Notes Ch. 10b Nervous System 1 Anatomy and Physiology

Description of Cells of the Nervous System Neurons vary shape and size, they can have different lengths and sizes of their axons and dendrites but every neuron has a cell body, dendrites, and an axon.

A neuron’s cell body has cytoplasm, mitochondria, lysosomes, a Golgi apparatus, and many microtubules. They also have fine threads called neurofilaments that go into the axon and supports it. In the cytoplasm are many chromatophilic substance (Nissl bodies), which is made mostly of rough endoplasmic reticulum. Each cell body also has a large, spherical nucleus.

Dendrites are highly branched, and provide receptive surfaces so they communicate with other neurons. Some dendrites have tiny, thorn like spines on their surfaces that provide a contact point for other neurons. A neuron can have many dendrites but only one axon. The axon will have a synaptic cleft - a space between the axon and other neurons.

In general neurons receive input through the dendrites and the cell body and send output in the form of an impulse conducted away from the cell body, down the axon. An axon also moves biochemicals and organelles. This is called axonal transport. This moves these items both directions in the axon.

In the PNS, neuroglia called Schwann cells cover the axons in a lipid-rich sheet. These Schwann cell membranes wind and wrap around axons. There are layers around the Schwann cells made of myelin. This gives the Schwann cell a higher proportion of lipid than other cell membranes. There are narrow gaps between the Schwann cells that are called nodes of Ranvier.

Axons that have myelin sheaths are called myelinated axons and those without are called unmyelinated axons. Myelinated axons conduct impulses very quickly compared to unmyelinated axons. White matter in the brain and spinal cord is made of myelinated axons. Gray matter is unmyelinated axons and neuron cell bodies.

Classification of Cells in the Nervous System There are 3 major groups of neurons based on structural differences. Each type of neuron is specialized to conduct an impulse in one direction.

Multipolar neuron - has many processes arising from its cell body Multipolar neuron - has many processes arising from its cell body. Only one is an axon and the rest are dendrites. These are mostly found in the brain or spinal cord. Ganglia are mass of multipolar neurons located outside the brain and spinal cord.

2. Bipolar neuron has only 2 processes, one coming from each end 2. Bipolar neuron has only 2 processes, one coming from each end. One is an axon and one is a dendrite. Bipolar neurons are found in specialized parts of the eyes, nose, and ears.

3. A unipolar neuron has a single process extending from its cells body. A short distance from the cell body this process divides into two branches which function as a single axon. ONe branch has dendrites and the other branch enters the brain or spinal cord.

Neurons can also be classified by functional differences based on if they carry information into the CNS, completely within the CNS, or out of the CNS.

Sensory neurons - conduct impulses from peripheral body parts into the brain or spinal cord. The dendrites of these neurons act as sensory receptors, detect changes in the outside world or in the body. An example would be getting external stimuli from eyes, ears, or touch receptors in the skin) and relaying that to the brain.

2. Interneurons - are found in the brain or spinal cord 2. Interneurons - are found in the brain or spinal cord. They are multipolar and form links with other neurons. They relay information from one part of the brain or spinal cord to another.

3. Motor neurons - conduct impulses from your brain or spinal cord to effectors. Example - when motor neurons stimulate muscle cells, the muscle cells contract or when motor neurons stimulate glands, the glands release secretions.

Classification of Neuroglia In embryos, neuroglia guide neurons to their positions and may stimulate them to specialize. Neuroglia produce growth factors that nourish neurons and remove excess ions and neurotransmitters.

Neuroglia of the CNS Astrocytes - star-shaped cells found between neurons and blood vessels, they provide support and hold structures together with abundant cellular processes. They have a nutritive function, regulate movement of substances from blood vessels to neurons.

2. Oligodendrocytes - smaller than astrocytes and have fewer processes 2. Oligodendrocytes - smaller than astrocytes and have fewer processes. They can send out the number of processes and helps myelinate axons.

3. Microglia - small cells with fewer processes 3. Microglia - small cells with fewer processes. They are scattered throughout the CNS. They help support neurons and phagocytize bacterial cells and cellular debris.

4. Ependyma - are cuboidal or columnar cells and may have cilia 4. Ependyma - are cuboidal or columnar cells and may have cilia. They are found in the spinal cord.

Many brain disorders are because of abnormal neuroglia Many brain disorders are because of abnormal neuroglia. They have found it is the cause of ALS, Huntington disease.

Neuroglia of the PNS Schwann cells - produce the myelin on peripheral myelinated neurons. Satellite cells - provide nutritional support and help regulate the concentrations of ions around neuron cell bodies in ganglia.

Read page 370 Neuroglia and Axon Regeneration