning Objectives

  • By the end of this section, you will be able to:

    • Describe the organization and functions of the sympathetic and parasympathetic nervous systems.

    • Describe the organization and function of the sensory-somatic nervous system.

Overview of the Nervous System

  • Peripheral Nervous System (PNS):

    • Connects the central nervous system (CNS) to the rest of the body.

    • CNS: Acts like a power plant, producing signals to control bodily functions.

    • PNS: Functions like wiring, transferring signals between the CNS and body.

Components of the PNS

  • Autonomic Nervous System:

    • Operates without conscious control.

    • Divided into:

      • Sympathetic Nervous System

      • Parasympathetic Nervous System

  • Sensory-Somatic Nervous System:

    • Transmits sensory information to the CNS and motor commands to the muscles.

Autonomic Nervous System

  • Structure:

    • Involves synapses between

      • Preganglionic neuron (CNS)

      • Postganglionic neuron (PNS)

    • Controls the lungs, heart, smooth muscles, and glands without conscious awareness

  • Functionality:

    • Maintains homeostasis by continuously monitoring and adjusting the body's internal conditions.

Sympathetic Nervous System

  • Role: Mediates the "fight or flight" response

    • Functions include:

      • Accelerated heart rate

      • Inhibition of digestion

    • Mechanism:

      • Preganglionic neurons originate in the spinal cord and release acetylcholine, activating postganglionic neurons.

      • Postganglionic neurons release norepinephrine on target organs.

  • Effects:

    • Increases heart rate, dilates bronchi, inhibits digestion, increases metabolism for energy.

Parasympathetic Nervous System

  • Role: Mediates the "rest and digest" response

    • Functions include:

      • Slowing heart rate

      • Stimulating digestion

    • Mechanism:

      • Preganglionic neurons are located near target organs and mainly release acetylcholine to activate them.

    • Helps reset the body after a sympathetic response by lowering heart rate and blood pressure.

Sensory-Somatic Nervous System

  • Components: Made up of cranial and spinal nerves

    • Contains sensory and motor neurons.

    • Responsibilities:

      • Sensory neurons transmit information from skin, muscles, and sensory organs to the CNS.

      • Motor neurons convey commands from the CNS to the muscles

  • Mechanism:

    • Sensory and motor neurons operate through a single synapse connecting directly to the CNS.

Cranial and Spinal Nerves

  • Cranial Nerves:

    • Humans have 12 cranial nerves that carry sensory and motor information, particularly for the head and neck.

    • Examples include:

      • Olfactory Nerve: Transmits smell.

      • Oculomotor Nerve: Controls eye movement.

      • Glossopharyngeal Nerve: Involved in taste and swallowing.

  • Spinal Nerves:

    • There are 31 spinal nerves in humans that transmit sensory and motor information; each has sensory neuron cell bodies in dorsal root ganglia.

Summary of Functionality

  • Sensory Neurons: Receive information from sensory receptors and transmit it to the CNS.

  • Motor Neurons: Relay commands from the CNS to skeletal muscles.

  • Integration: The combined action of the autonomic nervous system and sensory-somatic nervous system enables the body to respond effectively to internal and external stimuli.

Key Terms and Definitions

  1. Peripheral Nervous System (PNS): Connects the central nervous system (CNS) to the rest of the body; functions like wiring, transferring signals between the CNS and body.

  2. Central Nervous System (CNS): Acts like a power plant, producing signals to control bodily functions.

  3. Autonomic Nervous System: Operates without conscious control; regulates involuntary bodily functions.

    • Sympathetic Nervous System: Mediates the "fight or flight" response; accelerates heart rate, inhibits digestion.

    • Parasympathetic Nervous System: Mediates the "rest and digest" response; slows heart rate and stimulates digestion.

  4. Sensory-Somatic Nervous System: Transmits sensory information to the CNS and motor commands to the muscles; made up of cranial and spinal nerves.

  5. Preganglionic Neuron: Neuron originating in the CNS that synapses with a postganglionic neuron in the PNS.

  6. Postganglionic Neuron: Neuron in the PNS that receives signals from a preganglionic neuron.

  7. Homeostasis: The body's ability to maintain stable internal conditions.

  8. Acetylcholine: A neurotransmitter released by neurons in both the sympathetic and parasympathetic systems.

  9. Norepinephrine: A neurotransmitter released by postganglionic neurons in the sympathetic nervous system that affects target organs.

  10. Cranial Nerves: 12 nerves that carry sensory and motor information for the head and neck; examples include Olfactory, Oculomotor, and Glossopharyngeal nerves.

  11. Spinal Nerves: 31 nerves that transmit sensory and motor information; each has sensory neuron cell bodies in dorsal root ganglia.

  12. Sensory Neurons: Neurons that transmit sensory information from receptors to the CNS.

  13. Motor Neurons: Neurons that relay commands from the CNS to skeletal muscles.

Learning Objectives By the end of this section, you will be able to:

  • Describe the organization and functions of the sympathetic and parasympathetic nervous systems.

  • Describe the organization and function of the sensory-somatic nervous system.

Overview of the Nervous System

The nervous system is composed of two main components: the central nervous system (CNS) and the peripheral nervous system (PNS). While the CNS serves as the control center for processing information and coordinating bodily functions, the PNS acts as the communication relay between the CNS and the rest of the body.

Peripheral Nervous System (PNS):

  • Serves to connect the CNS to limbs and organs, thus enabling the brain and spinal cord to communicate with the rest of the body.

  • The PNS is analogous to wiring in a system, facilitating the transfer of signals, while the CNS can be likened to a power plant, generating the commands necessary for bodily functions.

Components of the PNS

The PNS is divided into two principal divisions:

  1. Autonomic Nervous System (ANS):

    • Operates without voluntary control, regulating involuntary bodily functions essential for survival.

    • Divided into:

      • Sympathetic Nervous System: Primarily responsible for the body's "fight or flight" response under stress.

      • Parasympathetic Nervous System: Engages in the "rest and digest" response, promoting relaxation and normal bodily function.

  2. Sensory-Somatic Nervous System:

    • Transmits sensory information from the body to the CNS and motor commands from the CNS to the skeletal muscles, facilitating voluntary movement and perception of the environment.

Autonomic Nervous System

Structure:

  • Comprised of two neuron types involved in the transmission of signals, with synapses occurring between:

    • Preganglionic Neuron: Originates in the CNS and releases neurotransmitters such as acetylcholine to activate postganglionic neurons.

    • Postganglionic Neuron: Resides in the PNS and transmits signals to target organs.

  • The ANS regulates the function of involuntary muscle and glands, including the heart, lungs, smooth muscles, and various glands.

Functionality:

  • Maintains homeostasis, the body's ability to maintain stable internal conditions, through continuous monitoring and adjustment of physiological parameters.

Sympathetic Nervous System

Role:

  • Prepares the body to respond to perceived threats or emergencies by invoking the "fight or flight" mechanism.

Functions Include:

  • Accelerated heart rate, increased blood flow to muscles, dilation of bronchial passages for enhanced oxygen intake, and inhibition of digestive processes to conserve energy for critical functions.

Mechanism:

  • Preganglionic neurons originate in the thoracic and lumbar regions of the spinal cord and predominantly release acetylcholine. This neurotransmitter stimulates postganglionic neurons to release norepinephrine, influencing target organs' responses.

Effects:

  • Physiological changes encompass increased heart rate, bronchodilation (enlargement of airways), inhibition of gastrointestinal activity, and the mobilization of energy stores for quick response.

Parasympathetic Nervous System

Role:

  • Mediates recovery and maintenance processes through the "rest and digest" response, restoring energy and promoting digestion.

Functions Include:

  • Slowing down the heart rate, increasing digestive secretions, and promoting nutrient absorption following meals.

Mechanism:

  • Preganglionic neurons are situated closer to the target organs, primarily releasing acetylcholine, facilitating increased activity in these organs. This action helps counterbalance the sympathetic responses and re-establish homeostasis.

Sensory-Somatic Nervous System

Components:

  • Composed of cranial and spinal nerves, integrating sensory input and motor commands.

Responsibilities:

  • Sensory Neurons: Transmit sensory data from skin, muscles, and sensory organs to the CNS for processing.

  • Motor Neurons: Carry motor commands from the CNS to skeletal muscles for movement.

Mechanism:

  • Operates through a direct synapse, whereby sensory and motor neurons connect directly to the CNS, facilitating quick reflex actions and responses.

Cranial and Spinal Nerves

  • Cranial Nerves: Humans possess 12 pairs of cranial nerves responsible for sensory and motor supply primarily to the head and neck.

    • Examples Include:

      • Olfactory Nerve: Involved in the sense of smell.

      • Oculomotor Nerve: Responsible for eye movement, pupil dilation, and lens shape adjustment.

      • Glossopharyngeal Nerve: Plays a role in taste sensation and swallowing.

  • Spinal Nerves: Comprising 31 pairs, these nerves are essential for transmitting sensory and motor information to and from the body. Each spinal nerve has sensory neuron cell bodies located in dorsal root ganglia.

Summary of Functionality

  • Sensory Neurons: Responsible for collecting information from various sensory receptors in the skin and muscles, transmitting this data to the CNS for interpretation.

  • Motor Neurons: Serve to relay CNS commands, stimulating skeletal muscles to produce movement.

  • Integration: The synergistic operation of the autonomic and sensory-somatic nervous systems enables the human body to respond adeptly to both internal needs and external challenges, ensuring survival and promoting health.