The Nervous System

General Nervous System Functions

• Control of the internal environment– Nervous system works with endocrine system

• Voluntary control of movement

• Programming spinal cord reflexes

• Assimilation of experiences necessary for memory and learning

Divisions of the Nervous System

Organization of the Nervous System

• Central nervous system (CNS)– Brain and spinal cord

• Peripheral nervous system (PNS)– Neurons outside the CNS

– Sensory division• Afferent fibers transmit impulses from receptors to CNS

– Motor division• Efferent fibers transmit impulses from CNS to effector organs

Relationship between motor and sensory fibers of the PNS and the CNS

Structure of a Neuron

• Cell body• Dendrites

– Conduct impulses toward cell body

• Axon– Carries electrical impulse away from cell body– May be covered by Schwann cells

• Forms discontinuous myelin sheath along length of axon

• Synapse– Contact points between axon of one neuron and

dendrite of another neuron

Parts of a neuron

Synaptic transmission

Resting Membrane Potential

• At rest all cells are negatively charged (including neurons)– Determined by concentration of ions (K+, Cl-, Na+)

• This negative charge is called Resting Membrane Potential (~-40 to -70 mv)

• The RMP is maintained by the semi-permeable cell membrane and intra vs extracellular concentration gradient of ions

Sodium Potassium Pump

• The concentration gradient across the cellular membrane is maintained by the Na+/K+ pump

• It uses ATP to actively pump Na+ out of the cell and K+ into the cell

• It takes energy (ATP) to maintain the pump and the gradient

Action potential

– Occurs when depolarization reaches threshold• Permeability of the membrane changes, allowing

Na+ into the cell, making the interior positively charged

– Repolarization• Change in membrane permeability, restoring resting

membrane potential

Action potential is produced by an increase in sodium conductance into the neuron

When a polarized nerve fiber is stimulated, sodium channels open, some sodium ions diffuse inward and the membrane is depolarized

When potassium channels open, potassium ions diffuse outward, and the membrane is repolarized

Neurotransmitters and Synaptic Transmission

• Neurons communicate across synapses using neurotransmitters– Released from presynaptic membrane– Binds to receptor on post synaptic membrane

• Excitatory transmitters– Cause depolarization (EPSP)

• Inhibitory transmitter– Cause hyperpolarization (IPSP)

Structure of a chemical synapse

Sensory Information

• Proprioceptors– Provide CNS with information about body

position and joint angle• Free nerve endings• Golgi-type receptors• Pacinian corpuscles

• Muscle chemoreceptors– Provide CNS with information regarding the

metabolic rate of muscular activity

Reflexes• Rapid, unconscious means of reacting to stimuli• Order of events:

– Sensory nerve sends impulse to spinal column

– Interneurons within spinal cord activate motor neurons

– Motor neurons control movement of muscles

• Reciprocal inhibition– EPSPs to muscles to withdraw from stimulus

– IPSPs to antagonistic muscles

The withdrawal reflex acts on the flexor muscle on one side of the body and the extensor on the other

Somatic Motor Function

• Somatic motor neurons of PNS– Responsible for carrying neural messages from

spinal cord to skeletal muscles

• Motor unit– Motor neuron and all the muscle fibers it

innervates

• Innervation ratio– Number of muscle fibers per motor neuron

A motor unit

Vestibular Apparatus and Equilibrium

• Located in the inner ear

• Responsible for maintaining general equilibrium and balance

• Sensitive to changes in linear and angular acceleration

Vestibular apparatus in maintenance of equilibrium and balance

Motor Control Functions of the Brain

• Brain stem• Cerebrum

– Cerebral cortex• Organization of complex movement• Storage of learned experiences• Reception of sensory information

– Motor cortex• Most concerned with voluntary movement

• Cerebellum– Monitoring complex movement

Motor Functions of the Spinal Cord

• Withdrawal reflex

• Contains groups of neurons capable of controlling certain aspects of motor activity

• Spinal tuning– Voluntary movement is translated into

appropriate muscle action

Structures and processes leading to voluntary movement

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

Neurotransmitters of the autonomic nervous system