Peripheral Nervous System (PNS)
- All neural structures outside the brain.
- Key components:
- Sensory receptors
- Peripheral nerves and associated ganglia
- Efferent motor endings
Organization of the Nervous System
Central Nervous System (CNS)
- Comprises the brain and spinal cord.
Peripheral Nervous System (PNS)
- Divided into:
- Sensory (afferent) division
- Motor (efferent) division
- Somatic nervous system
- Autonomic nervous system (ANS)
- Sympathetic division
- Parasympathetic division
- Visual representation reflects the structural organization of the entire nervous system.
Sensory Receptors
Definition: Specialized to respond to changes in the environment (stimuli).
Activation and Function:
- Activation leads to graded potentials that trigger nerve impulses.
- Sensation: Awareness of stimulus.
- Perception: Interpretation of the meaning of the stimulus.
Categories of Sensory Receptors:
1. Type of stimulus they detect
2. Location in the body
3. Structural complexity
Classification of Sensory Receptors by Type of Stimulus
Mechanoreceptors
- Respond to touch, pressure, vibration, and stretch.
Thermoreceptors
- Sensitive to changes in temperature.
Photoreceptors
- Respond to light energy (e.g., retina).
Chemoreceptors
- Respond to chemicals (e.g., smell, taste, changes in blood chemistry).
Nociceptors
- Sensitive to pain-causing stimuli (e.g., extreme heat or cold, excessive pressure, inflammatory chemicals).
Classification of Sensory Receptors by Location
Exteroceptors
- Respond to stimuli arising outside the body.
- Present in skin for touch, pressure, pain, and temperature.
- Includes most special sense organs.
Interoceptors
- Respond to stimuli arising in internal viscera and blood vessels.
- Sensitive to chemical changes, tissue stretch, and temperature.
Proprioceptors
- Respond to stretch in skeletal muscles, tendons, joints, and ligaments.
- Inform the brain of one’s movements.
Classification by Receptor Structure
Simple Receptors
- For general senses including tactile sensations (touch, pressure, stretch, vibration), temperature, pain, and muscle sense.
- Comprised of modified dendritic endings of sensory neurons.
Levels of Neural Integration:
1. Receptor Level—sensory receptors.
2. Circuit Level—processing in ascending pathways.
3. Perceptual Level—processing in cortical sensory areas.
Perceptual Processing Levels
Processing at different levels involves the following:
- Receptor Level:
- Distinguishes between different stimuli using graded potentials called generator potentials.
- Transduction occurs: a stimulus is changed to graded potential, which must reach threshold to generate action potentials.
- Circuit Level:
- Involves three neuron types:
1. First-order sensory neurons: Conduct impulses from receptor level to spinal reflexes or second-order neurons in CNS.
2. Second-order sensory neurons: Transmit impulses to third-order sensory neurons.
3. Third-order sensory neurons: Conduct impulses from the thalamus to the somatosensory cortex.
- Perceptual Level:
- Interpretation depends on the specific location of target neurons in the sensory cortex.
- Aspects of Sensory Perception:
- Perceptual detection: The ability to detect a stimulus (requires summation of impulses).
- Magnitude estimation: Intensity is coded in frequency of impulses.
- Spatial discrimination: Identifies site or pattern of stimulus.
- Quality discrimination: Identifies submodalities of a sensation (e.g., sweet or sour tastes).
- Pattern recognition: Recognizes familiar or significant patterns in stimuli (e.g., melody in music).
Nerves and Associated Ganglia
Definition of Nerve: Cordlike organ of the PNS, comprised of bundles of myelinated and nonmyelinated peripheral axons enclosed by connective tissue.
Types of Nerves:
- Spinal Nerves or Cranial Nerves, depending on origin.
- Most nerves are mixtures of afferent and efferent fibers, as well as somatic and autonomic (visceral) fibers.
- Classification of Nerves by Direction of Impulse:
- Mixed Nerves: Contain both sensory and motor fibers.
- Sensory (afferent) Nerves: Transmit impulses only toward the CNS.
- Motor (efferent) Nerves: Transmit impulses only away from the CNS.
Structure and Classification (cont.)
Pure sensory (afferent) or pure motor (efferent) nerves are rare; most nerves are mixed.
Types of Fibers in Mixed Nerves:
- Somatic afferent: Sensory fibers from skeletal muscle to the brain.
- Visceral afferent: Sensory fibers from organs to the brain.
- Somatic efferent: Motor fibers from the brain to skeletal muscle.
- Visceral efferent: Motor fibers from the brain to organs.
Motor Endings and Motor Activity
Motor Endings: PNS elements that activate effectors by releasing neurotransmitters.
- These elements innervate skeletal muscle, visceral muscle, and glands.
- Cerebellum and Basal Nuclei: Ultimate planners and coordinators of complex motor activities.
Hierarchy of Motor Control
Levels of Motor Control:
- Precommand Level (highest):
- Involves the cerebellum and basal nuclei, programming and modifying instructions based on feedback.
- Projection Level (middle):
- Comprises motor cortex (via pyramidal pathways) and brainstem nuclei.
- Conveys instructions to spinal cord motor neurons and sends copies to higher levels.
- Segmental Level (lowest):
- Exists in the spinal cord, containing central pattern generators (CPGs).
Functional Classification of Reflexes
Inborn (Intrinsic) Reflex: Rapid, involuntary motor responses to stimuli.
- Examples: Maintain posture, avoid pain, control visceral activities.
- Can be modified by learning and conscious effort.
Learned (Acquired) Reflexes: Results from practice or repetition.
- Example: Driving skills.
Reflex Arc Components
Components of a Reflex Arc:
1. Receptor: Site of stimulus action.
2. Sensory Neuron: Transmits afferent impulses to CNS.
3. Integration Center: Either monosynaptic or polysynaptic region within CNS.
4. Motor Neuron: Conducts efferent impulses from integration center to effector organ.
5. Effector: Muscle fiber or gland cell that responds to efferent impulses by contracting or secreting.
Spinal Reflexes:
- Integration center located in spinal cord (does not involve higher brain centers).
- Effectors are skeletal muscles.
- Testing of somatic reflexes is crucial clinically to assess the condition of the nervous system.
- Exaggerated, distorted, or absent reflexes signal potential degeneration/pathology of specific nervous system regions.
Stretch and Tendon Reflexes
Need for Proprioceptor Input: Nervous system must receive proprioceptor input regarding:
1. Length of muscle (from muscle spindles).
2. Amount of tension in muscle (from tendon organs).
Stretch Reflex:
- Maintains muscle tone in large postural muscles and adjusts it reflexively.
- Activates muscle contraction in response to increased muscle length (stretch).
- Operating mechanism:
- Stretch activates muscle spindles.
- Sensory neurons synapse directly with motor neurons in spinal cord to cause contraction of the stretched muscle (mediated by a-motor neurons).
- All stretch reflexes are monosynaptic and ipsilateral.
- Reciprocal Inhibition occurs: fibers synapse with interneurons that inhibit a-motor neurons of antagonistic muscles (e.g., in the patellar reflex: quadriceps contract while hamstrings relax).
The Tendon Reflex
Purpose: Helps prevent damage due to excessive stretch.
Response: Produces muscle relaxation (lengthening) in response to tension.
- Mechanism of action involves tension activation leading to:
- Afferent impulses transmitted to spinal cord.
- Contracting muscle relaxes, antagonist contracts (reciprocal activation).
- Additional information is relayed to cerebellum to adjust muscle tension accordingly.