Neurons Vary in size and structure, but have common features: Cell Body Dendrites Axon Mature neurons do not divide, but neural stem cells can divide and form neurons or neuroglial cells.

1. Cell Body Contains normal cellular structures (golgi apparatus, mitochondria, cytoplasm, cell membrane, etc.) Neurofibrils – fine threads that extend into the axon Nissl bodies (chromatophilic substances) Membranous sacs in the cytoplasm Similar to rough ER Ribosomes on Nissl bodies synthesize ______

2. Dendrites Usually short and highly branched (dendr = ?) The main receptive surfaces for receiving communication from axons of other neurons

3. Axons Arise from a slight elevation of the cell body, called the axonal hillock. Conduct nerve impulses away from the cell body Contains many mitochondria, microtubules, and neurofibrils Originates as a single structure, but may have branches, especially at the end to interact with receptive surfaces of other cells

PNS Axons Enclosed in myelin sheaths composed of many Schwann cells Myelin is a lipoprotein. Neurilemma sheath surrounds the myelin sheath Nodes of Ranvier – narrow gaps in the myelin sheath between the Schwann cells

Classification of Neurons Classification based on Structural differences: Bipolar neurons Unipolar neurons Multipolar neurons Classification based on Functional differences: Sensory neurons (afferent neurons) Interneurons (association or internuncial neurons) Motor neurons (efferent neurons)

Structural Differences Sketch the neurons below. Notes on the next 3 slides:

Structural Differences, cont….. Bipolar: 2 processes Axon Dendrite Found in specialized parts of the eyes, nose, and ears

Structural Differences, cont….. Unipolar: 1 process divides into 2 branches, which function as a single axon 1 branch (peripheral process) associated with dendrites Other branch (central process) enters brain or spinal cord

Structural Differences, cont….. Multipolar: Many processes arising from cell body: 1 axon Many dendrites Most neurons whose cell bodies lie in the brain or spinal cord are multipolar. Direction of impulse is ALWAYS from dendrites to axon.

Functional Differences Sensory (afferent) neurons From peripheral body parts to the brain or spinal cord Have specialized receptor ends at the tips of their dendrites OR Dendrites closely associated with receptor cells in the skin or sensory organs. Most are unipolar, but some are bipolar.

Functional Differences, cont….. Interneurons (association or internuncial neurons) Lie entirely in the brain or spinal cord Multipolar and link other neurons Transmit impulses from one part of the brain or spinal cord to another Motor (efferent) neurons Multipolar Carry nerve impulses from brain or spinal cord to effectors Stimulate muscles or glands