Chapter 7
The Nervous System:
Structure and Control of
Movement
Objectives
Discuss the general organization of the
nervous system
Describe the structure & function of a
nerve
Draw and label the pathways involved in a
withdraw reflex
Define depolarization, action potential, and
repolarization
Objectives
Discuss the role of position receptors in
the control of movement
Describe the role of vestibular
apparatus in maintaining equilibrium
Discuss the brain centers involved in
voluntary control of movement
Describe the structure and function of
the autonomic nervous system
General Nervous System
Functions
1. Control of the internal environment
– Nervous system works with endocrine system
2. Programming spinal cord reflexes
3. Voluntary control of movement
4. Assimilation of experiences necessary for
memory and learning
Organization of the Nervous
System
Central nervous system (CNS)
– Brain and spinal cord
Peripheral nervous system (PNS)
– Neurons outside the CNS
Sensory division
Motor division
Fig 7.1
Organization of the Nervous
System
– Sensory division
Afferent fibers transmit impulses from receptors to CNS
Sensation-heat, light, touch, smell, pressure
Blood and lymph vessels, internal organs
Skin, muscles, tendons
– Motor division
Efferent fibers transmit impulses from CNS to effector organs
Results in appropriate responses
– Muscle contraction
– Reflex activity
Divisions of the Nervous System
Fig 7.1
Relationship
Between
PNS and
CNS
Fig 7.2
Structure of a Neuron
Cell body - Soma
Dendrites: receptor areas, conduct
impulses toward cell body
Axon-nerve fiber
– Carries electrical impulse away from cell body
toward another neuron, organ
– Length – few millimeters to a meter
Basic Nerve Structure
Neuron
– Cell body (soma)
– Dendrites
– Axon
A. Motor neuron
– Away from soma
B. Sensory neuron
– Toward soma
Basic Nerve Structure
Large nerve fibers
– Skeletal muscles
– Myelin sheath
Composed of fat and
protein
Segments separated by
spaces-nodes of
Ranvier
Structure of a Neuron
Axon-nerve fiber
– May be covered by Schwann cells
Forms discontinuous myelin sheath along length of
axon
Gaps – Nodes of Ranvier
– Nerve impulse is propagated-bounces from node to node
– Saltatory conduction-increases conduction velocity
Large myelinated fibers-skeletal muscle
– 60 to 100 m/sec (135 to 225 miles/hr)
Non-myelinated fibers-mixed in with myelinated
– 6 to 10 m/sec
Structure
of a
Neuron
Electrical Activity in Neurons
Neurons are “Excitable Tissue”
– Irritability: ability to respond to a
stimulus and convert it to a neural
impulse
– Conductivity: transmission of the
impulse along the axon
Ion Concentrations
Ion
Sodium
Chloride
Potassium
ECF
150 mmoles/L
110
5
ICF
15
10
150
Ion [ ] set up potential difference across
membrane
– Resting membrane potential
Resting Membrane Potential
Resting membrane potential
– More Na+ outside
– More K+ inside
– Electrical gradient (difference) between inside and
outside cell
Action Potential
Cell membrane
depolarizes (polarity
is reversed)
– Na+ moves in
– K+ moves out
– With depolarization
Inside positive
Outside negative
– Starts action potential
propagated
Action Potential
Repolarization
– Na+ inside cell
– K+ pulled out of cell
(more negative)
– Outside becomes +
– Inside goes negative
Return to resting
Electrical Activity in Neurons
Resting membrane
potential
– At rest, the neurons are
negatively charged
– Determined by
concentrations of ions
(Na+, K+, Cl-) across
membrane
– Negative inside, K+ leaving
– Positive outside cell
membrane
– [ ] maintained by Na/K
pump
An Action Potential
Action potential
– Stimulus - neural
message-threshold
– Permeability of the
membrane changes,
Na+ gates open
– interior positively
charged
Action potential (nerve
impulse) occurs
Depolarization
Repolarization
Repolarization
– Change in membrane permeability, restoring
resting membrane potential
Na+ gates close-entry into cell is slowed
K+ leaves cell-returning inside to negative
Repolarization
Fig 7.6b
Synaptic Transmission
Neurons communicate across
synapses using neurotransmitters
– Released from presynaptic membrane
– Binds to receptor on post synaptic
membrane
Synaptic Transmission
Synapse: Contact points between axon of
one neuron and dendrite of another
neuron
– Nerve to nerve
– Nerve to muscleNeuromuscular junction
Myoneural junction
Motor end plate
Synaptic Transmission
Basic
Structure
of a
Chemical
Synapse
Synapse-two neurons
Axon terminals of
presynaptic neuron
Synaptic cleft-space
Receptors of second
neuron
Vesicles release
transmitter substance
NT diffuses across cleft
and impulse continues
Response?
Neuron Response
Type of transmitter released
– Excitatory-cause depolarization of membranes
Acetylcholine-neuron to muscle
Norepinephrine-peripheral sympathetic nerves
– Adrenergic receptors
Acetylcholine-peripheral parasympathetic nerves
– Cholinergic receptors
– Inhibitory-cause hyperpolarization of membranes
Gamma-aminobutyric acid (GABA)-spinal cord/brain
Glycine-spinal cord
Synaptic Transmission
Excitatory
postsynaptic
potentials (EPSP)
– Causes depolarization
which may or may not
reach threshold
– Sufficient amounts of
NT causes
depolarization to
threshold
– Action potential is
generated
Synaptic Transmission
Excitatory
postsynaptic
potentials (EPSP)
– Temporal summation:
Timing is important
Successive discharges
from the same terminal
Summing several
EPSPs from one
presynaptic neuron
Synaptic Transmission
Spatial summation: summing from several
different presynaptic neurons
– Additive effect of several stimuli
Stimulus
Inhibitory
postsynpatic
potentials (IPSP)
– Hyperpolarization
NT inhibits response
Inhibitory postsynaptic
potential (IPSP)
Sensory Information
Proprioceptors
– Proprioception: ability to determine
position of joint
– Kinesthesia: sensation of joint motion or
acceleration (rate of movement)
Muscle Chemoreceptors
– Sensitive to changes in the chemical
environment surrounding a muscle
Proprioceptors
Provide CNS with information about body
position and joint angle
– Free nerve endings – touch & pressure
– Golgi-type receptors – in ligaments & joints
– Pacinian corpuscles – in tissues around joints
Strongly stimulated at beginning of
movement then adapt
– Steady signal until move is completed
Proprioceptors
Joint receptors
– Provide body with information of orientation of
body parts
– Feedback about rates of limb movement
Muscle Chemoreceptors
Provide CNS with information regarding
the metabolic rate of muscular activity
– Hydrogen ion concentration
– Carbon dioxide (CO2)
– Potassium (K+)
Chemoreceptor feedback may regulate
cardiovascular and pulmonary exercise
responses
Reflexes
Rapid, unconscious means of reacting to
stimuli
Not dependent on higher brain centers
Rapid removal from pain source
Reflexes
Pathway for neural reflex:
1.Sensory nerve sends impulse to spinal
column
2.Interneurons activate motor neurons
3.Motor neurons control movement of muscles
Reciprocal inhibition
– EPSPs to muscles to withdraw from stimulus
– IPSPs to antagonistic muscles
A Reflex Arc Illustrating
Reciprocal Inhibition
Somatic Motor Function
Somatic motor neurons of PNS
– Responsible for carrying neural messages
from spinal cord to skeletal muscles
– Neuromuscular junction
– Axon splits into collateral branches, each
innervates muscle fiber
Fig 7.9
Somatic Motor Function
Motor unit
– Motor neuron and all the muscle fibers it
innervates
– Activation of single motor neuron
All or none law
All muscle fibers contract
Somatic Motor Function
Innervation ratio
– Number of muscle fibers per motor neuron
– Low ratio
Fine motor control
Eye movement
– High ratio
Gross motor control
Leg muscles
Illustration of a Motor Unit
Fig 7.9
Vestibular Apparatus and
Equilibrium
Located in the inner ear (Semi-circular
canals)
Responsible for maintaining general
equilibrium and balance
Sensitive to changes in linear and angular
acceleration
Motor Control Functions of the
Brain
Brain Stem
–
–
–
–
Medulla
Pons
Midbrain
Reticular formation
Cerebrum
– Cerebral cortex
Cerebellum
– Monitors complex
movement
Motor Control Functions of the
Brain
Cerebrum
– Cerebral cortex
Organization of complex movement
Storage of learned experiences
Reception of sensory information
– Motor cortex
Most concerned with voluntary movement
Cerebellum
- Monitors complex movement
- In response to proprioceptors
Autonomic Nervous System
Responsible for maintaining internal
environment
– Effector organs not under voluntary control
Smooth muscle, cardiac muscle, glands
Sympathetic division
– Releases norepinephrine (NE)
– Excites an effector organ
Parasympathetic division
– Releases acetylcholine (ACh)
– Inhibits effector organ
Motor Integration