section 1, chapter 10

Nervous System IBasic Structure and Function

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The nervous system is divided into 2 subdivisions

The Central Nervous System (CNS) consists of the brain and spinal cord.

The Peripheral Nervous System (PNS)• Consists of 12 pairs of cranial

nerves and 31 pairs of spinal nerves

• Nerves may be motor (efferent), sensory (afferent), or both (mixed)

Divisions of the PNS

The Somatic Nervous System is under voluntary control

Somatic motor controls skeletal muscles

Somatic sensory relays info regarding touch, pressure, and pain to the brain

The Autonomic Nervous System is under involuntary control

autonomic motor controls smooth muscles, cardiac muscles, and glands

autonomic sensory relays visceral info regarding pH, blood gasses, etc. to the brain

Figure 10.2. (a) overview of nervous system. CNS is grey, PNS is yellow. (b) CNS receives sensory input from PNS, and sends motor output to PNS. Somatic division of PNS is under voluntary control, while the autonomic division is under involuntary control.

The Autonomic Nervous System (ANS) is further divided into two branches.

The Sympathetic branch • prepares the body to respond to a stressful situation.• “Fight or Flight” Response

The Parasympathetic branch • Maintains normal body activities at rest• “Resting and Digesting”

Cells of the Nervous SystemNeurons• Integrate, regulate, and coordinate body functions

• Functions• Receive information - sensory • Conduct impulses - motor• Connect neurons - integrative

Neuroglia (glia = “glue”)• Neuroglia provide neurons with

nutritional, structural, and functional support

Neurons

Neurons vary in shape and size

3 Components of a neuron1. Dendrites receive impulse

2. Call body (soma)

3. Axon – transmits the impulse away from the cell body

Dendrites conduct information to the soma.

A cell may have a few dendrites, many dendrites, or no dendrites.

Dendritic Spines are additional contact points on some dendrites that increase the number of synapses possible by a neuron

Cell Body – SomaContains organelles such as the nucleus Mitochondria, Golgi Apparatus, etc.

The rough ER is often called chromatophilic substance (Nissl Bodies)

Dendrites

Axon

Axon Hillock is a specialized part of the soma that connects to the axon.

The axon hillock is often called the Trigger Zone because action potentials begin here.

Each neuron has only 1 axon, but it may divide into several branches, called collaterals

The end of the axon is called the axon terminal and it enlarges into a synaptic knob (bouton)

Microtubules called neurofibrils support long axons and aid in axonal transport (transport of biochemicals between the soma and the axon terminal)

Axon

Schwann Cells form the myelin sheath in the PNS. They wrap around the axons in a jelly-roll fashion to form a thick layer of fatty insulation.

The cytoplasm and the nucleus are pushed to the outermost layer, forming the neurolemma

Schwann cells are separated by gaps, called Nodes of Ranvier.

Myelination of axons occurs differently in the PNS than in the CNS.

Myelination of Axons

The myelin sheath is a thick fatty coating of insulation surrounding the axon that greatly enhances the speed of impulses.

Myelination of axons in the PNS.

Schwann cells still surround the axons of unmyelinated neurons, but they do not form the myelin sheath.

Unmyelinated axons with a Schwann cell.

A myelinated axon with a Schwann Cell.

Myelination of Axons in CNS

end of section 1, chapter 10

Within the CNS the myelin sheath is formed by Oligodendrocytes.

1 Oligodendrocyte may form the myelin sheath of several axons.

A mass of myelinated axons in the CNS forms the white matter.

A mass of cell bodies (which are unmyelinated) along with unmyelinated axons form the gray matter.

Gray and White Matter