The Nervous System

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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

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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

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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

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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

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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

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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

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Organization of Nervous

System