How Nerve Signals Maintain Homeostasis 9.1 The Importance of the Nervous System Page 412 - 417 1

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 PNS Brain and spinal cord Protected by All the other neurons 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

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

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) Neurons 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 Unipolar Multipolar 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? Dendrites The cell body The axon

Neuron Structure Cell body: Contains nucleus Dendrites: Receive information Conduct nerve impulses toward the cell body Axon: 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

Neurons Dendrites (“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 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).

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 cells White Matter: Nerve cells in the brain that contain myelinated fibres and a neurilemma Grey 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 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