Bio 111 Lecture 13
Nervous system is
master controlling and communicating
system of body
• Cells communicate via electrical and chemical signals – Rapid and specific
– Usually cause almost immediate responses
Nervous system has three overlapping functions
1.Sensory input
Information gathered by sensory receptors about internal and external changes
2.Integration
Processing and interpretation of sensory input
3. Motor output
Activation of effector organs (muscles and glands) produces a response
Nervous system is divided into two principal parts:
– Central nervous system (CNS)
Brain and spinal cord of dorsal body cavity § Integration and control center
– Interprets sensory input and dictates motor output
– Peripheral nervous system (PNS)
The portion of nervous system outside CNS
Consists mainly of nerves that extend from brain and spinal cord
– Spinal nerves to and from spinal cord – Cranial nerves to and from brain
Peripheral nervous system (PNS) has two functional divisions
– Sensory (afferent) division
§ Somatic sensory fibers: convey impulses from skin, skeletal muscles, and joints to CNS
§ Visceral sensory fibers: convey impulses from visceral organs to CNS
– Motor (efferent) division
§ Transmits impulses from CNS to effector organs
– Muscles and glands § Two sub-divisions
– Somatic nervous system
– Autonomic nervous system
Motor (Efferent) Divisions
Somatic nervous system
– Somatic motor nerve fibers conduct impulses from CNS to skeletal muscle
– Voluntary nervous system
Conscious control of skeletal muscles
• Autonomic nervous system
– Consists of visceral motor nerve fibers
– Regulates smooth muscle, cardiac muscle, and glands – Involuntary nervous system
– Two functional subdivisions
Sympathetic
Parasympathetic
Work in opposition to each other
6
look at pg
#7 in notes
Nervous Tissue Cells
Nervous tissue consists of two principal cell types
– Neuroglia (glial cells): small cells that surround and wrap
delicate neurons
– Neurons (nerve cells): excitable cells that transmit electrical signals
Four main neuroglia support CNS neurons
– Astrocytes
– Microglial cells
– Ependymal cells
– Oligodendrocytes
Neuroglia of the CNS
Astrocytes
– Most abundant, versatile, and highly branched of glial cells – Cling to neurons, synaptic endings, and capillaries
– Functions include:
- Support and brace neurons
- Play role in exchanges between capillaries and neurons
- Control chemical environment around neurons
- Respond to nerve impulses and neurotransmitters
- Influence neuronal functioning
- Participate in information processing in brain
Microglial cells
– Small, ovoid cells with thorny processes that touch and
monitor neurons
– Migrate toward injured neurons
– Can transform to phagocytize microorganisms and neuronal debris
Ependymal cells
– May be ciliated
Cilia beat to circulate CSF
– Line the central cavities of the brain and spinal column
– Form permeable barrier between cerebrospinal fluid (CSF) in cavities and tissue fluid bathing CNS cells
Autonomic Nervous control
is unconcious control you cannot tell your heart to stop
Oligodendrocytes
– Branched cells
– Processes wrap CNS nerve fibers, forming insulating myelin sheaths in thicker nerve fibers
Neuroglia of PNS • Two major neuroglia seen in PNS
• Satellite cells
– Surround neuron cell bodies in PNS and act like astrocytes
Schwann cells (neurolemmocytes)
– Surround all peripheral nerve fibers and form myelin sheaths in thicker nerve fibers, similar in function to oligodenrocytes
– Vital to regeneration of damaged peripheral nerve fibers (axons)
Neurons
Neurons (nerve cells) are structural units of nervous system
• Large, highly specialized cells that conduct impulses
• Special characteristics
– Extreme longevity (lasts a person’s lifetime)
– Amitotic, with few exceptions
– High metabolic rate: requires continuous supply of oxygen and glucose
• All have cell body and one or more processes
Neuron Cell Body
• Biosynthetic center of neuron
– Synthesizes proteins, membranes, chemicals
– Rough ER (chromatophilic substance, or Nissl bodies)
• Contains spherical nucleus with nucleolus
• Some contain pigments
• In most, plasma membrane is part of receptive region that receives input info from other neurons
Most neuron cell bodies are located in CNS
– Nuclei: clusters of neuron cell bodies in CNS – Ganglia: clusters of neuron cell bodies in PNS
Neuron Processes
Armlike processes that extend from cell body
– CNS contains both neuron cell bodies and their processes – PNS contains chiefly neuron processes
• Tracts
– Bundles of neuron processes in CNS
• Nerves
– Bundles of neuron processes in PNS
• Two types of processes
– Dendrites
– Axon
Dendrites
• Receptive (input/afferent) region of neuron
• Convey incoming messages (from action potential-triggered neurotransmitters from other neurons)
The axon Structure
– Each neuron has one axon that starts at
cone-shaped area called axon hillock
– Long axons are called nerve fibers
– Axons have occasional branches called axon collaterals
– Axons branch profusely at their end (terminus)
– Distal endings are called axon terminals and will release neurotransmitters
The axon: functional characteristics
– Axon is the conducting region of neuron
– Generates nerve impulses and transmits them along axolemma (neuron cell membrane) to axon terminal
Terminal: region that secretes neurotransmitters, which are released into extracellular space next to target cells (other neurons or muscle/gland cells)
Can excite or inhibit other neurons it contacts by making synapses with their dendrites
– Carries on many conversations with different neurons at same time
– Axons rely on cell bodies to renew proteins and membranes
– Axons quickly decay if cut or damaged
Myelin sheath
– Composed of myelin, a whitish, protein-lipid substance – Function of myelin
Protect and electrically insulate axon
Increase speed of nerve impulse transmission
– Myelinated Fibers
Segmented sheath surrounds most long
or large-diameter axons
Myelination in the PNS
– Myelin sheath gaps
Gaps between adjacent Schwann
cells (nodes of Ranvier)
Sites where axon collaterals can emerge
Nonmyelinated fibers
Thin fibers not wrapped in myelin; surrounded by Schwann cells but no myelin sheath
Myelin sheaths in the CNS
– Formed by processes of oligodendrocytes, not whole cells
– Each cell can wrap up to 60 axons at once
– Myelin sheath gap is present
– No outer collar of perinuclear cytoplasm
– Thinnest fibers are unmyelinated, but covered by long extensions of adjacent neuroglia
What is White Matter and What is Gray Matter?
– White matter: regions of brain and spinal cord with dense collections of myelinated fibers
Usually fiber tracts
– Gray matter: mostly neuron cell bodies and nonmyelinated
fibers
Classification of Neurons
Structural classification
– Three types grouped by number of processes
– Three types grouped by number of processes
1.Multipolar: three or more processes (1 axon, other processes are dendrites)
– Most common and major neuron type in CNS
2.Bipolar: two processes (one axon, one dendrite)
– Rare (ex: retina and olfactory mucosa)
3. Unipolar: one T-like process (two ‘axons’)
– Also called pseudounipolar
– Peripheral (distal) process: associated with sensory
receptor
– Proximal (central) process: enters CNS
Look At pg
#23 In Notes
Functional classification of neurons
– Three types of neurons grouped by direction in which nerve
impulse travels relative to CNS
1. Sensory
– Transmit impulses from sensory receptors toward CNS (afferent) – Almost all are unipolar
– Cell bodies are located in ganglia in PNS
2. Motor
– Carry impulses from CNS to effectors (efferent) – Multipolar
– Most cell bodies are located in CNS
3.
Interneurons (association neurons)
– Lie between motor and sensory neurons – Shuttle signals through CNS pathways
– Most are entirely within CNS
Membrane Potentials
Like all cells, neurons have a resting membrane potential (RMP) (required reading Section 11.4)
Changing the Resting Membrane Potential
• Membrane potential changes when:
– Concentrations of ions across membrane change – Membrane permeability to ions changes
• Changes produce two types of signals – Graded potentials
• Incoming signals operating over short distances – Action potentials
• Long-distance signals of axons
• Changes in membrane potential are used as signals to receive,
integrate, and send information
Changing the Resting Membrane Potential Cont’d
Terms describing membrane potential changes relative to resting membrane potential
– Depolarization: decrease in membrane potential (moves toward zero and above eg. From rmp of -70mV to -55mV)
• Inside of membrane becomes less negative than resting membrane potential
• Probability of producing action potential increases
– Hyperpolarization: increase in membrane potential (away
from zero eg -55mV to -70mV)
• Inside of membrane becomes more negative than resting membrane potential
• Probability of producing action potential decreases
Look at pg
#27 in Notes
Graded Potentials
Short-lived, localized changes in membrane potential
– The stronger the stimulus, the more voltage changes and
the farther current flows
• Triggered by stimulus that opens gated ion channels
– Results in depolarization or sometimes hyperpolarization
• Named according to location and function
– Receptor potential (generator potential): graded potentials
in receptors of sensory neurons
– Postsynaptic potential: neuron graded potential
• Once gated ion channel opens, depolarization spreads from one area of membrane to next
• Current flows but dissipates quickly and decays
– Graded potentials are signals only over short distances
Look at pg
#29 In Notes