The Nervous System. Introduction The nervous system is composed predominantly of neural tissue but also includes some blood vessels and connective tissue.

The Nervous System

Introduction The nervous system is composed predominantly

of neural tissue but also includes some blood vessels and connective tissue.

Neural tissue consists of two cell types: nerve cells (neurons) and neuroglial cells.

Neurons are specialized to react to physical and chemical changes in their surroundings.

Introduction Small cellular processes called

dendrites receive the input, and a longer process called an axon or nerve fiber, carries the information away from the cell in the form of bioelectric signals called nerve impulses.

Nerves are bundles of axons. Neuroglial cells nourish neurons and

perhaps even send and receive messages.

Typical Neuron

Introduction An important part of the nervous system at the

cellular level is not a cell at all, but the small spaces between neurons, called synapses.

Much of the effort of the nervous system centers on sending and receiving electrochemical messages from neuron to neuron at synapses.

Neurotransmitters are biological messenger molecules.

Two Groups of Nervous System Organs The organs of the nervous system can be

divided into two groups.1. Central nervous system (CNS)—consists

of the brain and spinal cord2. Peripheral nervous system (PNS)—

consists of the nerves that connect the central nervous system to other body parts.

General Functions of the Nervous System Three general functions: Sensory,

integrative, and motor. Sensory function derives from sensory

receptors at the ends of peripheral neurons. Integrative function brings sensory signals

together into perceptions. Motor function allows for movement.

Sensory Function Sensory receptors gather information by

detecting changes inside and outside the body. Ex: changes in temperature or light intensity

Sensory receptors convert environmental information into nerve impulses, which are then transmitted over peripheral nerves to the CNS.

Integrative Function When sensory signals arrive at the CNS,

they are integrated: brought together to create sensation, added to memory, or used to create thoughts about perception.

Integrative function allows us to make conscious or subconscious decisions about actions to be taken.

Motor Function When we act on a decision, we use motor

functions. Motor functions employ peripheral neurons, which

carry impulses from the CNS to responsive structures called effectors.

Effectors include muscles that contract and glands that secrete when stimulated by nerve impulses.

The motor portion of the peripheral nervous system can be subdivided into the somatic and the autonomic nervous systems.

Motor Function The somatic nervous system is involved in

conscious (voluntary) activities, such as skeletal muscle contraction.

The autonomic nervous system controls viscera, such as the heart and various glands, and thus controls subconscious (involuntary) actions.

General Functions The nervous system can detect changes in

the body, make decisions on the basis of the information received, and stimulate muscles or glands to respond so the nervous system can help maintain homeostasis.

Structure of a Neuron Terms: Cell body Neurofibrils Nissl bodies (chromatophilic

substance) Dendrites Axons Schwann cells Myelin sheath Nodes of Ranvier

Structure of a Neuron Neurons vary considerably in size and shape, but

they share certain features. Every neuron has a cell body, dendrites, and an

axon. A neuron’s cell body (soma or perikaryon)

contains granular cytoplasm, mitochondria, lysosomes, a Golgi apparatus, and many microtubules.

A network of fine threads called neurofibrils extends into the axons for support.

Structure of a Neuron Scattered throughout the cytoplasm are many

membranous packets of chromatophilic substance (Nissl bodies), which consist of rough endoplasmic reticulum.

The neuron cell body has a large spherical nucleus near the center with a conspicuous nucleolus.

Mature neurons generally do not divide; neural stem cells do.

Structure of a Neuron Dendrites are usually highly

branched, providing receptive surfaces to which processes from other neurons communicate.

Often the dendrites have tiny, thorn-like spines on their surfaces which are contact points for other neurons.

A neuron will have only 1 axon.

Structure of a Neuron The axon is a slender, cylindrical process

with a nearly smooth surface and uniform diameter.

It is specialized to conduct nerve impulses away from the cell body.

The cytoplasm of the axon includes many mitochondria, microtubules, and neurofibrils.

Structure of a Neuron The axon may give off

branches, called collaterals. Near its end, an axon may have

many fine extensions, each with a specialized ending called an axon terminal.

This ends as a synaptic knob very close to the receptive surface of another cell, separated by the synaptic cleft.

Structure of a Neuron The larger axons of peripheral neurons are encased in

lipid-rich sheaths formed by layers of cell membranes of neuroglial cells called Schwann cells, which wind tightly.

The layers are composed of myelin, which has a higher proportion of lipid than other surface membranes.

This coating is called a myelin sheath.

Structure of a Neuron Narrow gaps in the myelin sheath between

Schwann cells are called nodes of Ranvier. Axons that have myelin sheaths are called

myelinated axons and those that lack these sheaths are unmyelinated axons.

Groups of myelinated axons appear white; unmyelinated axons appear gray.

Masses of such axons impart color to the white matter or gray matter in the brain and spinal cord.

Types of Neurons and Neuroglial Cells Neurons are classified into 3 major groups

based on structure.1. Bipolar neurons have 2 nerve fibers (1 at

each end)2. Unipolar neurons have a single nerve fiber

that divides into 2 branches.3. Multipolar neurons have many nerve

fibers arising from their cell bodies.

Bipolar Neurons The 2 process of a bipolar

neuron are similar in structure; however, one is an axon and the other is a dendrite.

These neurons are found in specialized parts of the eyes, nose, and ears.

Unipolar Neuron Each unipolar neuron has a single process

extending from its cell body. A short distance from the cell body, this

process divides into 2 branches, which really function as a single axon.

One branch (peripheral process) is associated with dendrites near a body part.

The other branch (central process) enters the brain or spinal cord.

The cell bodies of some unipolar neurons aggregate in masses or nerve tissue called ganglia, outside the brain and spinal cord.

Multipolar Neuron Multipolar neurons have

many processes arising from their cell bodies.

Only 1 is an axon; the rest are dendrites.

Most neurons whose cell bodies lie within the brain or spinal cord are of this type.

Functional Classification of Neurons

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

3 Functional Classifications

1. Sensory Neurons (afferent neurons)

2. Interneurons (association or internuncial neurons)

3. Motor Neurons (efferent neurons)

Sensory Neurons Sensory neurons carry nerve

impulses from peripheral body parts into the brain or spinal cord.

These neurons have specialized receptor ends at the tips of their dendrites, or they have dendrites that are near receptor cells in the skin or sensory organs.

Interneurons Interneurons lie within the

brain or spinal cord. They are multipolar and

form links between other neurons.

They transmit impulses from one part of the brain or spinal cord to another.

Motor Neuron Motor neurons are multipolar and carry

nerve impulses out of the brain or spinal cord to effectors—structures that respond, such as muscles or glands.

2 specialized groups of motor neurons, accelerator and inhibitory neurons, innervate smooth and cardiac muscles.

Motor Neurons

Accelerator neurons increase muscular activities, whereas inhibitory neurons decrease such actions.

Motor neurons that control skeletal muscle are under voluntary (conscious) control.

Other motor neurons that control glands and smooth and cardiac are under involuntary control.

Classification of Neuroglial Cells Neurons and neuroglial cells are intimately

related, arising from the same neural stem cells and remaining associated throughout their existence.

In the embryo, neuroglial cells guide neurons to their positions and stimulate them to specialize.

Neuroglial cells also produce growth factors that nourish neurons and remove ions and neurotransmitters that accumulate between neurons, enabling them to continue transmitting information.

Classification of Neuroglial Cells The central nervous system contains the

following types of neuroglial cells:

1. Astrocytes

2. Oligodendrocytes

3. Microglia

4. Ependyma

Astrocytes Astrocytes are star-shaped. They are found between neurons and blood

vessels, where they provide support and hold structures together by means of abundant cellular processes.

Astrocytes aid metabolism of certain substances, such as glucose, important ions (potassium)within the interstitial space of nervous tissue.

Astrocytes Astrocytes also respond to injury of

brain tissue and form a special type of scar tissue, which fills spaces and closes gaps in the CNS.

Astrocytes also play an important role in the blood-brain barrier, which restricts movement of substances between the blood and the CNS.

Oligodendrocytes Oligodendrocytes resemble

astrocytes but are smaller and have fewer processes.

They commonly occur in rows along myelinated axons, and they form myelin in the brain and spinal cord.

Oligodendrocytes can send out many processes, each of which forms a myelin sheath around a nearby axon.

Microglia Microglial cells are small and have

fewer processes than other types of neuroglial cells.

They are scattered throughout the central nervous system, whey they support neurons and phagocytize bacterial cells and cellular debris.

They increase in number whenever the brain or spinal cord is inflamed.

Ependyma Ependyma cells are cuboidal or

columnar in shape and may have cilia.

They form the inner lining of the central canal that extends downward through the spinal cord and cover specialized capillaries called choroid plexuses.

They also form a one-cell-thick epithelial-like membrane that covers the inside spaces within the brain called ventricles.

Regeneration of Nerve Axons Injury to the cell body usually kills the neuron, and

because mature neurons do not divide, it is not replaced.

However, a damaged peripheral axon may regenerate.

Macrophages remove the fragments of myelin other debris and the proximal end of the injured axon develops sprouts that develop into a new axon.

Regeneration of Nerve Axons cont. The proximal end of the injured axon

develops sprouts shortly after the injury. One of these sprouts may grow into a tube

and any remaining Schwann cells form new myelin around the growing axon.

Growth is slow

Cell Membrane Potential A cell membrane is usually electrically

charged, or polarized, so that the inside is negatively charged with respect to the outside.

This polarization is due to an unequal distribution of positive and negative ions on either side of the membrane.

Distribution of Ions Potassium ions (K+) are the major

intracellular positive ion (cation), and sodium ions (Na+) are the major extracellular cation.

The distribution is created largely by the sodium-potassium pump (Na+/K+ pump), which actively transports sodium ions out of the cell and potassium ions into the cell.

Sodium/Potassium Pump

Resting Potential A resting nerve cell is one that is not being

stimulated to send a nerve impulse. Under resting conditions, nongated (always

open) channels determine the membrane permeability to sodium and potassium.

Resting Potential Sodium and potassium ions follow the laws

of diffusion and show net movement from high concentration to low concentration across a membrane.

The resting cell membrane is only slightly permeable to these ions, but is more permeable to potassium than to sodium.

Local Potential Changes Neurons are excitable—they can respond to

changes in their surroundings. Some neurons detect changes in temperature,

light, or pressure outside the body. Others respond to signals from inside the body,

often from other neurons. Such changes affect the membrane potential in

the region of the membrane exposed to the stimulus.

Local Potential Changes Environmental change affects the membrane

potential by opening a gated ion channel. If the membrane potential becomes more

negative than the resting potential, the membrane is hyperpolarized.

If the membrane becomes less negative (more positive), the membrane is depolarized.

Action Potential When a sufficiently large stimulus reaches the

membrane, an action potential results. Channels open and Na+ floods in, following its

concentration gradient. The membrane is now depolarized, which causes

the opening of K+ channels and potassium diffuses out and sodium and potassium have switched positions.

During the short time the sodium-potassium pump requires to restore the ions to their original positions, the neuron is refractory; it cannot fire again.

Action Potential If neurons are

depolarized sufficiently, the membrane potential reaches a level called the threshold potential which results in an action potential, and nerve impulse.

Action Potential The first part of the axon (initial segment) is called

the trigger zone. As Na+ ions come in, the membrane potential

changes from resting value and becomes positive on the inside (depolarization).

As K+ ions diffuse outward, the membrane becomes negatively charged once more (repolarized) and resting potential is reestablished.

All-or-None Response Nerve impulse conduction is an all-or-none

response. If a neuron responds at all, it responds

completely. Thus, a nerve impulse is conducted

whenever a stimulus of threshold intensity or above is applied to an axon.

Refractory Period For a very short time following passage of a nerve

impulse, a threshold stimulus will not trigger another impulse on an axon.

This is called the refractory period and has 2 parts.

During the absolute refractory period (1/2,500 of a second), the axon’s membrane cannot be stimulated.

This is followed by a relative refractory period during which the membrane is reestablishings its resting potential.

Impulse Conduction An unmyelinated axon conducts an impulse over

its entire surface. A myelinated axon functions differently. Myelin contains a high proportion of lipid that

excludes water and water-soluble substances. Myelin serves as an insulator and prevents almost

all flow of ions through the membrane that it encloses.

Impulse Conduction When a myelinated axon is stimulated to

threshold, an electric current flows away from the trigger zone through the cytoplasm of the axon.

As it reaches the first node, it stimulates the membrane to its threshold level and sends a current to the next node.

Because the actions potentials appear to jump from node to node, this is called saltatory conduction.

Neurotransmitters The nervous system produces at least 30 different

kinds of neurotransmitters. Some neurons release only 1 type; others

produce 2 or 3 kinds. Acetylcholine stimulates skeletal muscle

contractions. The enzyme acetylcholinase decomposes

acetylcholine in order to keep signal duration short.

Neurotransmitters A group of compounds called monoamines

includes the neurotransmitters epinephrine, norepinephrine, dopamine, and serotonin.

Norepinephrine creates a sense of feeling good and may excite or inhibit the autonomic nervous system.

Serotonin induces sleepiness.