How Nerve Signals Maintain Homeostasis

9.1 The Importance of the Nervous System

9.1: The Importance of the Nervous System

• The nervous system is an elaborate communication system that has more than 100 billion nerve cells in the brain alone.

• Memory, learning and language are all a part of the nervous system.

• Has two divisions: the central nervous system (CNS) and the peripheral nervous system (PNS).

• CNS– Brain and spinal cord – Protected by

• Meninges• skull and vertebrae

• PNS– All the other neurons– Serves limbs and organs– Sensory and motor branches– Exposed to toxins and more

susceptible to injuries

• CNS includes nerves of the brain and spinal cord and is the coordinating centre for incoming and outgoing information.

• The PNS includes the nerves that carry information between the organs of the body and the CNS– PNS is divided further into the somatic and

autonomic nerves.• Somatic: controls the skeletal muscle,

bones and skin.• Autonomic: special motor nerves that

control the internal organs of the body.–Autonomic can be subdivided into the

sympathetic and the parasympathetic.

Peripheral Nervous System

Somatic Nerves:• Controls skeletal

(voluntary) muscles, bones and skin

• Brings information from the external environment to the CNS

• Motor somatic nerves

Peripheral Nervous System

Autonomic Nerves:• Motor nerves that

control smooth (involuntary) muscles, organs

 

Autonomic nervous system

– Controls the internal organs of the body

– operate without conscious control

– constant interplay of balance between sympathetic and parasympathetic systems

autonomic nerves

http://itc.gsw.edu/faculty/gfisk/anim/autonomicns.swf

Sympathetic nervous system

– Prepares the body for

stress: • increases heart rate, • increases the release

of glucose, • dilates the pupils, • increases blood flow

to the skin, • causes release of

epinephrine

Parasympathetic nervous system

– Restores normal balance:

• decreases heart rate • stores glucose • constricts pupils• decreases blood flow

to the skin

– monitor external stimuli like temperature, light, and sound, etc.

– Use internal body receptors to detect variations in pressure, pH, CO2 concentration, levels of various electrolytes etc.

Nerves are responsible for sensory (input), integrative, and motor output.

Sensory Nerves

Integrative Nerves

– signals are brought together (integrated) to create sensations, to produce thoughts, or to add to memory.

– Decisions are made based on the sensory input.

Motor nerve output / response

–The nervous system responds to stimuli by sending signals to muscles, causing them to contract, or to glands, causing them to produce secretions. –Muscles and glands are called effectors

• They cause an effect in response to directions from the nervous system.

Anatomy of a Nerve Cell

Cells of NS

• Non-neural cells (neuroglial or simply glial)– Provide oxygen and nutrients to neurons– Provide support, insulation and protection

against pathogens– Remove dead neurons

• Neurons – receive stimuli – conduct action potentials

Non-neural Glial Cells• Astrocytes

– cover the surfaces of neurons and blood vessels. – provide structural support and help form the blood

brain barrier, thus playing a role in regulating what substances from the blood reach the neurons.

• Oligodendrocytes – form the myelin sheaths of axons in CNS

• Schwann cells – form a myelin sheath called the neurilemma around

each axon in PNS (sheath is 80% lipid + 20% protein)– very important in saltatory nerve conduction

Astrocyte

Types of Neurons

• Bipolar– has only two nerve

fibers (axon & dendrite)

• Unipolar – single nerve fiber

• Multipolar– many nerve fibers – One is an axon, rest

are dendrites

What are the 3 parts of all neurons?

1. Dendrites2. The cell body3. The axon

Neuron StructureCell body: Contains nucleusDendrites:• Receive information• Conduct nerve impulses

toward the cell bodyAxon:• Sends nerve impulses from

the cell body to other neurons (effectors)

Myelin Sheath:• White coat of fatty protein

that covers some axons

Schwann Cells:• Individual cells that compose the myelin sheath• Insulates the nerve cell

Nodes of Ranvier:• Areas between the sections of myelin sheath

NeuronsDendrites (“receivers”)

Cell Body

Axon terminals (“Transmitters “)

Functional Differences b/w Neurons

• Sensory (afferent) Neurons– carry nerve impulses from peripheral body parts into the brain or

spinal cord – PNS to CNS– Have specialized dendrite ends

• Interneurons (association)– within the brain or spinal cord – Link with other neurons

• Motor (efferent) Neurons– carry nerve impulses out of the brain or spinal cord to effectors – CNS to PNS– Motor impulses stimulate muscles to contract and glands to

release secretions

Sensory Neurons• Afferent neurons• Sense and relay stimuli (information) from the

environment to the CNS• Located in clusters outside of the spinal cord• Types of sensory receptors: thermoreceptors

(temp.), photo (vision), pain, pressure, mechano (hearing), chemo (taste and smell)

Interneurons/Association Neurons

• Neurons that link together neurons in the body

• Mainly in the spinal cord and brain

• human brain contains ~100 billion interneurons averaging 1000 synapses on each or some 1014 connections

Motor NeuronsEfferent Neurons•Relay information to the effectors

– Muscles, organs and glands– The axons connecting your spinal cord to your

foot can be as much as 1 m long (although only a few micrometers in diameter).

ANIMATIONS

• http://www.sumanasinc.com/webcontent/anisamples/neurobiology/neurobiology.html

Reflex Arcs

• Sensory neurons are stimulated by pricking rose thorns

• Signal travels to spinal cord along sensory neurons

• Synapse with interneurons within CNS

• Interneurons stimulate motor neurons

• Stimulus travels along motor axons to muscle

• Muscle contracts to withdraw hand

Reflex arcs:

–Sensory

–Integrative

–Motor

Reflex Arcs

The Speed of Nerve Impulses

• Myelinated nerve fibres speed up nerve impulses

• Nerve impulses jump from one node to another… speeding up nerve action (known as saltatory conduction)

Neuron Repair

Neurilemma:• Thin membrane that surrounds the axon• Promotes regeneration of damaged neurons• Not in all nerve cellsWhite Matter:• Nerve cells in the brain that contain myelinated fibres

and a neurilemmaGrey Matter:• Nerve cells in the brain and spinal cord that lack a myelin

sheath and neurilemma• Why are spinal and brain injuries often permanent?

How to Fix “Irreparable” Damage to the CNS

1. Reattach two torn nerves

- limited success

2. Grafts from the PNS

- more successful…CNS cells that are left alone however, had no regeneration

Stem Cells

• Cells that have not specialized into tissue cells

• Experiments are being done on replacing damaged cells using stem cells

• E.g. pp. 415…rats with reconnected spinal cords

• Page 417 # 1-6