Nervous System Anatomy and Function

Neuronal Signaling and Structure of the Nervous System

• The central nervous system (CNS) includes the brain and spinal cord. Terminologies include tracts/pathways/commissures and nuclei.
• The peripheral nervous system (PNS) includes nerves and nerve fibers (afferent and efferent). Terminologies include nerves and ganglia.
• The CNS controls skeletal muscles, and the autonomic nervous system (ANS) controls visceral organs. The ANS is divided into sympathetic (emergency) and parasympathetic (rest & digest) divisions.
• Neurons are the basic units of the CNS, comprising 10% of the cell count and 50% of the CNS volume They generate electrical signals and are integrators.
• Glial cells (neuroglia) support neurons, physically and metabolically. Account for 90% of CNS cells. Form myelin. Regulate extracellular fluid (ECF) composition in CNS. Contribute to the blood-brain barrier (BBB). Provide fuel (glucose) to neurons and remove waste (ammonia). Play a potential role in information signaling. Support immune defense in CNS. Regulate cerebrospinal fluid (CSF) production and flow. Support embryonic cells and are a source of growth factors.
• Types of Glia in CNS: Oligodendrocytes (myelin-forming cells). Astroglia (metabolically support neurons). Microglia (serve immune function).
• Types of Glia in PNS: Schwann cells (myelin-forming).
• Dendrites receive information, typically neurotransmitters, and undergo graded potentials.
• Axons undergo action potentials to deliver information, typically neurotransmitters, from the axon terminals.
• Myelinated neurons conduct action potentials most rapidly. Myelin sheath is a collection of Schwann cells and oligodendrocytes associated with the neuron.
• Axonal Transport: Important for maintenance of axonal structure and function. Involves transport of material between soma and axon terminal. Carried out by microtubules and motor proteins (kinesins & dyneins). Consists of anterograde (kinasin-powered) and retrograde (dynein-powered) types.
• Functional Classes of Neurons: Afferent neurons transmit information from tissues/organs to the CNS. Efferent neurons transmit information from the CNS to effector cells. Interneurons are the majority of neuron types, transmitting information within the CNS.
• Sensory receptors are specialized portions of the cell membrane or specialized cell types associated with afferent neurons.
• Neural pathways have long axons between brain regions/spinal cord with few synapses, leading to minimal information processing. Multineural (multisynaptic) pathways utilize many synapses, leading to significant information processing alterations.
• Neural development involves generation, axonal guidance, synapse formation, and fine-tuning of synapses.
• Axonal generation takes place primarily before birth.
• Axonal regeneration can occur outside the CNS.
• Membrane potentials in excitable cells: Basic principles of electricity. Resting membrane potential.
• Factors that determine resting membrane potential involve passive (electrochemical gradients) and active (membrane pumps) forces
• Active factors, such as the Na+/K+-pump, counteract passive forces.
• Action potentials, which involve transient changes in transmembrane potential, signify rapid alterations in membrane potential. They are important for long-distance communication, mainly mediated by voltage-gated Na+ & K+ channels.
• Voltage-gated Na+ channels open rapidly during depolarization, allowing rapid Na+ entry. Voltage-gated K+ channels open more slowly, causing K+ exit and repolarization.
• Refractory periods limit the frequency of action potential generation.
• Factors determining action potential propagation velocity include fiber diameter and myelination.

Synapses

• Synapses are anatomically specialized junctions in the CNS. Synapses can increase/decrease the likelihood of postsynaptic neuron action potentials by eliciting graded potentials.
• Excitatory synapses bring postsynaptic neuron membrane potential closer to threshold.
• Inhibitory synapses drive postsynaptic neuron membrane potential farther from threshold or stabilize at its present level.
• Types of Synapses: Electrical synapse – signals are transmitted via gap junctions, leading to rapid communication but being rare in the mammalian nervous system. Chemical synapse – signals are transmitted by chemical messengers (neurotransmitters).
• Presynaptic neuron releases neurotransmitters into the synaptic cleft.
• Postsynaptic neuron receives neurotransmitters, leading to changes in ion permeability.
• Removal of neurotransmitters from the synaptic cleft terminates the response of the postsynaptic cell, including active transport back into the axon terminal, diffusion away from the synapse, or transformation by enzymes into ineffective substances.
• An excitatory postsynaptic potential (EPSP) is a graded depolarization that moves the membrane potential closer to the threshold for an action potential. An inhibitory postsynaptic potential (IPSP) is a graded hyperpolarization that moves the membrane potential further from the threshold.
• The level of postsynaptic cell excitability is determined by the number of active synapses and the type of active synapses.
• Axo-axonal communication can modify synaptic communication, resulting in either presynaptic inhibition or facilitation.
• Several factors determine synaptic strength in terms of presynaptic, postsynaptic and general factors.

Brain Blood Supply & the Blood Brain Barrier

• Glucose is the sole brain energy substrate. Blood supply is vital for glucose and oxygen to brain; cessation results in neuron death, known as stroke
• The blood-brain barrier (formed by tightly sealed endothelial cells of brain capillaries) regulates the brain extracellular fluid (ECF) chemical composition and minimizes harmful substance access.

Divisions of the Peripheral Nervous System (PNS)

• The PNS is divided into afferent and efferent divisions.
• The somatic nervous system consists of a single neuron connecting the CNS to skeletal muscles, causing excitation.
• The autonomic nervous system has a two-neuron chain connecting the CNS to smooth and cardiac muscles, glands, and GI neurons. This system can mediate excitation or inhibition.
• Afferent division conveys sensory information from the periphery and internal organs to the CNS.
• The autonomic nervous system (ANS) is part of the efferent division, innervating smooth muscle, cardiac muscle, glands, and GI neurons.
• The ANS has 3 subdivisions:
• The sympathetic nervous system involves energy expenditure and reaction to emergencies.
• The parasympathetic nervous system facilitates rest and digestion.
• The enteric nervous system controls the gastrointestinal system.

Ventricular System & Cerebrospinal Fluid (CSF)

• The ventricular system is a network of interconnected cavities filled with CSF.
• The CSF is a filtrate of the blood that flows through the ventricles and spinal cord before being absorbed back into the blood.

The Brain

• The four interconnected ventricles of the brain are fluid-filled cavities.
• The brain is segmented into several parts with specific functions, including the cerebral cortex, thalamus, hypothalamus, and limbic system. The four main lobes of the cerebral cortex are frontal, parietal, temporal, and occipital lobes. The layers of the cerebral cortex integrate afferent and efferent signals.

Spinal cord

• The spinal cord is a component of the central nervous system. (CNS) with two main types of neurons (afferent & efferent).
• Afferent neurons carry sensory information, and efferent neurons carry motor commands.

Cranial Nerves

• Cranial nerves are part of the peripheral nervous system (PNS) connecting the brain to the head and neck.
• They perform various functions, including sensory perception, motor control, and glandular secretion.