Nervous Tissue

The nervous system serves as the body’s command center and communication network, with three primary functions:
- Senses changes both within the body and externally.
- Interprets sensory information received.
- Responds to sensory information by initiating movements or secretions.
These functions allow for maintaining homeostasis and ensuring efficient body function.

Composed of several organs:
- Brain and spinal cord.
- Receptors located in sense organs (e.g., eyes, ears).
- Nerves connecting to other body systems.
Nervous tissue contains two types of cells:
- Neurons: Responsible for intercellular communication.
- Neuroglia (glial cells): Essential for survival and functioning of neurons, and help preserve the structure of nervous tissue.

The nervous system is categorized anatomically into:
- Central Nervous System (CNS).
- Peripheral Nervous System (PNS).

Comprises the brain and spinal cord and includes nervous tissue, connective tissue, and blood vessels.
Functions of the CNS include:
- Processes and coordinates sensory data from inside and outside the body.
- Motor commands that control activities of peripheral organs (e.g., skeletal muscles).
- Higher brain functions such as intelligence, memory, learning, and emotion.

Includes all nervous tissue outside the CNS and the enteric nervous system (ENS).
Functions of the PNS are:
- Delivers sensory information to the CNS.
- Carries motor commands to peripheral tissues.

Nerves (or peripheral nerves) are bundles of axons with surrounding connective tissue and blood vessels that carry sensory information and motor commands:
- Cranial Nerves: Connect directly to the brain.
- Spinal Nerves: Connect to the spinal cord.

Afferent Division: Carries sensory information from receptors in peripheral tissues and organs to the CNS.
Efferent Division: Carries motor commands from the CNS to muscles, glands, and adipose tissue.

The efferent division is further split into:
- Somatic Nervous System (SNS): Controls skeletal muscle contractions (both voluntary and involuntary).
- Autonomic Nervous System (ANS): Controls subconscious actions, including contractions of smooth and cardiac muscles, and glandular secretions.
- Sympathetic Division: Has a stimulating effect on body systems.
- Parasympathetic Division: Generally has a relaxing effect on body functions.

Cell Body (Soma): Contains a large nucleus and nucleolus, surrounded by perikaryon (cytoplasm). Components include:
- Mitochondria (energy production).
- Rough endoplasmic reticulum (RER) and ribosomes (protein synthesis).
Dendrites: Short, highly branched processes that extend from the cell body. They include:
- Dendritic Spines: Fine processes on dendrites that receive information from other neurons, accounting for 80-90% of neuron surface area.
Axon: A single, long cytoplasmic process that propagates electrical signals (action potentials) away from the cell body; can range from a couple of mm to a meter in length.

Axolemma: Plasma membrane covering the axoplasm (cytoplasm of axon).
Initial Segment: Base of the axon where action potentials are initiated.
Axon Hillock: Thickened region connecting the initial segment to the cell body.
Collaterals: Branches from the axon allowing communication with multiple cells.
Telodendria: Fine extensions of the distal axon, ending in axon terminals (synaptic terminals).

Anaxonic Neurons: Small neurons with indistinguishable processes; found in the brain and special sense organs.
Bipolar Neurons: Characterized by one dendrite and one axon; found in special sense organs (sight, smell, hearing).
Unipolar Neurons (Pseudounipolar): Axon and dendrites fused, with cell body on one side; typical of most sensory neurons in the PNS.
Multipolar Neurons: Possess one long axon and multiple dendrites; common in CNS and include all motor neurons controlling skeletal muscles.

Sensory Neurons (Afferent Neurons): Detect environmental changes (heat, cold, chemicals) and transmit information to the CNS.
Motor Neurons (Efferent Neurons): Carry the instructions from the CNS to peripheral effectors.
Interneurons: Mostly found in the brain and spinal cord, located between sensory and motor neurons, facilitating connections and higher cognitive functions like memory and planning.

Neuroglia supports and protects neurons, comprising half the volume of the nervous system, with several types including:
- Astrocytes: Maintain the blood-brain barrier, provide structural support, regulate ion concentrations, and repair damaged tissue.
- Ependymal Cells: Line ventricles of the brain and central canal, produce and monitor cerebrospinal fluid (CSF).
- Oligodendrocytes: Myelinate CNS axons to increase speed of action potentials.
- Microglia: Move through nervous tissue to clean up debris and pathogens by phagocytosis.

Types include:
- Satellite Cells: Surround neuron cell bodies in ganglia, regulating O2, CO2, nutrient, and neurotransmitter levels.
- Schwann Cells: Myelinate peripheral axons and facilitate repair processes after injury, with each myelinating only one axon segment while many Schwann cells cover an entire axon.

Wallerian Degeneration: Process following axon injury where the distal part degenerates, and Schwann cells facilitate growth of new axon.
Nerve Regeneration in CNS: Limited by astrocytes, which produce scar tissue that inhibits regrowth.

Synapse: Specialized site where a neuron communicates with another cell, differentiated into presynaptic (sending) and postsynaptic (receiving) neurons.
- Electrical Synapses: Physical contact between cells.
- Chemical Synapses: Involves neurotransmitters bridging synaptic clefts; the most common type between neurons.

Chemical messengers within presynaptic cell’s synaptic vesicles that transmit signals to the postsynaptic membrane, later broken down by enzymes or reabsorbed for reuse.

These components of the nervous system work together to process sensory input, make decisions, and initiate appropriate responses to maintain the body’s homeostasis and function efficiently.

Familiarize with major brain regions, vesicles, and functions.
Understand brain protection, cerebrospinal fluid production, and circulation.
Differentiate anatomical structures such as the medulla oblongata and spinal cord, and identify functions of major brain components.
Recognize cranial reflexes tied to specific stimuli.