CNS 2

Central Nervous System Lecture 2

Reflex and Reflex Arc

Reflex: A rapid, predictable, and involuntary response to stimuli.
Reflex Arc: A direct pathway from a sensory neuron to an interneuron and then to an effector organ.
  - Components of Reflex Arc:
    1. Receptor: Senses the stimulus.
    2. Sensory Neuron: Transmits the sensory information.
    3. Integration Center: Processes the information (often an interneuron).
    4. Motor Neuron: Carries the response information to the effector.
    5. Effector Organ: Executes the response (e.g., muscle contraction or gland secretion).

Classification of Reflexes

Reflexes

Clinical Classification
    - Superficial Reflexes: Initiated by stimulating appropriate receptors on the skin; usually multisinaptic; involves moving away from stimulus (e.g., plantar reflex, corneal reflex).
    - Deep Reflexes: Initiated by stimulating receptors deep in muscle; essentially stretch reflexes (e.g., knee jerk, ankle jerk reflexes).
    - Visceral Reflexes: Reflexes where at least one part of the reflex arc is an autonomic nerve, initiated by receptors in visceral organs (e.g., pupillary reflex, carotid sinus reflex).
    - Pathological Reflexes: Not normally found; their presence indicates a pathological condition within the body (e.g., Babinski's sign).
Anatomic Classification
    - Segmental: End of afferent neuron and beginning of efferent neuron are in the same spinal cord segment (e.g., knee jerk reflex).
    - Intersegmental: End of afferent neuron and beginning of efferent neuron are in different spinal cord segments (e.g., crossed extensor reflex).
    - Suprasegmental: Centers of these reflexes lie above the spinal cord; interactions with suprasegmental components (e.g., postural reflex).
Classification by Number of Synapses
    - Monosynaptic: Sensory neurons synapse directly on motor neurons (e.g., stretch reflex).
    - Bisynaptic: One interneuron is interposed between sensory and motor neurons (e.g., reciprocal innervation).
    - Polysynaptic: More than two interneurons and synapses in the spinal cord (e.g., superficial reflex).
Physiological Classification
- Flexor Reflexes: Stimulation causes flexion of joints (e.g., thorn prick to the sole causes flexion of knee and hip joints).
- Extensor Reflexes: Stimulation causes extension of joints (e.g., stretch reflexes, which are the basis of tone and posture).
Conditioned vs Unconditioned Reflexes
- Conditioned Reflexes: Reflexes that develop after birth.
- Unconditioned Reflexes: Inborn reflexes that are present at birth.
Differences Between Conditioned and Unconditioned Reflexes
    - Unconditioned Reflex
      1. Inborn, inherited.
      2. Species-specific.
      3. Stable.
      4. Has a definite receptive field.
      5. Does not require the brain cortex.
      6. Permanent for life.
      7. Based on unconditioned reflex principles.
    - Conditioned Reflex
      1. Acquired (through life experience).
      2. Individual-specific.
      3. Unstable.
      4. Different receptive fields.
      5. Requires brain cortex involvement.
      6. Temporary.

Receptors

Definition: Specialized part in the plasma membrane of nerve cells that responds to sensory stimuli of the same modality.
Function: Converts different forms of stimuli into electrical signals (action potentials) in sensory nerves.

Properties of Receptors

Specificity (Muller’s Law): Every receptor has a particular type of stimulus called the adequate stimulus, to which it gives one type of sensation (response) regardless of how it is stimulated.
Adaptation: Decrease in receptor activity despite continuous receptor stimulation; many receptors stop responding when the stimulus is constant.
    - Adaptability Classification:
      a. Rapidly Adapting: e.g., light & touch receptors.
      b. Moderately Adapting: e.g., thermoreceptors (warm receptors adapt more than cold ones).
      c. Slowly Adapting: e.g., proprioceptors, pain receptors, deep pressure receptors, and baroreceptors.
Excitability: Does not obey the all-or-none law; the frequency or rate of receptor discharge is proportional to stimulus strength.
Summation: If subthreshold stimuli are applied successively within a short time, the stimuli are summed to produce a nerve impulse.

Peripheral Nervous System

Connects the Central Nervous System (CNS) to all organs of body systems.
Divided into two subdivisions:
- Somatic Nervous System: Voluntary control.
- Autonomic Nervous System: Involuntary control.

Functional Classification of the Peripheral Nervous System

Sensory (Afferent) Division: Nerve fibers that carry information to the CNS.
Motor (Efferent) Division: Nerve fibers that carry impulses away from the CNS.

Brain Overview

One of the largest and most metabolically active organs in the body.
Neurons proliferate only during prenatal development and a few months after birth; after this, they increase in size but not in number.
Comprises only 2% of total body weight, yet receives 15% of total blood flow and uses 20% of oxygen intake at rest (more during mental activity).
Interruption of blood flow for 1-2 minutes may impair function, and 4 minutes without oxygen can cause lasting damage.
Requires a continuous supply of glucose; decreased glucose levels lead to dizziness, convulsions, or unconsciousness.

Brain Regions

Cerebrum
Diencephalon
Cerebellum
Brain Stem

Subdivisions of the Brain

Cerebral Hemispheres (60% of brain mass): Associated with thought, creativity, communication.
Diencephalon: Controls moods, memory, manages the internal environment; includes:
   - Epithalamus
   - Thalamus
   - Hypothalamus
Cerebellum: Coordinates movement and balance.
Brain Stem: The smallest region; controls basic body functions and includes:
   - Midbrain
   - Pons
   - Medulla Oblongata

Brain Stem

Medulla Oblongata

Lowest portion of the brainstem; continuous with the spinal cord and contains all ascending and descending tracts from the spinal cord and brain (white matter).
Functions:
  - Autonomic reflex centers:
    - Cardiac Reflex Center: Controls the rate and force of heartbeat.
    - Vasomotor Control Center: Regulates blood vessel diameter, controls blood distribution to specific organs, and manages blood pressure.
    - Respiratory Centers: Regulate the breathing process.
  - Non-vital reflex centers (nuclei):
    - Responsible for speech, swallowing, vomiting, coughing, sneezing, and hiccupping.

Pons

Located just above the medulla oblongata; serves as a bridge connecting the spinal cord with the brain and various parts of the brain.
Functions:
  - Contains 2 centers that regulate breathing:
    - Pneumotaxic Center: Controls the rate and rhythm of respiration.
    - Apneustic Center: Controls the depth of respiration.
  - Contains nuclei affecting sleep and bladder control.

Midbrain

Formed of four lobes above and behind the pons (Corpora Quadrigemina):
  - Superior Colliculi (Upper 2 Lobes): Control center for some visual reflexes.
    - Includes pupillary reflex.
    - Coordinates eye movement with head and neck movement in response to visual stimuli.
  - Inferior Colliculi (Lower 2 Lobes): Control center for some auditory reflexes.
    - Includes reflex centers for head and trunk movement in response to auditory stimuli to locate sound.
    - Responsible for the startle response to loud noises.
  - Contains Substantia Nigra: Suppresses unwanted muscle contractions.

Reticular Formation (Reticular Activation System)

A diffuse system of interconnecting fibers extending through various brain areas, including the brainstem.
Comprises a large portion of the brainstem and extends into the spinal cord and diencephalon.
Functions of RAS:
  1. Regulates sleep and consciousness cycles; responsible for consciousness.
  2. Helps control muscle tone, balance, and posture during body movements.
  3. Filters sensory input (habituation).