Nervous System Overview

Nervous System

The nervous system is divided into the central nervous system (CNS) and the peripheral nervous system (PNS).
The CNS consists of the brain and spinal cord.
The PNS consists of all the nerves that connect the CNS to the rest of the body.
The brain is protected by the cranium, meninges, cerebrospinal fluid, and the blood-brain barrier.
The blood-brain barrier is a selectively permeable membrane that prevents harmful substances from entering the brain.
Cerebrum
- The cerebral cortex is the outer layer of the cerebrum and is responsible for higher-level cognitive functions.
- The cerebral cortex is divided into different regions, each specializing in different functions.
- Brodmann areas are regions of the cerebral cortex defined by their cytoarchitectonic structure.
- Areas: motor cortex (controls voluntary movement), somatosensory cortex (receives sensory information from the body), auditory cortex (processes auditory information), visual cortex (processes visual information), facial recognition area.
Hypothalamus
- The hypothalamus is a region of the brain responsible for regulating basic bodily functions such as temperature, hunger, and thirst.
Cerebellum
- The cerebellum is responsible for coordinating movement and balance.
Brain Imaging Techniques
- MRI (magnetic resonance imaging) creates images of the brain's structure.
- CT (computed tomography) creates images of the brain's structure.
- fMRI (functional magnetic resonance imaging) measures brain activity by detecting changes in blood flow.
- EEG (electroencephalography) measures brain activity by detecting electrical signals in the brain.
- PET (positron emission tomography) measures brain activity by detecting the distribution of radioactive tracers.

Sensory Receptors

Sensory receptors are specialized cells that detect stimuli from the environment.
Sensory receptors can be classified by their location:
- Peripheral endings: Neurons that send information from sensory receptors to the CNS.
- Separate cells: Cells that interact with neurons to send information to the CNS.
Sensory receptors convert sensory information into membrane potential.
Sensation is the process of converting sensory information into a conscious experience.
There are four steps to sensation:
- Stimulation of the sensory receptor.
- Transduction of the stimulus.
- Generation of action potentials.
- Integration of sensory input.
Receptive Field
- Receptive field density and overlap impact localization, meaning higher density and overlap allows us to detect sensory information more accurately.
- The intensity and duration of a stimulus are coded by the frequency and number of action potentials generated.
- Adaptation is the decline in the firing rate of a sensory receptor over time even in the presence of a continuous stimulus.
- Sensory Modality: The type of sensory stimulus that is perceived (i.e. light, sound, touch) is determined by the pathway the sensory information travels.

Types of Sensations

There are different types of somatic sensations, each detected by specific receptors:
- Tactile sensations: Touch, pressure, vibration.
- Thermal sensations: Hot and cold.
  - Hot and cold receptors: Detect changes in temperature.
- Pain sensations: Pain.
  - Nociceptors: Respond to painful stimuli.
- Proprioceptive sensations: Body position and movement.
  - Muscles, joints, tendons: Sensory receptors detect muscle length and tension.
- Stretch reflex: A reflex that causes a muscle to contract in response to being stretched, preventing overstretching.
- Tendon organ reflex: A reflex that causes a muscle to relax in response to excessive tension, preventing muscle damage.
Olfactory System:
- The olfactory system is responsible for the sense of smell.
- Detects odor molecules that bind to receptors within the olfactory epithelium.
- Very sensitive and fast-adapting receptors.
Gustatory System:
- The gustatory system is responsible for the sense of taste.
- Detects tastes through receptors on the tongue, where taste buds are located.
- Less sensitive and a narrower range of tastes compared to smell.

Visual System

Incoming light passes through the following structures:
- Pupil: Opening in the iris that regulates the amount of light entering the eye.
- Iris: Colored part of the eye that controls the size of the pupil.
- Lens: Focuses light onto the retina.
- Retina: Layer of light-sensitive cells at the back of the eye.
Photoreceptors in the retina detect light.
- Rods: Photoreceptors that are sensitive to low levels of light and are responsible for night vision.
- Cones: Photoreceptors that require higher levels of light and are responsible for color vision.
The fovea is the central region of the retina that contains the highest density of cones.
The fovea is the area of greatest acuity because:
- High cone density allows for better color discrimination.
- Lack of overlying nerve fibers allows light to pass directly to the photoreceptors.
- Direct connection to brain through the optic nerve.
Visual Pathway:
- Light stimuli are converted to action potentials by photoreceptors.
- Information is transmitted by the optic nerve to the thalamus.
- From the thalamus, information is relayed to the visual cortex in the occipital lobe.

Auditory and Vestibular Systems

The auditory system is responsible for hearing.
Sound waves travel from the external ear to the inner ear:
- External ear: Collects sound waves.
- Middle ear: Amplifies sound waves through the movement of ossicles (tiny bones).
- Inner ear: Contains the cochlea, which is filled with fluid and houses hair cells.
Cochlea:
- The movement of fluid in the cochlea causes the basilar membrane to vibrate.
- Movement of the basilar membrane stimulates hair cells, which generate action potentials that are transmitted to the brain.
- Cochlear tuning: Each hair cell is tuned to a specific frequency of sound, allowing us to distinguish different sounds.
The vestibular system is responsible for balance and spatial orientation.
Within the inner ear, the otolithic organs detect changes in linear acceleration and gravity.
Semicircular ducts detect changes in rotational movement.
Hair cells within these structures detect changes in movement and send signals to the brain.

Autonomic Nervous System

The autonomic nervous system controls involuntary bodily functions such as heart rate, respiration, digestion, and body temperature.
The autonomic nervous system has two divisions:
- Sympathetic nervous system: Prepares the body for "fight or flight" responses.
- Parasympathetic nervous system: Promotes "rest and digest" functions.
Sympathetic nervous system
- Adrenergic (releases neurotransmitters norepinephrine and epinephrine).
- Most effector tissue innervation.
- Chromaffin cells in the adrenal medulla release norepinephrine and epinephrine into the bloodstream.
Parasympathetic nervous system
- Cholinergic (releases neurotransmitter acetylcholine).
Agonists and Antagonists
- Drugs and poisons can target the receptors of the autonomic nervous system, acting as agonists (mimicking the effects of neurotransmitters) or antagonists (blocking the effects of neurotransmitters).

Somatic Nervous System

The somatic nervous system controls voluntary movement.
Neuromuscular junction:
- The connection between a motor neuron and a muscle fiber.
- Acetylcholine (neurotransmitter) released from the motor neuron binds to receptors on the muscle fiber, generating an end plate potential (EPP).
- The EPP triggers an action potential in the muscle fiber, leading to muscle contraction.
Electromyography
- Electromyography (EMG) is a technique that measures muscle fiber action potentials.
- Useful in diagnosing muscle disorders.

Skeletal Muscle

Skeletal muscle: Striated, voluntary muscle responsible for movement.
Muscle fiber (cell): The basic unit of skeletal muscle.
- Striations: Alternating light and dark bands within muscle fibers, resulting from the arrangement of protein filaments.
Types of Muscle:
- Type I (slow twitch): Red muscle fibers; contract slowly and are fatigue-resistant.
- Type IIa (fast twitch oxidative): Intermediate muscle fibers; contract quickly and are fatigue-resistant.
- Type IIb (fast twitch glycolytic): White muscle fibers; contract quickly and are easily fatigued.
Muscle Function:
- Muscle contraction: The process of muscle shortening caused by the sliding of protein filaments (actin and myosin).
- Muscle relaxation: The process of muscle lengthening caused by the removal of calcium ions from the muscle fiber.