APK 2100C Chapter 12

What are the three overlapping functions of the nervous system?

Sensory input, integration, and motor output.

Central Nervous System (CNS)

Consists of the brain and the spinal cord, which occupy the cranium and vertebral canal, respectively.

What is the integrating and command center of the nervous system?

The central nervous system (CNS).

Peripheral Nervous System (PNS)

Part of the nervous system outside the CNS, consists mainly of the nerves that extend from the brain and spinal cord.

Cranial nerves carry signals to and from the __________, whereas spinal nerves carry signals to and from the ____________.

brain; spinal cord

Ganglia

Included in the PNS, are areas where the cell bodies of neurons are clustered.

The nervous system receives ___________ and dictates __________.

sensory inputs; motor outputs

Sensory or afferent division

Signals are picked up by sensory receptors located throughout the body and carried by nerve fibers of the PNS into the CNS.

Motor or efferent division

Signals are carried away from the CNS by nerve fibers of the PNS to innervate the muscles and glands, causing these organs either to contract or to secrete.

Somatic body region

Consists of the structures external to the ventral body cavity. Ie. skin, skeletal musculature, bones, etc.

Visceral body region

Mostly contains the viscera within the ventral body cavity, which means the structures of the body's inner tube (digestive tube, lungs, heart, bladder, etc).

What are the four main subdivisions of the PNS?

1. Somatic Sensory

2. Visceral Sensory

3. Somatic motor

4. Visceral motor

Somatic motor

or voluntary motor, the motor innervation of the outer tube, specifically skeletal muscles.

Visceral motor

or autonomic nervous system (ANS), the involuntary motor innervation of the inner tube, specifically smooth muscle, cardiac muscle, and glands, as well as some outer tube structures.

General somatic senses

Senses whose receptors are spread widely throughout the outer tube of the body.

Proprioception

A sense that detects the amount of stretch in muscles, tendons, and joint capsules.

Special somatic senses

The somatic senses whose receptors are confined to relatively small areas rather than spread widely throughout the body.

Equilibrium

The sense of balance; measures both the position and the movement of the head.

General visceral senses

Stretch, pain, and temperature, which can be felt widely in the digestive and urinary tracts, reproductive organs, and other viscera. Sensations such as hunger and nausea are also this type of sensation.

Special visceral senses

Also referred to as the chemical senses, have their sensory receptors localized to the tongue and nasal cavity. Taste and small are considered this type of sensation.

Part of the PNS stimulates contraction of the skeletal muscles in the body.

Somatic motor

Part of the PNS regulates the contraction of smooth and cardiac muscle and secretion by the body's many glands.

Visceral motor

General visceral motor neurons make up the ____________, which controls the function of the visceral organs.

autonomic nervous system (ANS)

What two divisions make up the autonomic nervous system?

The sympathetic and parasympathetic divisions.

Sympathetic division

Gets the body ready for activity; "fight or flight"

Parasympathetic division

Conserves energy and promotes digestion; "rest and digest"

The nervous system consists mostly of ___________ tissue.

nervous

Nervous tissue

Cells are densely packed and tightly intertwined.

What two main types of cells make up nervous tissue?

Neurons and neuroglia.

Neurons

The excitable nerve cells that transmit electrical signals.

Neuroglia

Nonexcitable supporting cells that surround and wrap the neurons.

What are the basic structural units of the nervous system?

Neurons or nerve cells.

Signals are transmitted along the plasma membrane known as _____________.

neurilemma

Nerve impulses/action potentials

A reversal of electrical charge that travels rapidly along the neuronal membrane.

Cell body

Soma; all consist of a single nucleus surrounded by cytoplasm. The nucleus is spherical and clear and contains a dark nucleolus near its center.

Chromatophilic substance

Large clusters of rough endoplasmic reticulum and free ribosomes that stain darkly with basic dyes. These cellular organelles continually renew the membranes of the cell and the protein components of the cytosol.

Neurofibrils

Bundles of intermediate filaments that run in a network between the chromatophilic substance.

Neurofilaments

Keep the cell from being pulled apart when it is subjected to tensile forces.

Where are most neuron cell bodies located?

Within the CNS.

Where do ganglion lie?

Along the nerves in the PNS.

Processes

Extend from the cell bodies of all neurons.

What are the two types of processes?

Dendrites and axons.

Dendrites

Processes that branch from the cell body like the limbs on a tree. These function as receptive sites, providing an enlarged surface area for receiving signals from other neurons. They conduct electrical signals toward the cell body.

Axons

A neuron has only one of these, which arises from a cone-shaped region of the cell body. These are thin processes of uniform diameter throughout their length. These are impulse generators and conductors that transmit nerve impulses away form their cell body.

Avon hillock

Axons arise from these.

Both the axon and the axon hillock are missing ___________ and ____________.

chromatophilic substance; golgi apparatus

Axons must receive their protein components from the _______________.

cell body

Nerve fiber

Any long axon.

Axons with ____________ diameters conduct impulses faster than those with ____________ diameters.

larger; smaller

Axon collaterals

Extend from the axon at more or less right angles.

Terminal arborization

Where the axon branches profusely at its end.

Terminal boutons

A knob also known as an axon terminal.

Where is a nerve impulse typically generated?

Where the axon extends from the axon hillock and is conducted along the axon to the terminal boutons.

Neurotransmitters

Chemicals that are released into the extracellular space. These excite or inhibit the neurons or target organs that are in close contact with the axon boutons.

Synapse

Site at which neurons communicate.

Presynaptic neuron

The neuron that conducts signals toward a synapse.

Postsynaptic neuron

The neuron that transmits signals away from the synapse.

Axodendretic synapses

Synapses occurring between the terminal bouton of one neuron and the dendrites of another neuron.

Axosomatic synapses

Synapses occur between axons and neuron cell bodies.

Synaptic vesicles

Membrane-bound sacs filled with neurotransmitters.

What is abundant in the terminal and why?

Mitochondria because the secretion of neurotransmitters requires a great deal of energy.

Synaptic cleft

The separation between the plasma membranes of two neurons.

How does a synapse function?

When an impulse travels along the axon of the presynaptic neuron, the impulse stimulates the synaptic vesicles to fuse with the presynaptic membrane. The fused area then ruptures, causing the vesicle to release their neurotransmitter molecules, which diffuse across the synaptic cleft and bind to the postsynaptic membrane. This binding changes the membrane charge on the postsynaptic neuron, influencing the membrane's ability to generate a nerve impulse.

How are neurons grouped structurally?

By the number of processes that extend from the cell body.

Multipolar neurons

Have more than two processes. They usually have numerous dendrites and a single axon. Some small ones have no axon and rely strictly on their dendrites for conducting signals.

Bipolar neurons

Have two processes that extend from opposite side of the cell body. These are very rare and occur in some of the special sensory organs, where they mostly serve as sensory neurons.

Unipolar neurons

Have a short single process that emerges from the cell body and divides like an inverted T into two long branches. Sometimes are called pseudounipolar neurons.

How are neurons grouped functionally?

According to the direction the nerve impulse travels relative to the CNS.

Sensory neurons

Also known as afferent neurons make up the sensory division of the PNS. They transmit impulses toward the CNS from sensory receptors in the PNS. Most are pseuounipolar.

Motor neurons

Also known as efferent neurons make up the motor division of the PNS. These neurons carry impulses away from the CNS to effector organs. These are multipolar.

Interneurons

These lie between motor and sensory neurons. These multipolar neurons are confined entirely to the CNS.

What do neuroglia do?

They provide supportive scaffolding for neurons and they cover all nonsynaptic parts of the neurons, thereby insulating the neurons and keeping the electrical activities of adjacent neurons from interfering with each other.

How can neuroglia be distinguished from neurons?

Neuroglia are much smaller and have darker staining nuclei.

Can neuroglia cells divide throughout life?

Yes.

Astrocytes

Star-shaped and are the most abundant glial cells of the CNS. They have many radiating processes with bulbous ends. The functions are numerous and not fully understood.

What are three known functions of astrocytes?

1. Regulating neurotransmitter levels by increasing the rate of neurotransmitter uptake in regions of high neuronal activity.

2. Signaling increased blood flow through capillaries in active regions of the brain.

3. Controlling the ionic environment around neurons.

Microglial cells

The smallest and least abundant neuroglia of the CNS, have elongated cell bodies and cell processes with many pointed projections. They are phagocytes, the macrophages of the CNS. These are derived from blood cells called monocytes.

Ependymal cells

Form a simple epithelium that lines the central cavity of the spinal cord and brain. These cells provide a fairly permeable layer between the cerebrospinal fluid that fills this cavity and the tissue fluid that bathes the cells of the CNS. These cells bear cilia that help circulate the cerebrospinal fluid.

Oligodendrocytes

These cells have fewer branches than astrocytes. They line up in small groups and wrap their cell processes around the thicker axons in the CNS, producing insulating coverings called myelin sheaths.

What are the two types of neuroglia in the PNS?

Satellite cells and Schwann cells.

Satellite cells

Surround neuron cell bodies within ganglia.

Schwann cells

Surround all axons in the PNS and form myelin sheaths around many of these axons.

Myelin sheaths

Produced by oligodendrocytes in the CNS and Schwann cells in the PNS. These are segmented structures that are composed of the lipoprotein myelin and surround the thicker axons of the body. They form an insulating layer that prevents the leakage of electrical current from the axon, increases the speed of impulse conduction along the axon, and makes impulse propagation more energy-efficient.

The myelin sheaths in the PNS are formed by _____________ cells.

Schwann

Myelin sheath gaps

Also known as nodes of Ranvier, occur at regular intervals about 1 mm apart. Nerve impulses jump from the membrane of one myelin sheath gap to the next in a way that greatly speeds impulse conduction.

Nonmyelinated axons

Are thin, slowly conducting axon that lack a myelin sheath. Schwann cells surround such axons but do not wrap around them in concentric layers of membrane.

What forms the myelin sheaths in the brain and spinal cord?

Oligodendrocytes.

Gray matter

A gray-colored zone that surrounds the hollow central cavity of the CNS. In the spinal cord it is a butterfly-shaped region in which the dorsal half contains cell bodies of interneurons and the ventral half contains cell bodies of motor neurons. (Neuron cell bodies are clustered).

White matter

External to the gray matter. It contains no neuron cell bodies but millions of axons and neuroglia. The color comes from the myelin sheaths around many of the axons.

Tracts

Axons traveling to similar destinations form axon bundles.

Cortex

In the cerebrum and cerebellum there's an additional layer of gray matter located superficially.

Nerve

A cabelike organ in the peripheral nervous system.

Endoneurium

A delicate layer of loose connective tissue that surrounds Schwann cells.

Nerve Fascicles

Groups of axons are bound into bundles by a wrapping of connective tissue.

The wrapping of connective tissue that holds axons into bundles is known as ______________.

perineurium

The entire nerve is surrounded by a tough fibrous sheath, the ____________.

epineurium

Reflex arcs

Simple chains of neurons that cause our simplest, reflexive behaviors and reflect the basic structural plan of the nervous system.

Reflexes

Rapid, automatic motor responses to stimuli. They are unlearned, unpremeditated, and involuntary.

What are the five essential components of every reflex arc?

1. Receptor is the site where the stimulus acts. Receptors are located at the terminal end of the peripheral process of a sensory neuron.

2. The sensory neuron transmits the afferent impulses to the CNS.

3. The integration center consist of one ore more synapses in the gray matter of the CNS.

4. The motor neuron conducts efferent impulses from the integration center to an effector.

5. The effector is the muscle or gland cell that responds to the efferent impulses by contracting or secreting.

Monosynaptic reflex

The simplest of all reflexes. There is no interneuron between the sensory neuron and the motor neuron; thus there is only one synapse in the reflex arc.