9 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).

Vertebrate Nervous System

Central nervous System (CNS):

• The nerves in the brain and spinal cord

Peripheral Nervous System (PNS):

• Nerves that carry info. Between the organs and the CNS

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

• The PNS include the nerves that carry information between the organs of the body and the CNS– PNS can be 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

muscles, bones and skin

• Brings information from the external environment to the CNS

• Motor somatic nerves

What do the somatic nerves control?

Controls the skeletal muscles 

Peripheral Nervous System

Autonomic Nerves:• Motor nerves that

control organs

– monitor:• temperature, light, and

sound, ETC.

– Inside the body receptors detect variations in:

• pressure, pH, carbon dioxide concentration, and the levels of various electrolytes.

Is this sensory, integrative, or motor input?

SENSORY

Is this sensory, integrative, or motor input? – signals are brought together to:

• create sensations, to produce thoughts, or to add to memory.

– Decisions are made based on the sensory input. This is integration.

Integrative

Is this sensory, integrative, or motor input?– nervous system responds 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.

– This is the motor output or response

motor input

Anatomy of a Nerve Cell

Glial Cells:• Structural support and

metabolism of nerve cells

• Non-conducting

Neurons:• Sensory neurons• Interneurons• Motor Neurons

Sensory Neurons

• Afferent neurons

• Sense and relay stimuli (information) from the environment to the CNS

• Located in clusters outside of the spinal cord

Interneurons/Association Neurons

• Neurons that link together neurons in the body

• Mainly in the spinal cord and brain

• human brain contains ~100 billion (1011) interneurons averaging 1000 synapses on each or some 1014 connections

Motor Neurons• Efferent 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).

What are the three parts of all neurons?

1. Dendrites2. The cell body3. The axon

Neuron Structure

Dendrites:• 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

Neuron StructureSchwann Cells:

• Individual cells that compose the myelin sheath

• Insulates the nerve cell

Nodes of Ranvier:

• Areas between the sections of myeline sheath

ANIMATIONS

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

The Speed of Nerve Impulses

• Myelinated nerve fibres speed up nerve impulses

• Nerve impulses jump from one node to another… speeding up nerve action

Reflex Arcs

• What are the 3 overlapping nervous system functions?

–Sensory –Integrative –Motor

• Does this describe the autonomic or somatic nervous system?– Control the internal

organs of the body– operate without

conscious control– constant interplay of

balance between sympathetic and parasympathetic

autonomic nerves

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

Does this describe the sympathetic nervous system or parasympathetic?– 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

• sympathetic nervous system

parasympathetic nervous system–Restores normal

balance: • decreases heart

rate, • stores glucose, • constricts

pupils, • decreases

blood flow to the skin

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