The Nervous System

Chapters 39 & 40

Overview Three overlapping functions: sensory input,

integration, and motor output Sensory input – the conduction of signals from sensory

receptors to integration centers of the nervous system Integration – information from sensory receptors is

interpreted and associated with appropriate responses from the body

Motor output – the conduction of signals from the processing center to effector cells (muscle cells or gland cells) that carry out the response

Parts Central nervous system (CNS)

Brain and spinal cord Responsible for integration

Peripheral nervous system (PNS) Nerves which carry sensory input to the CNS

and motor output away from the CNS

Neurons Specialized for transmitting chemical and

electrical signals from one location to another Neurons are made up of:

Cell body: contains cytoplasm, nucleus, organelles Dendrites: carry information to the cell body; short,

numerous and very branched Axons: conduct impulses away from the cell body; long

and singular

Types of Neurons Sensory

Convey information about external and internal environments from sensory receptors to the CNS

Interneurons Integrate sensory input and motor output;

Located within the CNS

Motor Convey impulses from the CNS to effector cells

Supporting Cells Glial cells

Structurally reinforce, protect, insulate, and assist neurons

Do not conduct impulses Outnumber neurons 10-50 to 1

The Nature of Neural Signals Signal transmission along the length of a neuron

depends of voltages created by ionic changes across the plasma membranes

There is a difference in ion concentrations between the cell’s contents and the extracellular fluid – this is the membrane potential

All cells have this membrane potential Only neurons and muscle cells can change their

membrane potentials in response to stimuli

Action Potential An all-or-none change in the membrane potential Cells which can do this (nerve and muscle) are called

excitable cells The membrane potential of an excitable cell at rest

(unexcited state) is called a resting potential The presence of gated ion channels in these cells permits

them to change the plasma membrane’s permeability and alter the membrane potential in response to stimuli

An action potential is the rapid change in membrane potential of an excitable cell, caused by stimulus-triggered selective opening and closing of voltage-gated ion channels.

Refractory period A refractory period occurs after the changes in

membrane potential during which time the neuron is insensitive to stimuli.

The action potential is all-or-none: the neuron either fires or doesn’t fire

The nervous system distinguishes between strong and weak stimuli based on the frequency of action potentials generated

Action potentials travel along an axon

A neuron is stimulated at its dendrites or cell body, and the action potential travels along the axon to the other end of the neuron

Chemical or electrical communication between cells occurs at synapses

Synapses Gaps between neurons Electrical synapses:

Allows impulses to travel quickly and without loss of signal strength

Not very common Chemical synapses:

A neurotransmitter is released from the axon of one neuron into the gap

This in turn stimulates the dendrites of the next neuron The neurotransmitter chemical is quickly degraded by enzymes

and the components recycled Some neurotransmitters:

Acetylcholine, epinephrine, dopamine

Organization of the CNS Provides the basis for complex behavior in

vertebrates Spinal cord

Carries information to and from the brain Can integrate simple responses to some stimuli

(reflexes)

CNS… The Brain: Carries out complex integration for

homeostasis, perception, movement, intellect, and emotions

White matter – the inner region Gray matter – the outer region (opposite

orientation in the spinal cord)

The Brain Brain stem

Conducts data and control automatic activities essential for life

Cerebellum Controls movement and balance

Thalamus and hypothalmus Regulates homeostasis

Cerebrum The ‘thinking’ part of the brain

CNS… In both: Meninges – protective layers of connective

tissue Cerebrospinal fluid – fills the ventricles of

the brain and the central canal of the spinal cord; acts as a shock absorber and circulates hormones, nutrients, and white blood cells

The PNS Sensory division

Brings information from the sensory receptors to the CNS

Motor division Carries signals from the CNS to effector cells Two separate divisions:

Somatic nervous system – carry signals to skeletal muscles; voluntary

Autonomic nervous system – controls smooth and cardiac muscles; involuntary