Peripheral Nervous System

Organization of the Nervous System

The nervous system is organized into distinct levels that process sensory information and coordinate responses. The peripheral nervous system (PNS) connects the central nervous system (CNS) to limbs and organs, facilitating communication between the body and the brain.

• Receptor Level: Sensory reception and transmission to the CNS. Examples include joint kinesthetic receptors, muscle spindles, and free nerve endings for pain, cold, and warmth.

• Circuit Level: Processing in ascending pathways. Sensory information travels from receptors through the spinal cord, cerebellum, or reticular formation to the thalamus.

• Perceptual Level: Processing in cortical sensory centers. Pathways from the thalamus lead to the motor cortex or somatosensory cortex, where conscious perception occurs.

Additional info: The PNS includes sensory (afferent) and motor (efferent) divisions, further subdivided into somatic and autonomic systems.

Receptor Types by Stimulus

Sensory receptors are specialized to detect specific types of environmental stimuli, enabling the body to respond appropriately.

• Mechanoreceptors: Respond to mechanical forces such as touch, pressure (including blood pressure), vibration, and stretch.

• Thermoreceptors: Detect temperature changes.

• Photoreceptors: Located in the retina, respond to light.

• Chemoreceptors: Respond to chemicals in solution, including molecules smelled or tasted, or changes in blood/interstitial fluid chemistry.

• Nociceptors: Respond to damaging stimuli that result in pain.

Receptor Types by Body Location

Receptors are classified based on their location and the type of stimuli they detect.

• Exteroceptors: Detect external stimuli (outside the body), such as touch, pressure, pain, and temperature.

• Interoceptors: Detect internal stimuli, including chemical changes within the body.

• Proprioceptors: Located in skeletal muscles, tendons, joints, ligaments, and connective tissue coverings of bones and muscles; detect body position and movement.

Receptors and Sensations

Sensory receptors range from bare nerve endings to complex sense organs. When a receptor is excited by a stimulus, a sensation is generated and sent to the brain for processing. Awareness of sensations depends on the type and location of the stimulus.

• Sensory receptor: Specialized to detect stimuli.

• Sense organ: Combines nervous, muscular, epithelial, or connective tissue to enhance response.

• Sensation: Occurs when a receptor is excited by a stimulus.

Types of Sensory Information

Type (Modality)

Modality refers to the kind of environmental stimulus (e.g., vision, taste, pain, touch). The brain distinguishes modalities based on where sensory signals terminate.

Location

The location of a stimulus is determined by which sensory neuron sends a signal to the brain. Each neuron has a receptive field, and the brain cannot distinguish the exact point within the field that was stimulated.

Additional info: Smaller receptive fields allow for more precise localization of stimuli.

Intensity

Intensity is perceived as loudness, brightness, or pain severity. It is determined by:

• Which nerves are sending signals (sensitive or not)

• How many nerves are sending a signal

• The frequency of nerve signals

Duration

Duration refers to how long a stimulus lasts. All receptors are capable of sensory adaptation, where prolonged stimulation leads to decreased frequency of firing and reduced awareness.

• Example: Adapting to a hot bath or getting used to strong scents.

Sensory Transduction

Sensory information is sent to the brain via action potentials. Sense organs act as transducers, converting one form of energy (light, heat, chemicals, touch, sound) into another (action potentials). This process is called sensory transduction.

Pain

Pain is discomfort caused by tissue damage or noxious stimulation. It can be visceral (internal organs) or somatic (body wall). Specialized nerve endings called nociceptors respond to chemicals released by injured tissues. Nociceptors are concentrated in skin, mucous membranes, and most organs, but are absent in the brain.

Referred Pain

Referred pain occurs when visceral pain is perceived as originating from the skin. For example, pain in the left arm during a heart attack is referred pain.

Additional info: Referred pain is due to shared neural pathways between visceral and somatic regions.

Phantom Pain

Phantom pain is the sensation of pain from a limb that has been amputated. It is believed to result from the brain and spinal cord missing input sensations from the absent limb, triggering pain as a basic warning signal.

Spinal Gating

Spinal and brainstem neurons secrete pain-blocking chemicals called endorphins and enkephalins, which stop pain signals before reaching consciousness.

Nerve Repair

Nerve repair differs between the CNS and PNS. In the CNS, oligodendrocytes inhibit growth and astrocytes form scar tissue, preventing regeneration. In the PNS, Schwann cells facilitate regeneration by releasing growth factors and producing new myelin sheaths, while macrophages clean out dead axons and stimulate Schwann cell division.

Cranial Nerves

There are 12 cranial nerves, each with specific functions. They are identified by name or Roman numeral.

• I. Olfactory: Purely sensory, carries afferent impulses for smell.

• II. Optic: Purely sensory, carries afferent impulses for vision.

• III. Oculomotor: Mainly motor.

• IV. Trochlear: Primarily motor for eye muscles.

• V. Trigeminal: Main sensory nerve of the face.

• VI. Abducens: Primarily motor for eye muscles.

• VII. Facial: Chief motor nerve of the face.

• VIII. Vestibulocochlear: Mostly sensory for balance and hearing.

• IX. Glossopharyngeal: Mixed, innervates tongue and pharynx.

• X. Vagus: Mixed, mostly parasympathetic efferent (regulates heart rate, breathing, digestion).

• XI. Accessory: Mixed, primarily motor to trapezius and sternocleidomastoid.

• XII. Hypoglossal: Mixed, primarily motor for tongue and swallowing.

Somatic Reflexes

Reflexes are quick, involuntary reactions of glands or muscles to stimuli. They are characterized by being quick, involuntary, stereotyped, and always triggered by stimulation.

• Visceral reflexes: Involve cardiac muscle, smooth muscle, or glands (e.g., increased heart rate in fear, constriction of esophagus when swallowing).

• Somatic reflexes: Involve skeletal muscle (e.g., knee-jerk reflex, pulling hand away from a hot stove).

Reflex Arc Pathway

The reflex arc consists of:

• Receptor: Sensory nerve ending or sense organ in skin, muscle, or tendon.

• Afferent (sensory) neuron: Carries signal from receptor to spinal cord or brainstem.

• Integrating center: Synaptic contact between neurons in gray matter; may involve interneurons.

• Efferent (motor) neuron: Carries motor signal to skeletal muscle.

• Effector: Skeletal muscle or organ/cell that carries out the response.

Stretch Reflex

The "knee jerk" reflex is a monosynaptic reflex arc. A doctor taps the patellar tendon, stretch receptors (muscle spindles) detect the stimulus, and the signal travels through an afferent neuron to a motor neuron in the spinal cord, causing the quadriceps to contract.

Flexor (Withdrawal) Reflex

This protective reflex requires coordination of multiple muscles and utilizes a polysynaptic reflex. Afferent and efferent neurons synapse with interneurons, allowing both stimulation and inhibition of muscles (e.g., withdrawing from pain in the foot: hamstring excited, quadriceps inhibited).

Babinski’s Sign

If the primary motor cortex or corticospinal tract is damaged, the plantar reflex is replaced by Babinski’s sign, where the great toe dorsiflexes and smaller toes fan laterally. Infants exhibit this reflex until about 1 year of age.

Sample Question

The main sensory nerve of the face is the?

• A. abducens

• B. accessory

• C. trigeminal (Correct Answer)

• D. facial