The Nervous System

To return to the chapter summary click escape or close this document. Human Nervous System

Neurons  Cells that transmit impulses or electrical signals  3 types Sensory neurons carry information from the sense organs to spinal cord and brain Motor neurons carry impulses from brain and spinal cord to muscles and glands Interneurons connect sensory and motor neurons and carry impulses between them

Neuron structure  Cell body is largest part; contains the nucleus and most of the cytoplasm  Dendrites are short branches that extend out from the cell body; carry impulses from environment or other neurons toward the cell body  Axon long fiber that carries impulses away from the cell body

Axons  Usually surrounded by insulation called myelin sheath but leaves gaps, or nodes, between sections of the sheath  Impulses jump from node to node which increases the rate of impulse travel down the axon

To return to the chapter summary click escape or close this document. Neuron Membrane

The synapse  Location at which an impulse is passed from one neuron to another  The synaptic cleft (space) separates the axon terminal of one neuron from the dendrites of another  Terminals contain tiny sacs filled with neurotransmitters (chemicals) that carry the impulse across the synaptic cleft to the dendrites of the other neuron

To return to the chapter summary click escape or close this document. Neurotransmitters in Synapse

Neurotransmitters  Chemicals that are released when an impulse reaches the terminal of an axon  Chemicals move across the synaptic cleft and bind to receptors on dendrites of the next neuron  Takes a fraction of a second to bind and be released from receptors on the dendrites  Neurotransmitters are then either broken down by enzymes or recycled by the axon terminal

To return to the chapter summary click escape or close this document. Neurotransmitters

Central nervous system  Consists of the brain and spinal cord  Skull and vertebrae protect them  Both brain and spinal cord are wrapped in three layers of connective tissue called meninges  Space between the meninges and the nervous tissue is filled with cerebrospinal fluid: used as a shock absorber as well as a means of exchanging nutrients and wastes between nervous tissue and blood

To return to the chapter summary click escape or close this document.

Brain  Has four major parts Cerebrum-largest part used for voluntary, or conscious activities Cerebellum-second largest region located at the back of the skull Brain stem-connects the brain and spinal cord just below the cerebellum Thalamus and hypothalamus

To return to the chapter summary click escape or close this document. Human Brain

Cerebrum  Site of intelligence, learning, and judgement  Has right and left hemispheres connected by the corpus callosum  Each half deals with the opposite side of the body

Cerebrum  Has 2 layers Cerebral cortex (outer layer)-has gray matter (densely packed nerve cells); processes information from sense organs and controls body movements White matter (inner layer)-made of bundles of axons with myelin sheaths; connects the brain stem and cerebral cortex

Cerebellum  Coordinates and balances the actions of the muscles so that the body can move gracefully and efficiently

Brain stem  Two regions, pons and medulla oblongata, regulate the flow of information between brain and rest of the body  Also controls important functions such as blood pressure, heart rate, breathing, and swallowing

Thalamus and Hypothalamus  Located between brain stem and cerebrum  Thalamus-receives messages from sensory receptors and relays the information to the cerebrum for processing  Hypothalamus-control center for recognition and analysis of hunger, thirst, fatigue, anger, and body temperature; also controls coordination of nervous and endocrine systems

The spinal cord  Has 31 pairs of spinal nerves that branch out from the spinal cord  Connects the brain to different parts of the body  Processes some information directly, such as reflexes  Reflexes are fast, automatic responses to stimuli, processing by the spinal cord allows quick response to danger for survival

Peripheral nervous system  Consists of nerves and cells that are not part of the brain and spinal cord  These include cranial nerves that stimulate areas on the head and neck, spinal nerves, and ganglia or nerve cell bodies.  Two divisions Somatic nervous system Autonomic nervous system

Somatic nervous system  Regulates activities under conscious control  Movement of skeletal muscles  Also involved with reflexes and can act without conscious control due to reflex arcs (direct paths of impulses to create response without processing in brain)

Autonomic nervous system  Regulates activities that are involuntary  Can speed up or slow down heart rate and blood flow to muscles, stimulate sweat glands and adrenal glands, controls contractions of muscles in digestive system  Two parts Sympathetic and parasympathetic which have opposite effects on the same organ system (to maintain homeostasis) Ex: if sympathetic speeds up heart rate during exercise, parasympathetic will slow it down at rest

To return to the chapter summary click escape or close this document. Organization of Nervous System