Peripheral Nervous System: Structure, Function, and Integration (BIO 168 Chapter 13 Study Guide)

Peripheral Nervous System (PNS)

Overview of the Peripheral Nervous System

The Peripheral Nervous System (PNS) connects the Central Nervous System (CNS) to the rest of the body, facilitating sensory input and motor output. It is divided into sensory and motor divisions, each with distinct functions and structures.

• Sensory (Afferent) Division: Transmits sensory information from receptors to the CNS.

• Motor (Efferent) Division: Sends motor commands from the CNS to effectors (muscles and glands).

Somatic Sensory Division: Carries information from skin, muscles, and joints.

Visceral Sensory Division: Transmits information from internal organs.

Somatic Motor Division: Carries signals to skeletal muscles; voluntary movement

Visceral Motor (Autonomic) Division: Carries signals to heart and smooth muscles; Regulates involuntary functions (e.g., heart rate, digestion).

Structure of Peripheral Nerves

Peripheral nerves are bundles of axons surrounded by connective tissue. They are classified based on the type of information they carry:

• Peripheral Nerve: Bundle of axons located on the outside of the brain and spinal cord; Connect the CNS to the rest of the body

• Mixed Nerve: Contains both sensory and motor fibers.; Carries signals both to and from the CNS

• Motor Nerve: Contains only motor fibers.; Sends commands from CNS to muscles or glands

• Sensory Nerve: Contains only sensory fibers.; Brings information from receptors to the CNS

Spinal Nerves: Emerge from the spinal cord; anterior root carries motor information, posterior root carries sensory information. The posterior root ganglion houses sensory neuron cell bodies.

Cranial Nerves: Originate from the brain; may be sensory, motor, or mixed.; 12 pairs, controls structures of the head and neck

Detailed Structure of a Nerve

• Posterior Root: Sensory fibers entering the spinal cord.

• Posterior Root Ganglion: Cluster of sensory neuron cell bodies.; Houses and processes incoming sensory information

• Anterior Root: Motor fibers exiting the spinal cord.

• Spinal Nerve: Mixed peripheral nerve Formed by the union of anterior and posterior roots.; Carries sensory in and motor out

• Epineurium: Outermost connective tissue covering the nerve.

• Perineurium: Surrounds bundles (fascicles) of axons.

• Nerve Fascicle: Bundle of axons within a nerve.

• Endoneurium: Surrounds individual axons.

• Nerve Axon: The long, slender projection of a neuron that carries electrical signals away from the body

Spinal Nerves

Structure and Organization

Spinal nerves are organized into roots, nerves, rami, plexuses, and ganglia. Each structure plays a role in transmitting signals and organizing nerve fibers.

• Anterior Root: Carries motor fibers out.

• Posterior Root: Carries Sensory fibers in.

• Spinal Nerve: Mixed nerve formed by joining anterior and posterior roots.

• Ramus Communicans: Branch connecting spinal nerve to sympathetic ganglion.

• (Dorsal- Back muscles and skin)

• (Anterior- Front of body and limbs)

• Sympathetic Ganglion: Part of the autonomic nervous system.

• Anterior Ramus: Supplies limbs and anterior trunk.

• Posterior Ramus: Supplies posterior trunk.

Spinal Nerve Plexuses

Spinal nerve plexuses are networks formed by anterior rami of spinal nerves. There are four main plexuses:

• Cervical Plexus- Neck; breathing (Phrenic Nerve)

• Brachial Plexus- upper limb movement (Radial, median, ulnar nerve)

• Lumbar Plexus- Front of the leg (Femoral Nerve)

• Sacral Plexus- back of the legs + feet (Sciatic Nerve, the largest)

Spinal Nerve Pairs: There are 31 pairs, grouped as follows:

Group	Number of Pairs	Numbering
Cervical	8	C1–C8
Thoracic	12	T1–T12
Lumbar	5	L1–L5
Sacral	5	S1–S5
Coccygeal	1	Co1

Sensation: Role of the PNS

Types of Sensory Receptors

Sensory receptors detect changes in the environment and transmit information to the CNS. They are classified by location and stimulus origin:

• Exteroceptors: Located near the body surface; detect external stimuli (e.g., touch, temperature).

• Interoceptors: Located within internal organs; detect internal stimuli (e.g., blood pressure).

• Proprioceptors: Located in muscles, tendons and joints; detect body position and movement.

Receptors are also classified by the type of stimulus detected:

• Mechanoreceptors: Detect mechanical forces (e.g., pressure, vibration).

• Thermoreceptors: Detect temperature changes.

• Chemoreceptors: Detect chemical changes (e.g., pH, CO2).

• Photoreceptors: Detect light (found in the eyes).

• Nociceptors: Detect pain.

Structure and Function of Sensory Neurons

• Pseudounipolar Neurons: Sensory neurons with a single process that splits into two branches; cell bodies are located in the posterior root ganglion.

• Sensory Transduction: The process by which a sensory receptor converts a stimulus into an electrical signal.

• Sensory Receptor: Specialized cell or structure that detects stimuli.

• Encapsulated Nerve Endings: Sensory nerve endings surrounded by connective tissue.

• Free Nerve Endings: Sensory nerve endings without connective tissue covering.

Rapidly Adapting Sensors: Respond quickly to stimuli but stop firing if the stimulus remains. Slowly Adapting Sensors: Continue to respond as long as the stimulus is present.

Dermatomes and Referred Pain

• Dermatome: Area of skin supplied by a single spinal nerve. Used to diagnose spinal cord injuries. (If injured the dermatome it supplies loses sensation) EX: Loss of sensation in the thumb= May indicate C6 nerve damage

• Referred Pain: Pain perceived at a location other than the site of origin, due to shared neural pathways.

Sensory Pathway

Sensory information travels from receptors in the PNS to the CNS via the following pathway:

• Detection by sensory receptor

• Transmission via sensory neuron

• Processing in the CNS

Movement: Role of the PNS

Motor Neurons and Movement

Motor control involves two types of neurons:

• Upper Motor Neurons: Located in the CNS; initiate voluntary movement.

• Lower Motor Neurons: Located in the PNS; directly supply muscle with nerves muscles.

Movement occurs through a coordinated process involving sensory input, integration in the CNS, and motor output.

1. Stimulus causes sensation

2. CNS detects and send out motor canal

3. Effector moves

Reflex Arcs: Integration of Sensory and Motor Functions

Reflex Responses and Reflex Arcs

Reflexes are rapid, automatic responses to stimuli. A reflex arc is the neural pathway that mediates a reflex.

• Reflex: Involuntary response to a stimulus./ pre programmed

• Reflex Arc: Pathway consisting of sensory receptor, sensory neuron, integration center, motor neuron, and effector. produces a reflex

• Stimulus: Change in the environment that triggers a response.

• Integration: Processing of sensory input in the CNS.

• Motor Response: Action by an effector (muscle or gland).

• Somatic Reflexes- Voluntary movement: Blinking

• Visceral Reflexes: Internal organs, involves brain stem

Somatic Reflexes: Involve skeletal muscles. Visceral Reflexes: Involve smooth muscle, cardiac muscle, or glands.

Types of Reflexes

• Stretch Reflex: Causes muscle contraction in response to be stretching (e.g., knee-jerk reflex).

• Flexion (Withdrawal) Reflex: Causes withdrawal from painful stimuli.

• Crossed-Extensor Reflex: Involves extension of the opposite limb during withdrawal.

Difference: The flexion reflex withdraws the affected limb, while the crossed-extensor reflex stabilizes the opposite limb.

Example: Knee-Jerk Reflex

• Tap on patellar tendon stretches quadriceps.

• Sensory neuron transmits signal to spinal cord.

• Motor neuron causes quadriceps contraction.

Additional info: Reflex arcs are essential for rapid protective responses and are used clinically to assess nervous system function.