Topic: 6.5
Option A
Nerve Signals Maintain
Homeostasis
 Both the nervous system and the endocrine system
control actions of the body and maintain homeostasis.
 Responses to changes in environment are made by
electrochemical messages (relayed to and from the
brain) or chemical messages (hormones - carried by
the blood)
 The nervous system is an elaborate communication
system that contains about 85 billion nerve cells,
(called neurons) that transmit nerve impulses
(electrical signals)
 Memory, learning, and language are functions of the
nervous system.
Vertebrate Nervous Systems
 2 main divisions:
 CNS– central nervous system:
 the nerves of the brain and spinal cord
- coordinating centre for incoming & outgoing info
 PNS – peripheral nervous system:
 all other nerves (called neurons)
 nerves that carry info in the form of electrical impulses
between organs and the CNS
Controls skeletal
muscle, bones, and skin
Controls internal
organs
Relays info
about
environment
to CNS
Initiates
appropriate
response
Nerve Cells
 2 types of cells: glial cells and neurons
 GLIAL CELLS (neuroglial cells): nonconduction;
provide structural support and metabolism of the
nerve cells.
 NEURONS: functional units of nervous system;
conduct nerve impulses (action potentials)
 3 types: sensory neurons, interneurons, motor neurons
Nerve
 A nerve is made of several individual neurons grouped
together into a single structure
 (Kind of like a telephone cable: a protective sheath
surrounding many individual wires)
Nerve Cell
Sensory Neurons
 Also known as afferent neurons
 Relay info from receptor cells and then pass it on to an
interneuron
 Located in clusters called ganglia outside the CNS
Receptors
 Specialized cells that receive stimuli from the (internal
or external) environment and pass on the information
to sensory neurons.
 Ex: photoreceptors in your eyes respond to light
 Ex: chemoreceptors in your nose and on your tongue are
sensitive to chemicals
 Ex: baroreceptors are pressure receptors in your skin
that detect the fit of your clothes
 Ex: thermoreceptors in your skin respond to different
temperatures
Interneurons/ Relay Neurons
 Link neurons within the body
 Found mainly in the brain and the spinal cord
 They receive nerve impulses from sensory neurons and
pass them on to other parts of the CNS or motor
neurons.
Motor Neurons
 Also known as efferent neurons
 Relay info from the CNS to the effectors
 (Effectors: muscles, organs, and glands – they
produce responses to the stimuli)
Nerve Cell Parts
DENDRITES: projections of cytoplasm
Receive info (from sensory neurons or other nerve cells
or receptors) and conducts nerve impulses toward the
cell body.
Cell Body: contains the nucleus, ER, Golgi, ribosomes,
lysosomes, mitochondria
Nerve Cell Parts
 AXON: an extension of the cytoplasm
 Carries info and conducts nerve impulses away from
the cell body (to other neurons or to effectors)
 Very thin – 100 axons could fit in a single hair strand
 MYELIN SHEATH – a white coat of fatty protein and
multiple phospholipid bilayers that covers many axons
 Prevents the loss of ions from the nerve cell
 If an axon has a myelin sheath, it is “myelinated”
 The myelin sheath is made of special glial cells called
Schwann cells.
 Nodes of Ranvier: regularly occurring gaps between
adjacent Schwann cells of the myelin sheath.
 Nerve impulses move much faster along a myelinated
nerve than a nonmyelinated one.
 The nerve impulse can jump from one node of Ranvier
to the next – this is called saltatory conduction.
 (Conduction through a unmyelinated sheath is known
as continuous conduction)
 Ex: Speed of conduction of 100m/s v..s 1m/s
MS – Multiple Sclerosis
 MS is caused by destruction of the myelin sheath.
 The myelin sheath hardens and produces scarlike
tissue that prevents normal transmission of info.
 MS symptoms: double vision, speech difficulty, jerky
movements, partial paralysis
 Nerve fibers in the PNS contain a thin membrane
called the neurilemma which surrounds the axon
 This promotes the regeneration of damaged axons.
 When you get a paper cut, neurons are severed.
 Feeling gradually returns to your fingers as the severed
neurons are rejoined
 Nerves within the brain that contain myelin and
neurilemma are called white matter (b/c of whitish
appearance)
 Nerves that lack both are called grey matter.
 These are found with the brain and the spinal cord
 Damage to grey matter is usually permanent
Nerve Circuits
 STIMULUS: a change in the environment (internal or
external) that is detected by receptor cells and elicits a
response.
 RESPONSE: a change in an organism often carried out
by a muscle or a gland (called an effector)
Nerve Circuits
 EX: When you put your hand on a hot stove, the stimuli of
the heat ultimately cause you to move your muscles in
your arm to remove your hand from the stove.
 Receptor  sensory neuron interneuron  brain 
interneuron  motor neuron  effector
 Sometimes, these messages can be sent even quicker –
you can move your hand from the hot stove before your
brain even receives that information. This is a REFLEX.
 Reflexes are involuntary and often unconscious.
REFLEX ARC
 The simplest nerve pathway is the reflex arc.
 Most reflexes occur without brain coordination.
 Reflex arcs contain five essential components:
1. the receptor
2. the sensory neuron
3. the interneuron
4. the motor neuron
5. the effector
Withdrawal Reflex
ex hand on a hot surface
1.
Pain receptors in the finger detect the pain and
activate sensory neurons.
2. The sensory neurons carry the impulse to the spinal
cord via the dorsal root of a spinal nerve. The
impulse is passed to a interneuron (relay neuron) in
the grey matter of the spinal cord.
Withdrawal Reflex
ex hand on a hot surface
3. The interneuron passes the impulse to a motor
neuron.
4. The motor neuron carries the impulse out of the
spinal cord via the ventral root to the muscles in the
arm, which is the effector
5. The muscles (effector) contract and pull the arm
away from the hot object.
 Draw and label a diagram of a reflex arc for a pain
withdrawal reflex.
 See page 536
 http://www.pennmedicine.org/encyclopedia/em_Disp
layAnimation.aspx?gcid=000105&ptid=17
 http://www.pennmedicine.org/encyclopedia/em_Disp
layAnimation.aspx?gcid=000054&ptid=17