Nervous System Functions and Components

Functions of the Nervous System

• Sensory Input:
  • Gathers information from stimuli through receptors.
  • Detects changes inside and outside the body.
• Integration:
  • Processes and interprets sensory input, making decisions based on it.
• Motor Output:
  • Responds to stimuli by activating effector organs, particularly muscles and glands.

Organization of the Nervous System

• Central Nervous System (CNS):
  • Comprises the brain and spinal cord.
• Peripheral Nervous System (PNS):
  • Extends out from the CNS.
• Pathways:
  • Sensory pathways: Carry information to the CNS.
  • Motor pathways: Carry commands from the CNS.

Nervous System Components

• Somatic Nervous System:
  • Controls voluntary movements of skeletal muscles.
• Autonomic Nervous System:
  • Controls involuntary actions of smooth muscle, cardiac muscle, and glands.
  • Sympathetic Division:
  • Responsible for 'fight or flight' responses.
  • Activates under stress.
  • Neurotransmitter: Norepinephrine (NE).
  • Effects:
    • Increases heart rate.
    • Dilates bronchioles.
    • Inhibits digestion.
  • Parasympathetic Division:
  • Responsible for 'rest and digest' functions.
  • Neurotransmitter: Acetylcholine (ACh).
  • Effects:
    • Slows heart rate.
    • Constricts bronchioles.
    • Stimulates digestion.

Neuroglia Types

• Microglia: Tiny cells involved in immune response.
• Oligodendrocytes: Have few branches; insulate axons in the CNS.
• Astrocytes: Most abundant, support, and anchor neurons.
• Ependymal Cells: Line ventricles and central canal.
• Schwann Cells: Surround peripheral nerve fibers.
• Satellite Cells: Support neuronal cell bodies in the PNS.

Action Potential

• Mechanism:
  • Begins at the axon hillock, travels down the axon to the axon terminals.
  • Phases:
  • Depolarization: Neuron becomes less negative.
  • Repolarization: Returns to resting potential.
  • Hyperpolarization: Becomes more negative than resting potential.
  • All-or-none response: Action potentials maintain strength throughout.

Nerve Impulse Conduction at Synapse

• Mechanism:
  • Neurotransmitters are released from presynaptic neuron, cross the synaptic cleft, bind to receptors on the postsynaptic neuron, triggering the next action potential.
• Types of Synapses:
  • Axodendritic: Between axon and dendrite.
  • Axosomatic: Between axon and cell body.
  • Axoaxonic: Between axons.
• Electrical Synapses:
  • Ions flow directly between cells via gap junctions; fast and two-way communication.
• Chemical Synapses:
  • Utilize neurotransmitters for one-way signaling.

Neurotransmitter Classification

• Excitatory:
  • Cause depolarization; promote action potentials (e.g., Glutamate).
• Inhibitory:
  • Cause hyperpolarization; inhibit action potentials and can promote calmness (e.g., GABA).
• Some neurotransmitters, like ACh, can function as both depending on context.

Components of a Reflex Arc

• Receptor: Detects the stimulus.
• Sensory Neuron: Carries impulse to the CNS.
• Integration Center: Processes information in the spinal cord.
• Motor Neuron: Sends impulse to effector.
• Effector: Muscle or gland that responds.

Ascending and Descending Tracts

• Ascending Tracts: Carry sensory information to the brain.
• Descending Tracts: Carry motor commands from the brain.

Structure and Function of a Nerve

• Structure: Bundle of axons (nerve fibers) wrapped in connective tissue.
  • Epineurium: Outermost layer.
  • Perineurium: Middle layer.
  • Endoneurium: Innermost layer.
• Function: Transmits electrical signals between CNS and body.
• Functional Types:
  • Sensory Nerves: Carry signals to CNS.
  • Motor Nerves: Carry commands from CNS.
  • Mixed Nerves: Contain both fibers.

Spinal Nerves

• 31 pairs of mixed nerves (both sensory and motor).
• Arise from the spinal cord and exit through intervertebral foramina.
• Each consists of anterior (motor) and posterior (sensory) roots.

Nerve Plexuses

• Definition: Complex networks of mixed nerves formed from adjacent spinal nerves.
• Major Plexuses:
  • Cervical Plexus: Serves neck and diaphragm.
  • Brachial Plexus: Serves shoulders and arms.
  • Lumbar Plexus: Serves lower abdomen and thighs.
  • Sacral Plexus: Serves buttocks, legs, and feet.

Meninges

• Three protective layers covering the brain and spinal cord.
  • Dura Mater: Outermost, tough layer.
  • Arachnoid Mater: Middle, web-like layer.
  • Pia Mater: Innermost, delicate layer.
• Function: Protect CNS, carry blood supply, and contain cerebrospinal fluid.

Cranial Nerves

1. I Olfactory: Smell (Nose)
2. II Optic: Vision (Eyes)
3. III Oculomotor: Eye movement, pupil constriction (Eye muscles)
4. IV Trochlear: Eye movement (Superior oblique muscle)
5. V Trigeminal: Facial sensation, chewing (Face, scalp, jaw)
6. VI Abducens: Eye movement (Lateral rectus muscle)
7. VII Facial: Facial expression, taste (Face muscles, anterior tongue)
8. VIII Vestibulocochlear: Hearing and balance (Ear)
9. IX Glossopharyngeal: Taste, swallowing (Posterior tongue, pharynx)
10. X Vagus: Autonomic control of organs (Thoracic/abdominal organs)
11. XI Accessory: Shoulder & neck movement (Trapezius, sternocleidomastoid)
12. XII Hypoglossal: Tongue movement (Tongue muscles)

Parts of the Brain

• Cerebrum: Largest part; responsible for reasoning, memory, and sensory perception.
• Lobes:
  • Frontal: Conscious thought, voluntary movement.
  • Parietal: Absorbs data and handles abstract mental functions.
  • Occipital: Visual processing.
  • Temporal: Manages sensory memories related to sound.
• Diencephalon: Houses thalamus (relay station) and hypothalamus (homeostasis).
• Brainstem: Includes midbrain, pons, medulla oblongata; controls vital functions.
• Cerebellum: Coordinates muscle movements and balance; processes sensory input about body position.

Gray Matter vs White Matter

• Gray Matter: Contains neuron cell bodies; found in cerebral cortex and nuclei.
• White Matter: Contains myelinated axons; connects different brain areas for communication.

Limbic System

• Function: Emotional center involved in motivation, memory, and behavior.
• Includes the hippocampus (memory), amygdala (emotion), and cingulate gyrus.

Reticular Formation

• Function: Regulates consciousness, alertness, and sleep-wake cycles; filters sensory input and helps with attention.

Additional Brain Functions and Structures

• Thalamus: Relays sensory and motor signals to the cortex.
• Hypothalamus: Synthesizes hormones; controls hunger, thirst, and body temperature.
• Pons: Sends info from cerebrum to cerebellum; involved in respiration and posture.
• Medulla Oblongata: Controls autonomic functions (blood pressure, heart rate, respiration).
• Pineal Gland: Releases melatonin, regulates sleep cycles.