Shabba

Chapter 12
Neural Tissue
An Introduction to the Nervous System
The Nervous System
Includes all neural tissue in the body
Neural tissue contains two kinds of cells
1.Neurons
Cells that send and receive signals
2.Neuroglia (glial cells)
Cells that support and protect neurons
Organs of the Nervous System
Brain and spinal cord
Sensory receptors of sense organs (eyes, ears, etc.)
Nerves connect nervous system with other systems
12-1 Divisions of the Nervous System
Anatomical Divisions of the Nervous System
Central nervous system (CNS)
Peripheral nervous system (PNS)
The Central Nervous System (CNS)
Consists of the spinal cord and brain
Contains neural tissue, connective tissues, and blood vessels
Functions of the CNS are to process and coordinate:
Sensory data from inside and outside body
Motor commands control activities of peripheral organs (e.g., skeletal muscles)
Higher functions of brain intelligence, memory, learning, emotion
The Peripheral Nervous System (PNS)
Includes all neural tissue outside the CNS
Functions of the PNS
Deliver sensory information to the CNS
Carry motor commands to peripheral tissues and systems
Nerves (also called peripheral nerves)
Bundles of axons with connective tissues and blood vessels
Carry sensory information and motor commands in PNS
Cranial nerves connect to brain
Spinal nerves attach to spinal cord
Functional Divisions of the PNS
Afferent division
Carries sensory information
From PNS sensory receptors to CNS
Efferent division
Carries motor commands
From CNS to PNS muscles and glands
Receptors and effectors of afferent division
Receptors
Detect changes or respond to stimuli
Neurons and specialized cells
Complex sensory organs (e.g., eyes, ears)
Effectors
Respond to efferent signals
Cells and organs
The efferent division
Somatic nervous system (SNS)
Controls voluntary and involuntary (reflexes) muscle skeletal contractions
The efferent division
Autonomic nervous system (ANS)
Controls subconscious actions, contractions of smooth muscle and cardiac muscle, and glandular secretions
Sympathetic division has a stimulating effect
Parasympathetic division has a relaxing effect
12-2 Neurons
Neurons
The basic functional units of the nervous system
The structure of neurons
The multipolar neuron
Common in the CNS
Cell body (soma)
Short, branched dendrites
Long, single axon
The Cell Body
Large nucleus and nucleolus
Perikaryon (cytoplasm)
Mitochondria (produce energy)
RER and ribosomes (produce neurotransmitters)
Cytoskeleton
Neurofilaments and neurotubules in place of microfilaments and microtubules
Neurofibrils: bundles of neurofilaments that provide support for dendrites and axon
Nissl bodies
Dense areas of RER and ribosomes
Make neural tissue appear gray (gray matter)
Dendrites
Highly branched
Dendritic spines
Many fine processes
Receive information from other neurons
8090% of neuron surface area
The axon
Is long
Carries electrical signal (action potential) to target
Axon structure is critical to function
Structures of the Axon
Axoplasm
Cytoplasm of axon
Contains neurofibrils, neurotubules, enzymes, organelles
Axolemma
Specialized cell membrane
Covers the axoplasm
Axon hillock
Thick section of cell body
Attaches to initial segment
Initial segment
Attaches to axon hillock
Collaterals
Branches of a single axon
Telodendria
Fine extensions of distal axon
Synaptic terminals
Tips of telodendria
The Structure of Neurons
The synapse
Area where a neuron communicates with another cell
Presynaptic cell
Neuron that sends message
Postsynaptic cell
Cell that receives message
The synaptic cleft
The small gap that separates the presynaptic membrane and the postsynaptic membrane
The synaptic terminal
Is expanded area of axon of presynaptic neuron
Contains synaptic vesicles of neurotransmitters
Neurotransmitters
Are chemical messengers
Are released at presynaptic membrane
Affect receptors of postsynaptic membrane
Are broken down by enzymes
Are reassembled at synaptic terminal
Recycling Neurotransmitters
Axoplasmic transport
Neurotubules within the axon
Transport raw materials
Between cell body and synaptic terminal
Powered by mitochondria, kinesin, and dynein
Types of Synapses
Neuromuscular junction
Synapse between neuron and muscle
Neuroglandular junction
Synapse between neuron and gland
Structural Classification of Neurons
Anaxonic neurons
Found in brain and sense organs
Bipolar neurons
Found in special sensory organs (sight, smell, hearing)
Unipolar neurons
Found in sensory neurons of PNS
Multipolar neurons
Common in the CNS
Include all skeletal muscle motor neurons
Anaxonic Neurons
Small
All cell processes look alike
Bipolar Neurons
Are small
One dendrite, one axon
Unipolar Neurons
Also called pseudounipolar neurons
Have very long axons
Fused dendrites and axon
Cell body to one side
Multipolar Neurons
Have very long axons
Multiple dendrites, one axon
Three Functional Classifications of Neurons
1.Sensory neurons
Afferent neurons of PNS
2.Motor neurons
Efferent neurons of PNS
3.Interneurons
Association neurons
Functions of Sensory Neurons
Monitor internal environment (visceral sensory neurons)
Monitor effects of external environment (somatic sensory neurons)
Structures of Sensory Neurons
Unipolar
Cell bodies grouped in sensory ganglia
Processes (afferent fibers) extend from sensory receptors to CNS
Three Types of Sensory Receptors
1.Interoceptors
Monitor internal systems (digestive, respiratory, cardiovascular, urinary, reproductive)
Internal senses (taste, deep pressure, pain)
2.Exteroceptors
External senses (touch, temperature, pressure)
Distance senses (sight, smell, hearing)
3.Proprioceptors
Monitor position and movement (skeletal muscles and joints)
Motor Neurons
Carry instructions from CNS to peripheral effectors
Via efferent fibers (axons)
Two major efferent systems
1.Somatic nervous system (SNS)
Includes all somatic motor neurons that innervate skeletal muscles
2.Autonomic (visceral) nervous system (ANS)
Visceral motor neurons innervate all other peripheral effectors
Smooth muscle, cardiac muscle, glands, adipose tissue
Two groups of efferent axons
Signals from CNS motor neurons to visceral effectors pass synapses at autonomic ganglia dividing axons into:
Preganglionic