Neurons

Messages in the nervous system provide information about the environment, enabling responses.
Synchronization of millions of neural signals facilitates thinking and communication.
Essential to understand the workings of the brain and nervous system.

Neurons:
- Unique cells in the body that conduct electrochemical signals.
- Irregular shape with numerous extensions.
Components of Neurons:
  - Axon:
    - Carries messages away from the cell.
    - Diameter: 5 to 100 micrometers; Length: up to 1 meter.
  - Dendrites:
    - Short extensions, branching like trees from the neuron cell body.
    - A single neuron can possess 1 to 20 dendrites, significantly increasing surface area for signal reception.
  - Dendritic Spines:
    - Small, nubby structures on dendrites that enhance contact area for receiving signals.
    - A neuron can connect with up to 10,000 other neurons.

Activation of receptors in dendrites sends tiny electrical currents to the axon hillock.
Action Potential:
- An electrical impulse generated in the neuron.
- Travels down the axon to transmit neural messages.
Axon Terminals:
  - Branches at the end of an axon that connect to other neurons via synapses.
  - Synapse:
    - Gap (10 to 20 nanometers) between axon terminal and receiving neuron's dendrites.
  - Neurotransmitters:
    - Chemicals that carry messages across synapses.
    - Released by presynaptic neurons, bind to postsynaptic receptors, triggering a new electrochemical process.

Nerve impulses can travel up to 120 meters in one second.
The nervous system is divided into:
- Central Nervous System (CNS): Includes the brain and spinal cord, encased in the skull and vertebral column.
- Peripheral Nervous System (PNS): Comprises all other nervous system components.

Cerebrum:
  - Composed of two cerebral hemispheres, the largest part of the brain.
  - Covered by the Cerebral Cortex:
    - Gray matter layer approximately 3 millimeters thick.
    - Beneath is the White Matter, which contains fiber tracks for communication to/from the cortex.
Lobes of the Cerebral Cortex:
  - Frontal Lobe:
    - Functions in thinking, speech, emotion, movement planning and production.
  - Occipital Lobe:
    - Receives and interprets visual input.
  - Temporal Lobe:
    - Receives auditory input and is involved in emotion, speech understanding, form, color perception, and smell.
  - Parietal Lobe:
    - Interprets sensory messages related to pain, touch, and spatial positioning of limbs.
    - Integrates auditory and visual inputs with sensory messages.

Diencephalon:
  - Contains the Thalamus: Relay center for sensory information (excluding smell).
  - Contains the Hypothalamus:
    - Regulates hormones and autonomic nervous system functions.
Brain Stem:
  - Responsible for fundamental functions:
    - Breathing control, pain perception, body temperature regulation, organizing simple movements.
  - Contains the Reticular Formation: Web-like neural network controlling cardiovascular and respiratory functions, consciousness, and sleep.
  - Damage to the brain stem can be fatal due to its critical control functions.

Cerebellum:
- Coordinates voluntary movements, maintains posture, and learns motor skills.
Spinal Cord:
  - Transition point to the peripheral system through the medulla.
  - Enclosed by spinal vertebrae, containing axons that extend to muscles and internal organs through peripheral nerves.
  - Fiber Pathways: Made up of nerve bundles (fiber tracks).

Somatic Nervous System:
- Controls skeletal muscle movements.
Autonomic Nervous System:
- Regulates internal organs and glands; unconscious bodily functions.

Sensation:
- Involves receiving information through sensory organs.
- Transduction: Conversion of physical/chemical stimuli into neural signals by sensory receptors.
Signal conduction occurs via nerve cells to the CNS.
All nerve impulses are electrical signals; their destination neurons help the brain differentiate between sensations.

Vision: Dominant sense with 70% of sensory receptors located in the eyes; significant cortical involvement (almost 50%).
Rods and Cones:
- Rods: Sensitive to light intensity.
- Cones: Detect color (three types: red, blue, green).
Signals from rods and cones travel through the optic nerve to the thalamus, then processed in the cerebral cortex.

Sound generated by vibrating objects; perceived as sound waves by the ear.
Ear Structure:
  - Outer Ear: Collects sound waves; funnels them to the eardrum.
  - Middle Ear: Contains ossicles, which transmit vibrations.
  - Inner Ear: Contains cochlea, the organ of Corti, responsible for hearing perception via hair cells (cilia).
Auditory information travels through the auditory nerve to the brain for processing in levels (primary to higher-order auditory cortex).

Gustation (Taste):
  - Provides information on food quality, potentially harmful substances.
  - Taste buds cluster around Lingual Papillae: Approximately 10,000 taste buds present.
  - Basic tastes: sweet, sour, salty, bitter, and umami (Japanese for 'delicious'); enhanced by amino acids (e.g., glutamate).
Olfaction (Smell):
  - More complex than taste; about 2,004 odors detectable.
  - Olfactory receptors located in nasal cavity; olfactory neurons have cilia for odor detection.
  - Signals sent to the cerebral cortex and limbic system for emotional response and memory association.

Other than the traditional five senses (vision, hearing, taste, smell, touch), additional senses include temperature, pressure, equilibrium, and pain.
Mechanoreceptors: Sensitive to touch and pressure located in skin and internal organs.
Chemoreceptors: Respond to chemical changes (taste and smell receptors).
Thermoreceptors: Detect heat/cold to regulate body temperature.
Nociceptors: Specialized receptors for pain.

Approximately 1,000,000 motor neurons in the spinal cord control muscle movements across the body.
Motor neurons activate muscle fibers through axonal transmission; number of fibers stimulated varies by movement precision needs.

Sleep represents about one-third of human lifespan; is a regulated, active process essential for brain restoration rather than just physical rest.
Ascending Reticular Activating System (ARAS): Responsible for arousal and activation of the cerebral cortex during wakefulness.
Anterior hypothalamus promotes sleep, while the posterior hypothalamus is involved in wakefulness control.