Neuron Classification and Structure Quiz

Neuron Classification and Function

• Neurons are structurally classified as multipolar, bipolar, or unipolar based on the number of processes extending from the cell body.
• Multipolar neurons, the most common type in humans, have three or more processes, including one axon and multiple dendrites.
• Bipolar neurons possess two processes—an axon and a dendrite—found in special sense organs like the retina and olfactory mucosa.
• Unipolar neurons have a single short process that divides into proximal and distal branches, functioning mainly as sensory neurons in the PNS.
• Functionally, neurons are grouped as sensory, motor, and interneurons based on the direction of nerve impulse transmission relative to the CNS.
• Sensory neurons, mainly unipolar, transmit impulses from sensory receptors towards the CNS, with long peripheral processes.
• Motor neurons, multipolar, carry impulses away from the CNS to effector organs like muscles and glands.
• Interneurons, also multipolar, act as association neurons within the CNS, facilitating signal integration in neural pathways.
• Neurons have a resting membrane potential due to differences in ion concentration and permeability across their membranes.
• Membrane ion channels, including chemically gated, voltage-gated, and mechanically gated channels, regulate ion movement across cellular membranes.
• Electrochemical gradients, comprising concentration and electrical gradients, determine the direction of ion movement across the membrane.
• The resting membrane potential is generated by the balance between the chemical and electrical gradients, driving the net flow of ions in and out of the cell.

Neuron Structure and Function

• Nerve impulses are generated and transmitted along the axolemma, typically away from the cell body.
• In motor neurons, the nerve impulse is generated at the initial segment of the axon and conducted to the axon terminals.
• Axon terminals release neurotransmitters into the extracellular space, which either excite or inhibit neurons, muscle, or gland cells.
• Axons lack rough endoplasmic reticulum and a Golgi apparatus, relying on the cell body for protein synthesis and efficient transport mechanisms.
• Axonal transport occurs in both anterograde (away from the cell body) and retrograde (toward the cell body) directions, facilitated by motor proteins and microtubules.
• Myelin sheaths protect and electrically insulate nerve fibers, increasing the transmission speed of nerve impulses.
• Myelin sheaths in the peripheral nervous system (PNS) are formed by Schwann cells, while in the central nervous system (CNS), they are formed by oligodendrocytes.
• Myelin sheaths in the PNS contain myelin sheath gaps called Nodes of Ranvier and an outer collar of perinuclear cytoplasm (neurilemma).
• Nonmyelinated nerve fibers associated with Schwann cells in the PNS are thin fibers and are covered by the long extensions of adjacent glial cells in the CNS.
• Myelinated fibers conduct nerve impulses rapidly, while nonmyelinated fibers conduct impulses more slowly.
• In the PNS, myelin sheaths are formed by Schwann cells, which wrap themselves around the axon in a jelly roll fashion, gradually squeezing out cytoplasm to form the myelin sheath.
• Myelin sheaths in the CNS lack an outer collar of perinuclear cytoplasm, and the smallest-diameter axons are nonmyelinated and covered by the long extensions of adjacent glial cells.

Functional and Structural Areas of the Cerebral Cortex

• The Cerebral Cortex is divided into functional and structural areas, including the primary motor and somatosensory cortex, multimodal association areas, and anterior, posterior, and limbic association areas.
• The Anterior Association Area, also known as the prefrontal cortex, is involved in intellect, complex learning, recall, personality, and contains working memory.
• The Posterior Association Area plays a role in recognizing patterns, faces, spatial localization, sensory integration, and language comprehension.
• The Limbic Association Area, which includes the cingulate gyrus, parahippocampal gyrus, and hippocampus, is responsible for emotional impact and memory formation.
• Lateralization of Cortical Functioning involves the division of labor between the cerebral hemispheres, with the left hemisphere dominant for language, math, and logic in about 90% of people.
• The remaining 10% may have reversed or shared hemisphere functions, and right-cerebral-dominant individuals are often left-handed.
• The two cerebral hemispheres have almost instantaneous communication and complete functional integration, with each hemisphere having specific abilities.
• Cerebral White Matter, the second region of each cerebral hemisphere, is responsible for communication within the cortex and with lower CNS centers, consisting of myelinated fibers classified as association, commissural, or projection fibers.
• Association fibers connect different parts of the same hemisphere, while commissural fibers connect corresponding gray areas of the two hemispheres, with the largest commissure being the Corpus Callosum.
• Projection fibers transmit sensory information to the cerebral cortex and motor output from it, connecting the cortex to the rest of the nervous system and the body's receptors and effectors.
• The Basal Nuclei, deep within the cerebral white matter, are involved in the control of movement, and include the caudate nucleus, putamen, and globus pallidus.
• The caudate nucleus, putamen, and globus pallidus are the primary structures forming the basal nuclei, and are primarily involved in movement control.

Development and Structure of the Adult Brain

• The adult brain is composed of four main regions: Cerebral Hemispheres, Diencephalon, Brain Stem, and Cerebellum.
• Gray matter consists of nonmyelinated neurons and neuron cell bodies, while white matter consists mostly of myelinated axons with some nonmyelinated axons, primarily in fiber tracts.
• The brain ventricles are filled with cerebrospinal fluid and lined by ependymal cells, and they are continuous with each other and with the central canal of the spinal cord.
• The cerebral hemispheres are the most visible parts of the brain and account for about 83% of total brain mass.
• The cerebral hemispheres consist of cortex, white matter, and basal nuclei, and are divided into lobes - frontal, parietal, temporal, occipital, and insula.
• The cerebral cortex is the "Executive Suite" of the nervous system, composed of gray matter and containing billions of neurons arranged in six layers.
• Modern imaging techniques, such as PET scans and functional MRI scans, reveal that specific motor and sensory functions are localized in discrete cortical areas called Domains.
• The basic pattern of the central nervous system (CNS) is a central cavity surrounded by gray matter, external to which is white matter.
• The third ventricle is continuous with the fourth ventricle via the cerebral aqueduct that runs through the midbrain.
• The cerebral hemispheres fit snugly in the skull, with the occipital lobes located well superior to the posterior cranial fossa.
• Each of the cerebral hemispheres has three basic regions: a superficial cerebral cortex of gray matter, internal white matter, and basal nuclei.
• Many higher mental functions, such as memory and language, appear to be spread over large areas of the cortex in overlapping domains.