Transcript The Nervous System - Plain Local Schools

The Nervous System
Functions of the Nervous System
• 1. Communication and coordination system in
the body
• 2. Seat of intellect and reasoning
• The goal is to monitor changes in the
environment inside and outside the body,
interpret the changes, and initiate a response
in an effort to maintain homeostasis.
• Electrochemical messages called Nerve
impulses race through your body every
moment, traveling along special routes, or
Nerves, at high speeds.
• This is how communication happens
Divisions of the Nervous system
• Two main groups
• Central Nervous System (CNS) which includes
the brain and spinal cord
• Peripheral Nervous System (PNS) which includes
the nerves and sensory receptors
• The PNS is divided into the Autonomic Nervous
System (ANS) which controls unconscious
activities and the Somatic Nervous System (SNS)
which oversees conscious activities
Nervous tissue
• The functional cells of nervous tissue are
called neurons, which receive support from
nearby neuroglial cells (connective part)
• Each neuron consists of a cell body and
branches. The cell body contains the nucleus
and most of the cytoplasm, and the branches
include many dendrites which carry impulses
toward the cell body, and a single axon which
carries impulses away.
Nervous tissue
Nervous tissue
• In many neurons, the axon is covered with
numerous neuroglial cells known as Schwann
cells, which provide a white-colored
protective sheath that is mostly fat.
• This fat layer is called the myelin sheath and it
insulates and protects the axon (some axons
are nearly one meter – about 3 feet – long).
Types of neurons
• Sensory (afferent) neurons carry nerve
impulses from body parts into the brain or
spinal cord. They have receptors at the end of
their dendrites
• Motor (efferent) neurons carry nerve
impulses out of the brain or spinal cord to
effectors
• Interneurons (mixed nerves) lie within the
brain or spinal cord and link other neurons
Cell membrane potential
• The surface of a cell membrane is usually
electrically charged, or polarized, with respect
to the outside
• This polarization is due to an unequal
distribution of positive and negative ions
between sides of the membrane
• Important in the conduction of muscle and
nerve impulses
• Because of the active transport of sodium and
potassium ions, cells throughout the body
have a relatively greater concentration of
sodium ions (Na+) outside and a relatively
greater concentration of potassium ions (K+)
inside
• The cytoplasm of these cells has many large
negatively charged particles that cannot
diffuse across the cell membranes.
• Sodium and potassium ions follow the laws of
diffusion and show mvmt from high to low
concentration as permeability permits
• The difference in electrical charge between two
regions is called a potential difference and in a
resting nerve cell this is called resting potential
• When permeability changes in the region of cell
membrane that is being stimulated, channels
open and allow sodium to diffuse inward
• This mvmt is aided by the negative electrical
condition on the inside of the membrane, which
attracts the positively charged sodium ions
• Now the membrane loses its negative charge and
becomes depolarized
• At almost the same time, membrane channels
open and allow potassium ions to pass through
and diffuse outward so the inside again becomes
negatively charged and repolarized
• This rapid sequence of depolarization and
repolarization takes about one-thousandth of
a second and is called Action Potential
• www.youtube.com/watch?v=yQ-wQsEK21E
• A wave of action potentials moves down the
fiber to the end. This delivers a nerve impulse
The Synapse
• Neurons have the ability to conduct nerve
impulses very quickly, but how does one cell
communicate with another cell?
• Adjacent neurons communicate by releasing
chemicals across tiny gaps that separate them,
called synapses (synaptic cleft)
• The chemicals, known as neurotransmitters,
are released by a neuron when a nerve
impulse reaches its distal end
• The neurotransmitters then diffuse across the
synaptic cleft to contact the adjacent cell
• Contact with the next neuron may stimulate it
to trigger a nerve impulse, or it may inhibit it
(pain meds)
The Reflex Act
• The simplest type of nervous response is the
reflex act, which is unconscious and
involuntary (examples)
• Every reflex act is preceded by a stimulus (any
change in the environment)
• Special structures called receptors pick up
these stimuli
• Afferent neurons take the message to the
interneuron
• The interneuron interprets the info and
decides the action
• The efferent neuron takes the message to the
responding organ
• Reaction to the stimulus is called the response
and if there is mvmt the muscles are called the
effectors
The Reflex Arc
• A simple reflex is one in which there is only a
sensory nerve and a motor nerve involved
• Classic example is the knee-jerk reflex
• Also the withdrawal reflex
Central Nervous System
• The “central station” for incoming and
outgoing nerve impulses
• Includes the brain and spinal cord
• Both are protected by bones (cranium and
vertebral column) and a thick set of
membranes called the meninges (located
between the soft nervous tissue and the hard
bones, and is several layers thick)
Meninges
• Outer layer is tough, white fibrous connective
tissue and cantains many nerves and bv - dura
mater
• As it continues down the spinal cord it does not
attach directly to the vertebrae but is separated
by an epidural space (filled with adipose and
connective tissue to pad around the spinal cord)
• Middle layer is thin weblike arachnoid (lacks
nerves and bv)
• Inner layer is thin and plastered to the nervous
tissue pia mater (contains many nerves and bv)
• Between middle and inner is subarachnoid
space that is filled with a slightly yellowish
fluid called cerebrospinal fluid (CSF)
• CSF is also found in the ventricles of the brain
(small spaces within the brain’s center)
The Spinal Cord