BIOM- L15 - Nervous System and Nervous Tissue

Functions of the Nervous System

1.Sensory input

2.Integration

3.Motor output

4.Mental activity

5.Homeostasis

1.Sensory input

To monitor stimuli occurring inside and outside the body

•Performed by the sensory receptors

2.Integration

•To process and interpret sensory input and decide if action is needed

•Performed by the brain/spinal cord

3.Motor output

•A response to integrated stimuli

•The response is performed by muscles or glands

4.Mental activity

•eg thinking, remembering etc

Homeostasis

•internal balance

Basic Divisions of the Nervous System

1) 1.Central Nervous System (CNS)

2) 1.Peripheral Nervous System (PNS)

1.Central Nervous System (CNS)

•Brain and spinal cord

•Integrating and command center

•Analyses sensory input, decides on what to do about it, sends message to effectors (muscles and glands)

1.Peripheral Nervous System (PNS)

Nerves extending from

•brain (called cranial nerves) and spinal cord (called cranial nerves) and their ganglia*

Peripheral nerves link all regions of the body to the CNS

•Peripheral nerves link all regions of the body to the CNS via:

•Sensory input from periphery to CNS

•Motor output from CNS to periphery

Sensory (aka afferent) signals are picked up by

sensory receptors

Sensory (aka afferent) signals are Carried from

•PNS to CNS for integration (analysis)

•Sensory input may be:

•General sensory

Special sensory

General sensory

eg pain, pressure, tickle, itch, proprioception

Special sensory

eg hearing, vision, taste, smell

Motor (aka efferent) signals are carried away from

CNS by PNS nerves, to the effectors (muscles and glands)

Nervous tissue is made up of 2 types of cells:

1) Neurons

2) Neuroglia

1. Neurons

•Nerve cells

Cells specialized to

•transmit electrical signals

•Cannot undergo mitosis

2. Neuroglia

•More numerous than neurons

•Support cells

•Can divide to produce more cells

•Several types

Neurons are the

basic structural unit of the nervous system

Neurons are Specialized cells, conduct

•electrical signals

•Called a Nerve impulse or Action Potential

•Major regions of neurons:

Cell body

•Processes

Neurons: Cell body -contains

organelles

•Processes-fibers that extend from the cell body; two kinds:

•Dendrites

•Axons

•Dendrites

-receiving end

•Axons

-sending end

Neurons: Cell Body, Metabolic center of the cell

Contains organelles

-eg Nucleus, mitochondria etc

Also contains specialized organelles:

-Nissl bodies or Substance-

-Neurofibrils-

Nissl bodies or Substance

specialized rough endoplasmic reticulum

Neurofibrils-

cytoskeleton, acts to maintain cell shape

Neurons: Processes, •Fibers extending from the cell body, two types:

Dendrites: highly branching,

•receive and then conduct impulses toward the cell body

Axons-a single, sending process (though is highly branched at its end), connects to cell body at

axon hillock, conducts those impulses away from the cell body (towards the cell it is communicating with)

The axon ends at the

axon terminals, which contain sacs called vesicles

These vesicles are filled with messenger chemicals called

neurotransmitters

Axon terminals are separated from the next neuron (or the muscle/gland cell it is communicating with) by a gap called the

Synaptic cleft

Synapse-functional junction between neurons; the actual

communication between cells

Nerve impulses aka Action Potential, 1) Generated at the axon hillock, travel along the

axon to the axon terminal

2)This triggers the release of neurotransmitters across the

synaptic cleft

3)The dendrite of the next neuron has receptors that are stimulated by the

neurotransmitter when it binds to them

4) •A nerve impulse is started in the receiving dendrite

This ensures the continuation of the

•'message' to the next neuron

•This communication occurs at the synapse

Synapse-functional junction between neurons; the

actual communication between cells (neuron to neuron or muscle, gland, organ)

•Presynaptic neuron

Conducts

•signal toward a synapse

•Sending neuron

•Postsynaptic neuron

Transmits electrical activity away from

•a synapse

•Receiving neuron

Electrical Synapses-Some nerve impulses are transmitted electrically through tiny tunnels in adjacent neuronal cell walled called

gap junctions

eg intercalated discs of cardiac muscle

Chemical Synapses-Most nerve impulses are transmitted via

chemical messengers (the neurotransmitters), since the electrical signal can't jump the synaptic cleft between neurons

Structural Classification of Neurons

•Unipolar

•Bipolar

•Multipolar

Unipolar-one short, single process, with

•lots of dendritic branching

•eg mainly sensory neurons (PNS)

Bipolar—possess two processes;

•one axon and one dendrite

•eg found only in special sensory organs eg eye and nasal cavity

Multipolar-more than two processes;

•numerous dendrites

and one axon

•e.g. CNS and most motor neurons

Functional classification is based on

what information the neuron is carrying

1.Sensory (afferent) neurons- carries sensory input from PNS to CNS

Tend to be

•unipolar neurons

1.Motor (efferent) neurons-carries motor input from CNS to PNS and effectors (muscles, organs, glands)

Tend to be

•multipolar neurons

1.Interneurons (association neurons)-in CNS; analyzes sensory input, decides upon motor response of effectors

Most are

multipolar

Sensory input carried by

sensory neuron

Integration

Carried out by interneuron

aka association neuron

motor output

Carried by

motor neuron

2. Neuroglia, •Not electrically excitable (do not transmit electrical signals)

•Make up about half the volume of the nervous system

•Can undergo mitosis

•6 types total

•4 in Central Nervous System (CNS)

•2 in Peripheral Nervous System (PNS)

Astrocytes:

• Star-shaped

• Most abundant

Forms

blood-brain barrier (BBB) around blood vessels in the brain

Ependymal Cells:

Produce and circulate

cerebrospinal fluid (CSF)

Microglia:

Small

•macrophages

•Remove bacteria and cell debris in CNS

Oligodendrocytes:

Produce

myelin sheath in CNS

Schwann cells:

Produce

•myelin sheath in PNS

•aka Neurolemmocyte

Satellite cells:

Surround the cell body of a

•neuron

•Insulate, regulate chemical environment

•The myelin sheath is produced by Schwann cells (PNS) and oligodendrocytes (CNS)

Surrounds the axons of most neurons, with occasional gaps called

Nodes of Ranvier

Oligodendrocytes

____ produces myelin sheath around several axons

One

Schwann cells

______ produces myelin around each axon

Several

Myelin Sheath, •Whitish, fatty material (mostly lipids)

•Covers and insulates axons

•Allows for electrical impulses to move along the axon faster

•Not all axons are myelinated (have a myelin sheath)

Some are

unmyelinated; conduct impulses more slowly than those that are myelinated

White matter:

Collection of

•myelinated axons

•Myelin is white in appearance; therefore, this tissue appears white

Gray matter:

Collection of

•dendrites and cell bodies

•Not myelinated, therefore not white, appears grey

Nerves

•Bundles of axons wrapped in connective tissue

Carry

•sensory and motor info

•In the PNS

Tracts

•Bundles of axons wrapped in connective tissue

Carry

•sensory and motor info

•In the CNS

Endoneurium—layer of delicate connective tissue surrounding the

individual axon

Perineurium—connective tissue wrapping surrounding a

nerve fascicle

Nerve fascicles—groups of axons bound into

•bundles

Epineurium—whole nerve is surrounded by

tough fibrous sheath

•Reflexes: Predictable, rapid, autonomic motor responses to stimuli; composed of:

1.Receptor

2. Sensory neuron

3. Integration center

4. Motor neuron—

5. Effector

1.Receptor

—site where stimulus acts

2. Sensory neuron

transmits afferent (sensory) impulses to the CNS

3) Integration center

—consists of one or more synapses in the CNS; interneurons

4) Motor neuron

conducts efferent (motor) impulses from integration center to an effector

5) Effector

—muscle or gland cell (contracting of a muscle or secretion from glands)

Example of a Reflex: Patellar (Knee-Jerk) Reflex 1) Receptor

tap patellar tendon with hammer, receptors in quadriceps muscle stretched

Example of a Reflex: Patellar (Knee-Jerk) Reflex 2) Sensory neuron

carries stretch message directly to motor neuron

Example of a Reflex: Patellar (Knee-Jerk) Reflex 3) Integration center-

in patellar reflex, processing is in spinal cord

Example of a Reflex: Patellar (Knee-Jerk) Reflex 4) Motor neuron

carries message back to effector

Example of a Reflex: Patellar (Knee-Jerk) Reflex 5) Effector

quadriceps muscle stimulated to contract

•Reflexes are often used for diagnosing disorders of the nervous system and locating injured tissue

If a reflex is absent or abnormal, the damage may be

somewhere along a particular conduction pathway

Biceps reflex, Contraction of the biceps muscle when its

•tendon is tapped,

•Tests C5 and C6 nerve roots

Triceps reflex

Contraction of the triceps muscle and slight extension of the upper limb when the tendon of the muscle is

•tapped directly, with the limb flexed and fully supported and relaxed

•Tests C6 and C7 nerve roots

Ankle-jerk reflex

Plantar extension of the foot elicited by a tap on the

Achilles tendon, while the patient is seated on a bed or chair, with feet hanging freely,

Tests S1 nerve root

In the CNS, there is little or no repair due to:

-Inhibitory influences from neuroglia, particularly oligodendrocytes

-Absence of growth-stimulating cues that were present during fetal development

-Rapid formation of scar tissue

In the PNS repair is possible if

•The cell body is intact

•Schwann cells are functional

•The neurolemma (a thin coating found on PNS nerve processes) is intact

•Scar tissue formation does not occur too rapidly

•But regeneration in PNS is not fast, perfect, or always possible

Slow regrowth means process may take

•2 years

•Some nerve fibers connect with the wrong muscle fibers; some die