Human Morph & Func ch 12

Briefly describe the nervous system.

communication and control system

What are some functions of the nervous system?

• Collect information

• Processes and evaluate information

• Initiate response to information

How does the nervous system collect information?

Receptors detect stimuli and send sensory signals to spinal cord and brain

How does the nervous system organs evaluate and process information?

Brain and spinal cord determine response to sensory input

How does the nervous system initiate response to information?

Brain and spinal send motor output via nerves to effectors (muscles or glands)

What is the central nervous system comprised of?

brain and spinal cord

What is the peripheral nervous system comprised of?

nerves and ganglia

Describe the sensory nervous system.

afferent nervous system that receives sensory information from receptors and transmits it to CNS

- somatic and visceral

Describe sensory somatic nervous system.

detects stimuli we consciously perceive

Describe visceral nervous system.

detects stimuli we typically do not perceive

• For example, signals from the heart or kidneys

Describe motor nervous system.

efferent nervous system that initiates motor output and transmits it from CNS to effectors

- somatic and autonomic

Describe motor somatic nervous system.

sends voluntary signals to skeletal muscles

Describe autonomic nervous system.

(visceral motor) sends involuntary commands to heart, smooth muscle, and glands

• Has sympathetic and parasympathetic divisions

What is a nerve?

a bundle of parallel axons in the PNS w/ connective tissue wrappings: Epineurium, Perineurium, Endoneurium

Describe epineurium.

thick layer of dense irregular connective tissue that encloses the entire nerve

Describe Perineurium.

layer of dense irregular connective tissue that wraps fascicle

Describe Endoneurium.

delicate layer of areolar connective tissue that separates and electrically insulates each individual axon

Describe the vascularization of nerves.

Blood vessels branch through epineurium and perineurium to become capillaries; allow for exchange between axons and blood

How are nerves structurally classified?

• Cranial nerves: extend from brain

• Spinal nerves: extend from spinal cord

How are nerves functionally classified?

sensory, motor, mixed

Describe sensory nerves.

contain sensory neurons sending signals to CNS

Describe motor nerves.

contain motor neurons sending signals from CNS

Describe mixed nerves.

most commonly named, contain both sensory and motor neurons w/ individual axons in these nerves transmit

only one type of information

What is a ganglion?

a cluster of neuron cell bodies in the PNS

What is a neuron?

the structural unit of the nervous system

Describe the following neuron characteristic: excitability.

responsiveness to a stimulus

• Stimulus causes change in cell's membrane potential

Describe the following neuron characteristic: conductivity.

ability to propagate electrical signal

• Voltage-gated channels along membrane open sequentially

Describe the following neuron characteristic: secretion.

release of neurotransmitter in response to conductive activity

• Messenger is released from vesicle to influence target cell

Describe the following neuron characteristic: extreme longevity.

cell can live throughout person's lifetime

Describe the following neuron characteristic: amitotic.

After fetal development, mitotic activity is lost in most neurons

Describe the Cell body (soma) of a neuron.

• Plasma membrane encloses cytoplasm

• Contains nucleus

• Initiates some graded potentials, receives others from dendrites; conducts these potentials to axon

• Contains chromatophilic substance (Nissl bodies) made of ribosomes (free and bound)

Describe the dendrites of a neuron.

• Short, unmyelinated processes branching off cell body

• Receive input and transfer it to cell body

Describe the axon of a neuron.

• Long process emanating from cell body

• Makes contact with other neurons, muscle cells, or glands

• function to conduct action potentials and then release neurotransmitter at terminals

How does the axon attach to the cell body?

via axon hillock

What does the axon split into?

branches called axon collaterals

What are axon terminals also called?

telodendria

What do axon terminals house?

synaptic vesicles containing neurotransmitter

Describe the cytoskeleton of neurons.

Composed of microfilaments, intermediate filaments,

microtubules

What is the function of intermediate filaments?

(neurofilaments) aggregate to form bundles, neurofibrils, that provide tensile strength

What is anterograde transport?

Moves newly synthesized material from cell body toward axon terminals

What is retrograde transport?

Moves used materials from axon for breakdown and recycling in soma to cell body

What is a multipolar neuron?

most common type of neuron, many dendrites, 1 axon

What is a bipolar neuron?

sense neurons w/ one dendrite and one axon

What is a unipolar neuron?

most sensory neurons w/ two dendrites on one long axon

What is an anaxonic neuron?

interneurons w/ no axons, only dendrites

Describe sensory neurons.

mainly unipolar, conduct input from somatic and visceral receptors to CNS

Describe motor neurons.

mainly multipolar, conduct output from CNS to somatic and visceral effectors

Describe interneurons.

mainly multipolar, receive, process, and integrate information from many other neurons and communicate between sensory and motor neurons

• Located within CNS; make up 99% of our neuron

What is a synapse?

place where a neuron connects to another neuron or an effector

- chemical (more common) and electrical

Describe electrical synapses.

Presynaptic and postsynaptic neurons bound together by gap junctions

• Fast: no synaptic delay in passing electrical signal

• Located in limited regions of the brain and eyes

Describe chemical synapses.

• Presynaptic neuron's axon terminal produces signal

• Postsynaptic neuron receives signal

What is a synaptic cleft?

small fluid-filled gap between the two neurons

Describe synaptic communication.

1. Neurotransmitter molecules released from vesicles of axon terminal into cleft

2. Neurotransmitter diffuses across cleft and binds to postsynaptic receptors

3. Binding of neurotransmitter to receptor initiates postsynaptic potential (a graded potential)

What is synaptic delay?

time it takes for all of these events to occur

What are glial cells?

Nonexcitable, support cells found in CNS and PNS

What are some general characteristics of glial cells?

• Capable of mitosis

• Protect and nourish neurons

• Provide physical scaffolding for nervous tissue

• Guide migrating neurons during development

• Critical for normal function at neural synapses

What are the functions of oligodendrocytes?

- myelinates and insulates CNS axons

- allows faster action potential propagation along axons in the CNS

What are the functions of microglia?

- phagocytize substances in the CNS

- protect CNS by engulfing infectious/harmful agents

What are the functions of the ependymal cells?

- Line the ventricles of the brain and the central canal of the spinal cord

- assist in production and circulation of CSF via choroid plexus

What some functions of astrocytes?

- helps form the blood-brain barrier

- regulates interstitial fluid composition

- provides structural support and organization of CNS

- assists w/ neuronal development

- alters synaptic activity

- replicates to occupy space of dying neurons

Describe astrocytes.

star-shaped cells w/ perivascular feet

How do astrocytes develop the blood-brain barrier?

by wrapping feet around brain capillaries to control which substances have access to brain

What are microglia?

Small cells that wander CNS and replicate in infection

Describe oligodendrocytes.

Large cells with slender extensions that wrap around axons of neurons, forming myelin sheath

What are satellite cells?

arranged around neuronal cell bodies in a ganglion, electrically insulate, and regulate the exchange of nutrients and wastes

What are schwann cells (neurolemmocyte)?

Elongated, flat cells that ensheath PNS axons with myelin, allowing for faster action potential propagation

Define myelination.

process of wrapping an axon with myelin

What is myelin?

several layers of membrane of glial cells

What is the purpose of high lipid content in myelin?

gives it glossy-white appearance and insulates axon

What kind of cells are glial cells?

Schwann cells in PNS; sometimes also referred to as oligodendrocytes in CNS

What is neuron function dependent on?

on transmission of intracellular electrical signals

How does the plasma membrane facilitate electrical signals?

by establishing ion concentration gradients

Describe the function of proteins in neurons.

Membrane proteins that maintain a concentration gradient by moving substances against their concentration gradient using energy

What kind of pumps do neurons have?

sodium-potassium pumps and calcium pumps in their membranes

What are channels?

Protein pores in the membrane that allow ions to move down their concentration gradients (into or out of the cell)

Describe leak channels.

always open for continuous diffusion

What is the difference between chemically gated channels and voltage-gated channels?

c: normally closed, but open when neurotransmitter binds (cell body)

v: normally closed, but open when qmembrane charge changes

What are modality-gated channels?

Normally closed, but open in response to specific type of sensory stimulus

• Found in membranes of sensory neurons that respond to changes in their environment

What pumps and channels are found in the entire plasma membrane of a neuron?

• Leak channels (For example, K+ leak channels)

• Na+/K+ pumps

• Maintain resting membrane potential

What pumps/channels are found in the Receptive segment (dendrite and cell body) of a neuron?

Chemically gated channels (For example, chemically gated Cl- channels)

What pumps/channels are found in the initial segment (axon hillock) and the Conductive segment (axon and its branches) of a neuron?

Voltage-gated Na+ channels and voltage-gated K+ channels

What pumps/channels are found in the Transmissive segment (axon terminals)?

Voltage-gated Ca2+ channels and Ca2+ pumps

What is electrical energy?

movement of charged particles

Describe voltage (V).

Amount of difference in electrical charge between two places, represents potential energy

• Measured in volts or millivolts

Describe current (I).

Movement of charged particles across barrier separating them

• Can be harnessed to do work

Define resistance (R).

Opposition to movement of charged particles (that is, the barrier)

• An increase in resistance lowers the current

What is Ohm's Law?

Current = voltage/resistance (I = V/R)

• Current increases with larger voltage and smaller resistance

How does Ohm's Law relate to neurons?

• Charged particles are ions, and current is generated when ions diffuse through channels

• Voltage exists across the membrane due to unequal distribution of ions

• The membrane offers resistance to ion flow, and this resistance changes due to the actions of gated channels

• Resistance decreases when channels open

What are some characteristics of neurons at rest?

• Ions are unevenly distributed across the plasma membrane due to the actions of pumps

• Calcium concentration gradient exists at axon terminal

• Gated channels are closed in the functional segments of the cell

• There is an electrical charge difference across the membrane (called a membrane potential)

How are ions unevenly distributed across the plasma membrane?

• Higher concentration of K+ in cytosol verses interstitial fluid (IF)

• Higher concentrations of Na+, Cl-, Ca2+ in interstitial than in cytosol

Describe the electrical charge difference across the membrane of a neuron.

• Cytosol is relatively negative compared to IF

• Resting membrane potential (RMP) is typically -70 mV

• This difference can be measured with microelectrodes (one inside cell; one outside) and a voltmeter

Why is K+ diffusion the most important factor in setting RMP?

K+ diffuses out of the cell due to its concentration gradient and is limited by the electrical gradient

What would happen if K+ were the only ion that leaked?

RMP would be where the K+ concentration and electrical gradients are at equilibrium (-90 mV)

How does Na+ influence RMP?

Na+ diffuses in due to its concentration gradient and the electrical gradient

• This small Na+ leakage means RMP is less negative (-70 mV)

Describe the role of Na+/K+ pumps in maintaining RMP.

by pushing 3 positive charges out and pushing in only 2, the pump contributes about -3 mV (of the - 70 mV total)

Describe the receptive segment of a neuron.

binding of neurotransmitter released from presynaptic neurons leads to the production of graded potentials.

What are graded potentials?

small, short-lived changes in the RMP

How are graded potentials established in the receptive segment?

by the opening of chemically gated ion channels

What are graded potentials in the postsynaptic neuron referred to as?

postsynaptic potentials

What is a postsynaptic potential resulting in depolarization called?

excitatory postsynaptic potential (EPSP)