The Nervous SystemChapter 35

Maintaining Homeostasis

•Homeostasis is the process by which organisms keep internal conditions relatively constant despite changes in external environments.

–Homeostasis in the body is maintained by feedback inhibition.

Maintaining Homeostasis

•Feedback inhibition (negative feedback) = the process in which a stimulus produces a response that opposes the original stimulus.

Maintaining Homeostasis

•An Example of Feedback Inhibition

Thermostat senses temperature change and switches off heating system

Thermostat senses temperature change and switches on heating system

Room temperature increases

Room temperature decreases

35.2 The Nervous System

An Overview• The NS interprets stimuli and

coordinates other organ systems to respond to stimuli

• A nerve impulse, or action potential, is an electrical signal that travels like a wave over the length of a nervous system cell called a neuron

Cells of the NS• Neurons include:

– Sensory neurons transmit impulses from the environment to the spinal cord and brain

– Motor neurons carry instructions from the brain and spinal cord to muscles and glands

– Interneurons are the most abundant and transmit signals from one neuron to another

Anatomy of a Neuron• The cell body contains the nucleus and

other organelles• Dendrites receive stimuli and conduct

impulses towards the cell body• The axon carries impulses away from the

cell body

Dendrite

Cell Body

Axon

• The myelin sheath is composed of cells called Schwann cells and insulate the axon

• Nodes of Ranvier are the gaps between neighboring Schwann cells

• Axon terminals are the ends of the axon

Direction of impulse

Myelin SheathNodes of Ranvier

Axon Terminals

The Synapse• A synapse is the space between a neuron

and another cell

• Neurotransmitters are chemicals that are stored in the axon terminals and released into a synapse when triggered by an impulse– send message to the next neuron

• An impulse begins when a neuron is stimulated by another neuron or by the environment.

35.3 Divisions of the Nervous System

2 Major Divisions:

• Central Nervous System (CNS) = the brain and spinal cord

• Peripheral Nervous System (PNS) = cranial and spinal nerves

The CNS

• The central nervous system relays messages, processes information, and analyzes information.

• meninges = the protective membrane covering the brain and spinal cord

• cerebrospinal fluid (CSF) = fluid that circulates around the CNS and provides cushioning

• entire CNS encased in bone

Pons

Pituitary gland

Hypothalamus

Cerebrum

Medulla oblongata Spinal cord

Cerebellum

Pineal gland

Thalamus

The Human Brain

• Adult human brain is composed of ~100 billion neurons

• The cerebrum is the largest and most prominent region of the human brain. – controls the voluntary, or conscious,

activities of the body– divided into right and left hemispheres,

which control sensory and motor functions of the opposite side of the body

• The second largest region of the brain is the cerebellum. – coordinates and balances the actions of the

muscles so that the body can move gracefully and efficiently

• The brain stem connects the brain and spinal cord. – It has two regions: the pons and the medulla

oblongata. Each region regulates information flow between the brain and the rest of the body.

– Blood pressure, heart rate, breathing, and swallowing are controlled in the brain stem.

• The spinal cord is the main communications link between the brain and the rest of the body. – certain information, including some

kinds of reflexes, are processed directly in the spinal cord.

– A reflex is a quick, automatic response to a stimulus.

Concept Map

is divided into

Section 35-3The Nervous

System

Central nervous system

Peripheral nervous system

Brain Spinal Cord

which consists of which consists of

Spinal Nerves