Nervous System II: Peripheral Nervous System

Organization of the Nervous System

  • The nervous system consists of neural tissue, which is composed of specialized cells that facilitate communication.
  • The nervous system is organized into two main divisions:
    • Central Nervous System (CNS): brain and spinal cord.
    • Peripheral Nervous System (PNS): nerve cells and ganglia.
  • A ganglion is a cluster of nerve cell bodies located outside the central nervous system.
  • The CNS integrates and coordinates both voluntary and involuntary functions.
  • The PNS connects the body to the CNS.

Peripheral Nervous System (PNS) Divisions

  • The PNS has two main divisions:
    • Somatic Nervous System: controls voluntary movement and sensation.
    • Autonomic Nervous System: regulates involuntary activities to maintain a stable internal environment.
  • The autonomic nervous system is further divided into:
    • Sympathetic Nervous System: active during stressful situations (fight-or-flight).
    • Parasympathetic Nervous System: active during restful conditions to conserve energy (rest-and-digest).

Peripheral Nervous System Functions

Somatic Nervous System

  • The somatic nervous system deals with voluntary actions, serving two main functions:
    • Input: Carries sensory signals from the body and external world to the CNS, resulting in conscious sensation. It can also receive sensory input regarding pain from internal body organs.
    • Output: Carries motor signals from the brain, dictating the desired response to the appropriate location.
  • Examples of functions include:
    • Receiving and delivering sensory input like touch, taste, and sound (e.g., tasting a veggie burger, seeing a cute dog).
    • Controlling voluntary movement (e.g., picking up the veggie burger, petting the cute dog).

Autonomic Nervous System

  • The autonomic nervous system deals with involuntary actions, serving two main functions:
    • Input: Receives information from receptors within the body.
    • Output: Carries motor signals that control autonomic activities to the appropriate organ.
  • The two subdivisions of the autonomic nervous system are:
    • Sympathetic: Fight or Flight.
    • Parasympathetic: Rest and Digest.
  • The autonomic nervous system helps maintain homeostasis in our bodies and carries out certain reflexes.
    • Examples include speeding up or slowing down heart and breathing rates and increasing or decreasing digestive functions.

Sympathetic Nervous System

  • Responds to stress or fear.
  • When the body is under stress, such as seeing a bear on a hike, some responses include:
    • Increasing blood pressure and heart rate to deliver more blood to muscles.
    • Decreasing digestive activity to conserve energy.
    • Dilating pupils to let in more light to see better.
    • Stimulating adrenal glands to secrete hormones that prolong other sympathetic responses.

Parasympathetic Nervous System

  • Works when there is no threat and the body can relax.
  • For example, after avoiding the bear, the parasympathetic nervous system will:
    • Decrease blood pressure and heart rate as the body no longer needs as much oxygen.
    • Increase digestive activity.
    • Constrict pupils.

Peripheral Nervous System Structures

  • The PNS is composed of nerves (clusters of neurons) that extend out from the brain and the spinal cord.
    • Spinal nerves carry signals to and from the spinal cord.
    • Cranial nerves carry signals to and from the brain.
  • These nerves serve as communication lines (sensory and motor) and link every part of the body to the CNS.

Cranial Nerves

  • There are 12 pairs of cranial nerves.
  • They primarily serve the head and neck, with the exception of the vagus nerve, which extends into the thoracic and abdominal cavities.
  • Some are mixed nerves, some are sensory nerves, and some are motor nerves.

Spinal Nerves

  • There are 31 pairs of spinal nerves.
  • Spinal nerves are named according to where they emerge from:
    • Cervical
    • Thoracic
    • Lumbar
    • Sacral
    • Coccygeal
  • The spinal nerves are mixed nerves, containing nerves that are part of the afferent and efferent pathways.
  • They are formed by the combination of the ventral and dorsal roots of the spinal nerves.
  • The dorsal horns contain sensory neurons entering from the dorsal root. The cell bodies are located inside the dorsal root ganglion.
  • The ventral horns contain cell bodies for motor neurons of the somatic nervous system. These cells send out their axons through the ventral root of the spinal cord.
  • The dorsal and ventral roots fuse to form the spinal nerves.
  • Recall:
    • Grey matter serves as a reflex centre.
    • White matter transmits messages to and from the CNS.

PNS & CNS Combined: Somatosensory Pathway

  • 3 neuron chain:
    1. Sensory neuron from receptor cell to spinal cord
    2. Interneuron goes up spinal cord to thalamus
    3. Finally, the signal is relayed by a neuron from the thalamus to somatosensory cortex
  • After a signal is transmitted to the brain, there is often a motor output that follows.

PNS & CNS Combined: Reflex Types

  • Somatic reflexes:
    • Reflexes that stimulate the skeletal muscles.
    • Involuntary, although skeletal muscle is normally under voluntary control.
    • Example: pulling your hand away from a hot object.
    • Incorporates spinal cord and PNS.
  • Autonomic reflexes:
    • Regulate the activity of smooth muscles, the heart, and glands in response to external stimuli.
    • All involuntary reflexes.
    • Example: regulation of smooth muscles, heart and blood pressure, glands, digestive system.
    • Incorporates brainstem, spinal cord, and PNS.

PNS & CNS Combined: Pathway of a Somatic Reflex Arc

  • Reflexes occur over pathways called reflex arcs, which are composed of five elements:
    1. Sensory receptor: Receives a stimulus.
    2. Sensory neuron: Carries message from receptor to the integration center.
    3. Integration center (CNS): Grey matter in spinal cord processes information and directs motor output.
    4. Motor neuron: Carries message to an effector.
    5. Effector organ: Is the muscle or gland to be stimulated.
  • Spinal reflexes are immediate, involuntary reactions brought on by the spinal cord that are advantageous for a person’s safety.

Reflex arc pathway:

1. Stimulus causes a painful sensation picked up by sensory receptor.
2. Sensory neuron carries information to spinal cord.
3. Interneurons in spinal cord integrate information and send information to motor neurons.
4. Motor neuron stimulates appropriate muscle.
5. Muscles contract, removing appendage or limb from stimulus.
  • Example: flexor (withdrawal) reflex. The pathway includes:
    1. Sensory receptor
    2. Sensory (afferent) neuron
    3. Interneuron
    4. Motor (efferent) neuron
    5. Effector organ