Nervous SystemChapter 7

The Nervous system is the master controlling and communicating system of the body.

3 step process◦Sensory-uses sensory receptors to monitor changes inside

and outside of the body.

◦ Integration-processes and interprets the information and makes decision about what to do with the information (integration)

◦Motor-activation of muscles or glands in response to the stimuli

NERVOUS SYSTEM

Organization of the Nervous System

All nervous system organs are separated into two classifications by structure.

◦Central Nervous System

◦Peripheral Nervous System

Central Nervous System(CNS)

Made up of the brain and spinal cord

Main purpose is to interpret the incoming sensory information and relay tissue instructions based on past experiences or conditions.

Peripheral Nervous System(PNS)

Anything in the nervous system outside of the brain and the spinal cord.

Consists of mainly nerves◦2 types

Spinal Nerves- Carry impulses to and from the spinal cord.

Cranial Nerves- Carry impulses to and from the brain.

Functional ClassificationsThe functional classification is only

concerned with the peripheral nervous system

◦Sensory/afferent- send information from the sensory receptors to the CNS.

◦Motor/efferent-carry impulses from the CNS to the muscles and glands and initiate a response.

Motor Divisions

Somatic nervous system◦allows conscious, voluntary movement of

skeletal muscles.◦Not all muscular activity is voluntary

Skeletal muscle reflexes-stretch reflex◦When a muscle spindle is stretched, it sends a message

to the brain telling the brain to contract the muscle to prevent tearing. Patellar-tendon reflex

Stretch Reflex

Motor Divisions

Autonomic Nervous System◦Regulates events that are automatic or

involuntary Activity of cardiac muscles and smooth muscle Separated into two parts

◦Sympathetic-fight or flight, produce reactions under stress

◦Parasympathetic-rest and digest, all other autonomic functions, blood vessel dilation, pupil dilation.

Nervous Tissues◦ Astrocytes-

· Abundant, star-shaped cells

· Brace neurons to their nutrient

supply

· Form barrier between capillaries and neurons

· Control the chemical environment of the brain by picking up excess ions and recapturing released

neurotransmitters.

· Microglia-· Spiderlike phagocytes that dispose of debris (dead cells)

· Ependymal· Line the cavities of the brain and spinal cord.

· Their cilia help circulate CSF that fills those cavities

· CSF forms a protective cushion for the CNS

Oligodendrocytes-◦Produce a myelin sheath around nerve fibers in

the CNS◦Insulation/protection for nerves

Nervous Tissue: Support Cells

· Satellite cells· Protect neuron cell bodies by cushioning

cells

· Schwann cells· Form myelin sheath in the peripheral

nervous system

Structure of a Motor Neuron

Neuron Anatomy

· Neurons = nerve cells

· Cells specialized to transmit messages

· Major regions of neurons

· Cell body – nucleus and metabolic center of the cell

· Processes – fibers that extend from the cell body

Neuron Anatomy

· Body of the Cell· Metabolic center of the neuron· Nissl substance – specialized rough

endoplasmic reticulum· Neurofibrils – intermediate cytoskeleton

that maintains cell shape

Neuron Anatomy

· Extensions outside the cell body· Dendrites -carry messages toward the

cell body

· Axons –carry messages away from the cell body to another neuron

Axons and Nerve Impulses

Axons transmit their information at their terminal ends.

All axons branch out at their end forming thousands of axonal terminals.

Once the impulse reaches the axonal terminal it stimulates the release of neurotransmitters into the extracellular space.

Synapse

In Between each axonal terminal is a small gap called a synapse.

Neurons never touch other neurons.

Neurons

Most long nerve fibers are covered with a fatty material called myelin. ◦It protects and insulates the fibers and

increases the transmission rate.Axons outside of the CNS are insulated

(myelinated) by Schwann cells.◦Wrap themselves around axons for insulation.◦When it is wrapped around the axon, the

myelin sheath encloses the axon.

Neurons

The neurilemma is in between the myelin sheath and the Schwann cells.

The Myelin sheath is formed by many different Schwann cells, this leaves gaps of uncovered surface area that are called Nodes of Ranvier.

Multiple sclerosis

Myelin sheaths around the fibers are gradually destroyed.

Once destroyed they harden and become “scleroses”

This decreases the persons ability to control their muscles and their mobility decreases.

Central Nervous System

Clusters of neuron cell bodies and collections of nerve fibers are named nuclei when in the CNS.

They are well protected in the body within the skull or the spinal column.

These cells do not go through cell division after birth. If a cell dies, it is not replaced. Thus the need for the bony protective coverings.

CNS anatomy

Ganglia- small collection of cell bodies in the CNS.

Tracts- bundles of nerves running through

White matter-dense collections of myelinated tracts (fibers)

Gray matter-unmyelinated fibers and cell bodies

CNS

Sensory Neurons◦Afferent-go toward the brain/spinal cord for

processing◦Transmit information about outside stimuli to

the CNS◦Cutaneous receptors-skin◦Proprioceptors- muscle/tendon◦Nociceptors- pain impulses

Proprioceptors

Detect the amount of stretch or tension in skeletal muscles, tendons or joints.

Information is sent to the brain so that it can make adjustments for any changes in posture/balance.

CNS

Motor Neurons-◦Efferent, carry impulses from the CNS to the

muscles/glands for action.◦Relay the action message to the muscles

Association Neurons-◦Also known as interneurons.◦They connect the motor and sensory neurons in

neural pathways.

Sensory Receptors

Naked Nerve Endings- pain and temperature

Messner’s Corpuscles- touch receptorsPacinian Corpuscles-Deep PressureGolgi Tendon Organs (GTOs)-

proprioception (contraction)Muscle Spindle-proprioception (stretch)

Nerve Impulses-Phyisology

2 major functions◦Irritability- ability to respond to a stimuli and

convert it into a nerve impulse

◦Conductivity-ability to transmit the impulse to other neurons, muscles or glands.

Physiology

When at rest, the plasma membrane is polarized, meaning there are fewer positive ions sitting on the inner face of the membrane than on the outside.

The major positive ions on the inside of the cell are potassium (K+), and the positive ions on the outside, are sodium (Na+)

If the inside is more negative than the outside, the neuron remains inactive.

Physiology

Many types of stimuli are used to excite the neurons, to activate and create an impulse. ◦Light excites eye receptors◦Sound excited some ear receptors◦Pressure for cutaneous receptors, etc.

Physiology-depolarization

Regardless of the stimuli, the result is all the same, permeability of the cell membrane changes for a brief period.◦Once the neuron is activated, the sodium gates

of the plasma membrane open and allow the sodium (Na+) into the cell.

◦Law of diffusion- higher concentration of Na+ outside the membrane

◦Once inside the polarity of the inside of the cell changes, this process is called depolarization.

Depolarization cont.

If the stimulus is strong enough, and the rush of sodium is great enough, the neuron is activated through depolarization.◦Once depolarized the neuron will transmit the

nerve impulse (action potential).**This process is all or nothing**. The

impulse will either be sent all the way through the neuron, or not sent at all.

Repolarization

Almost immediately after the Na+ ions rush in, the membrane permeability changes◦ It returns to being impermeable to Na+, but

permeable to K+, just as before.Now the K+ ions are free to float back out

into the tissue fluid. This happens rapidly, the quick outflow of these ions restores the electrical conditions of the cell. Returning to its resting (polarized) state. This process is called repolarization

Action potential

Conductivity of Neurons

Once an impulse is sent through the neuron and it reaches the axonal terminal, tiny vesicles containing the neurotransmitter chemical fuse with the axonal membrane, releasing the chemical transmitters.

These chemicals travel across the synapse and bind to the next neuron.

This will more often than not restart the action potential in that next neuron.