Brainstem

Medulla, Pons, Midbrain, and Reticular Formation

The brainstem is a critical structure connecting the spinal cord to the higher brain regions. It is composed of the medulla oblongata, pons, and midbrain, and contains the reticular formation, which is essential for consciousness and vital functions.

• Medulla Oblongata: Controls autonomic functions such as heart rate, blood pressure, and respiration.

• Pons: Contains nuclei and tracts that relay information between the cerebellum and cerebrum. It works with the medulla in the regulation of breathing.

• Midbrain (Mesencephalon): Junction between lower brainstem and diencephalon. Main functions include controlling eye movement and relaying auditory and visual information. Contains the substantia nigra, important for movement regulation.

• Reticular Formation: A network of small clusters of neurons interspersed among ascending and descending tracts throughout the brainstem. It is important in maintaining consciousness, arousal, attention, and alertness. It also assists in vital functions such as heart rate and blood pressure.

Example: Damage to the reticular formation can result in coma or loss of consciousness.

Cerebellum

Structure and Function

The cerebellum is the largest brain structure after the cerebrum and is located posterior to the brainstem. It is divided into two hemispheres and is responsible for coordinating movement and balance.

• Receives sensory information related to movement and position.

• Regulates execution of movement by sending feedback signals to motor areas of the cerebral cortex via connections to the thalamus and pons.

• Corrects errors in both voluntary movements and posture.

Example: Cerebellar damage can cause ataxia, a loss of coordination and balance.

Diencephalon

Thalamus, Hypothalamus, Pineal Gland, and Pituitary

The diencephalon is located between the brainstem and cerebrum and includes several key structures involved in sensory relay, endocrine function, and homeostasis.

• Thalamus: Acts as a relay center, receiving sensory information from the optic tract, spinal cord, and motor information from the cerebellum, then projecting it to the cerebrum for processing.

• Hypothalamus: Regulates homeostasis, autonomic functions, and endocrine activity. Controls body temperature, osmolarity, reproductive functions, and interacts with the limbic system for emotional behavior.

• Pineal Gland: Releases melatonin, which is involved in circadian rhythms and sleep/wake cycles.

• Pituitary Gland: Divided into anterior (endocrine tissue) and posterior (neural tissue) lobes. The hypothalamus controls the release of hormones from both lobes, affecting growth, metabolism, and reproduction.

Example: The hypothalamus stimulates the release of vasopressin from the posterior pituitary to regulate water balance.

Cerebrum

Basal Ganglia, Limbic System, and Cerebral Cortex

The cerebrum is the largest and most distinctive part of the brain, responsible for higher processing and intelligence. It is divided into two hemispheres and four lobes, connected by the corpus callosum.

• Basal Ganglia (Nuclei): Includes the globus pallidus, putamen, and caudate nucleus. Regulates initiation and termination of movement, receives input from the cerebral cortex, and provides output to motor areas.

• Limbic System: The "emotional brain" involved in pain, pleasure, docility, affection, anger, learning, and memory. Major components are the cingulate gyrus, amygdala, and hippocampus.

• Cerebral Cortex: The outermost layer, serving as the integrating center for the CNS. Contains sensory areas (perception), motor areas (movement), and association areas (integration and voluntary behavior).

Example: The hippocampus is essential for the formation of new memories.

Cerebral Lateralization

Functional areas in the two hemispheres are not symmetrical. For example, language centers are typically located in the left hemisphere.

Sensory Physiology

General Properties of Sensory Systems

Sensory systems provide information about the environment and the internal state of the body. Sensory neurons have transducers (receptors) that convert physical stimuli into intracellular signals, usually by changing membrane potential through gated channels.

• Transduction: Conversion of stimulus energy into a change in membrane potential.

• Receptive Fields: The specific area where a sensory neuron is activated by a stimulus.

• Integration: Sensory information is processed and integrated in the CNS.

• Coding and Processing: Sensory systems encode modality, location, intensity, and duration of stimuli.

Equation:

Types of Sensory Receptors

Sensory receptors are specialized to detect different types of stimuli.

• Chemoreceptors: Detect chemical changes (e.g., oxygen, pH).

• Mechanoreceptors: Detect pressure, cell stretch, vibration, acceleration, and sound.

• Photoreceptors: Detect photons of light (vision).

• Thermoreceptors: Detect varying degrees of heat.

• Nociceptors: Detect pain and tissue damage.

Example: Proprioceptors provide information about body position and movement.

Table: Sensory Receptor Types and Stimuli

Receptive Fields

Each sensory neuron is activated by stimuli within a specific area called its receptive field. Overlapping receptive fields allow for more precise localization of stimuli.

• Small receptive fields: Allow for fine discrimination (e.g., fingertips).

• Large receptive fields: Less precise localization (e.g., back).

Example: Two-point discrimination is better in areas with small receptive fields.

Additional info:

• Neurophysiology experiments (as shown in the first image) often use electrical stimulation at different frequencies (e.g., 1Hz, 2Hz) to study neuronal activity and synaptic responses.

• Basal ganglia and substantia nigra are involved in movement disorders such as Parkinson's disease.