Summary
Highlights
The nervous system is divided into the Central Nervous System (CNS), comprising the brain and spinal cord, and the Peripheral Nervous System (PNS), which includes all nerves extending outwards. Its functions include sensory input, information integration, muscle and gland control, maintaining homeostasis, and establishing mental activity and emotions.
The sensory division conducts action potentials from sensory receptors to the CNS. The motor division carries action potentials to organs like muscles and glands. The somatic nervous system transmits signals from the CNS to skeletal muscles. The autonomic nervous system controls smooth and cardiac muscles, while the enteric nervous system is solely involved with the digestive tract.
Neurons receive stimuli, conduct action potentials, and send signals. Glial cells are supportive cells that enhance neuron function, found in both CNS and PNS. Key parts of a neuron include the cell body (containing the nucleus), dendrites (receiving chemical signals), and the axon (a long tail-like structure for sending signals). Neurons can be multipolar (many dendrites, one axon), bipolar (one dendrite, one axon), or pseudo unipolar (one process branching into two).
Glial cells include astrocytes (stimulate or inhibit neuron signaling, form blood-brain barrier), ependymal cells (line brain cavities, produce cerebrospinal fluid), microglia (immune-like function, remove debris), oligodendrocytes (provide myelin in CNS), and Schwann cells (provide myelin in PNS). The myeline sheath, made of proteins and fatty substances, insulates axons, increasing the speed and efficiency of action potentials. Gaps in the myelin sheath are called Nodes of Ranvier. Disorders like multiple sclerosis affect myelin, leading to muscle dysfunction.
Myelinated axons are fully wrapped in a myelin sheath, while unmyelinated axons are often embedded within glial cells but not fully covered. The color of nervous tissue depends on myelination: gray matter has little myelin, while white matter consists of bundles of myelinated axons.
Nerve cells communicate through excitable responses to stimuli. The speed of action potential conduction varies based on axon diameter: larger diameters result in faster conduction velocities (e.g., 120 m/s for larger axons vs. 15 m/s for medium axons).
Synapses are spaces where neurons interact. The axon terminal of the transmitting neuron is the presynaptic terminal, and the receiving neuron's membrane is the postsynaptic terminal. Neurotransmitters are chemical signals released by neurons at the presynaptic terminal to communicate with other neurons. Acetylcholine and norepinephrine are well-known examples, with acetylcholinesterase breaking down acetylcholine.
Converging pathways involve multiple neurons sending signals to a single neuron. Diverging pathways involve one neuron sending signals to multiple other neurons.
The spinal cord, protected by the vertebral column, extends from the medulla oblongata. Damage to the spinal cord can lead to paralysis, with higher damage causing more extensive paralysis. It contains gray matter (butterfly-shaped, centrally located, less myelin) and white matter (outer areas, more myelin). Different regions of gray matter (posterior, anterior, and lateral horns) house neurons with specific functions, such as sensory, somatic, and autonomic.
Reflexes are involuntary actions that allow quick reactions to stimuli without conscious thought. A reflex arc is the neuronal pathway for a reflex, consisting of five steps: receptor receives stimulus, sensory neuron processes it, interneurons relay the signal, motor neurons communicate with muscles, and finally, effector organs react. Examples include the stretch reflex (like the knee-jerk reflex) and the withdrawal reflex.
The brain has four major regions: brain stem, cerebellum, diencephalon, and cerebrum. The brain stem includes the medulla oblongata (controls heart rate, breathing, involuntary actions like coughing), pons (bridge between cerebrum and cerebellum, aids in breathing, chewing, swallowing), and midbrain (coordinates eye movement, pupil diameter adjustment, head turning towards noises).
The cerebellum, known as the 'little brain,' is attached to the brain stem and primarily controls balance and coordination. The diencephalon comprises the thalamus (largest part, regulates sensory input, responsible for moods), epithalamus (reacts to smell, houses the pineal gland), and hypothalamus (controls the pituitary gland, responsible for many bodily functions).
The cerebrum is the largest part of the brain, divided into left and right hemispheres by the longitudinal fissure. The cerebral cortex, made of gray matter, is responsible for thinking, communication, memory, understanding, and voluntary movement. It features gyri (folds) and sulci (shallow indentations/fissures). More wrinkles in the brain are correlated with higher IQ. The left hemisphere controls the right side of the body and is analytical (language, math), while the right hemisphere controls the left side and is creative (music, art, abstract ideas). Both hemispheres work together, connected by the corpus callosum.
The cerebral lobes include the frontal lobe (voluntary motor functions, mood, aggression), parietal lobe (processes sensory input), occipital lobe (controls vision), and temporal lobe (hearing, smelling, memory). Sensory speech (comprehension) is processed by Wernicke's area in the parietal lobe, while motor speech (formulation) is processed by Broca's area in the frontal lobe.
Brain disorders are diagnosed using electroencephalograms (EEGs) to monitor electrical activity. Different brain waves include alpha waves (awake, quiet), beta waves (intense mental activity), delta waves (deep sleep), and theta waves (typically in children's brains).
Working memory stores information for short periods, relevant to current tasks. Short-term memory lasts longer and can be consolidated into long-term memory through a process of creating new and stronger synaptic connections. Long-term memory includes declarative memory (facts, feelings, names, dates) and procedural memory (motor skills, muscle memory). The limbic system influences long-term declarative memory, emotions, and motivation.
The brain and spinal cord are covered by meninges. The outermost layer is the dura mater (toughest). The second layer is the arachnoid matter, with the subdural space between it and the dura mater. The innermost layer is the pia mater, tightly bound to the brain and spinal cord surface. The subarachnoid space (between arachnoid and pia mater) contains blood vessels. Cerebrospinal fluid (CSF), produced by ependymal cells, provides a fluid cushion for the CNS.
The autonomic nervous system controls involuntary functions of smooth muscles, cardiac muscles, and glands. It has two divisions: the sympathetic division (for physical activities, e.g., running) and the parasympathetic division (for involuntary functions like digestion). The enteric nervous system consists of neurons located within the digestive tract, with sensory neurons connecting it to the CNS, and sympathetic and parasympathetic neurons connecting the CNS to the digestive tract.