Chapter 48: Nervous System and Functions. Nervous system Components Neurons are the basic nerve cell or smallest entity of communication - all animals.

Chapter 48: Nervous System and Functions

Nervous system Components

• Neurons are the basic nerve cell or smallest entity of communication

- all animals groups except sponges have a neural network of some kind

- differences aren’t the neurons but how they are organized- Nerve nets serve most invertebrates for contraction and

expansion of cavity- Cephalization involves clustering neurons near brain to

form more complex systems*CNS (Central Nervous System ) – consists of brain and spinal cord*PNS (Peripheral Nervous System) – consists of segmented

clusters of neurons (ganglia) and nerves that run throughout the body

Nervous Systems

Neuron Structure

• Neuron consists of 3 main structures:Cell body – contains nucleus and organellesDendrites – branched extension of cell body that receives

signals from other neuronsAxons- extension from base of neuron that transmits signal

to other neurons*Covered by a layer in segments called a

myelin sheath ( increases conduction speed)• Synapse – space between end of one axon and dendrite of next

cell - Neurontransmitters – chemical messengers that navigate

across synapse to continue message of neuron

Neuron Structure

Supporting Cells• Glia – supporting cells necessary for integrity of

nervous systemKinds of glia:1. Astrocytes – provide structural support and

regulate concentrations of ions and neurotransmitters2. Radial glia – form tracks that give rise to CNS and can serve as stem cells to generate new neurons

and glia3. Oligodendrocytes and Schwann cells – form

the myelin sheaths around axons*Multiple sclerosis – deterioration of myelin

sheaths and progressive loss of body function

Supporting Cells

Info Processing• 3 Stages of processing information:

1. Sensory input – sensory neurons transmit along nerves (light, sound, touch, smell)

2. Integration – interneurons analyze inputs and relay from brain to motor output

3. Motor output – motor neurons are effector cells that act on response from body

Reflexes

Membrane Potential

• All cells have an electrical or voltage difference across their plasma membrane known as membrane potential

• Resting potential – membrane of neuron that is not transmitting signals

- refers to the passive diffusion of ions across the membrane

Ex: Na and K in and out cell: Na/K pump• Basis of all electrical signals in the nervous system is

that membrane potential can change from resting when membrane’s permeability to an ion changes

Gated Ion channels

• Neurons can have gated ion channels which open and close in response to stimuli

Stretch-gated – sense stretch and open when membrane is mechanically deformed

Ligand-gated – messenger/receptor specific, usually related to chemicals like neurotransmitters found at synapses and bind to a channel

Voltage gated – open and close when membrane potential changes

Gated Ion Channels

Action Potentials• Action potential – all or none response that takes

place when membrane potential has reached its threshold

*Gated channels open or close in response to stimuli that changes membrane potential

- Na/ K channels are the main ions involved in action potentials

- Normal resting potential is a negative charge (-60 – 80 mV), due to constant shuffling of potassium ions

Action Potential• Depolarization occurs with membrane permeability

changing to open Na channels- Na rushes into cell giving cell a positive charge- Threshold is reached creating an action potential send along the axon

• K channels that were inactivated change and open to allow Potassium to leave the cell creating a negative charge or Repolarizing effect

• Continued release of K from open channel eventually causes a hyperpolarizing effect

- Na and K channels are closed causing a period of inactivity known as a refractory period

Action Potentials

Conduction of Action Potentials

• Conduction can be long distance signal

• Signal maintains strength without diminish by regenerating itself along axon

• Depolarization (Na)wave is in one direction only; followed by a repolarization wave (K)

Speed of Signal

• Factors that contribute to conduction speed:1. Diameter of axon – larger offers less resistance – faster speed2. Myelinated axons increase speed along the nodes (Nodes of Ranvier)

- Saltatory conduction - signal jumps from node to node regenerating the depolarizing and repolarizing effect

Saltatory Conduction

Neuron Communication

• Information traveling from neuron to neuron has two possible transmissions

1. Electrical synapses – contain gap junctions and allow electric current to flow directly

from cell to cell2. Chemical synapses – involve the release of neurotranmitters that have been packaged in synaptic vessicles, fuse with the membrane,

and by exocytosis cross the synaptic cleft to post-synaptic cell

Direct Synaptic Transmission

• Direct transmission involved ligand gated ion channels responding to specific neurotransmitters

- binding to receptor opens or activates channel allowing ions to diffuse across membrane- creates a postsynaptic potential by

changing membrane permeability

Direct Transmission

Two effects of postsynaptic potentials1. Excitatory postsynaptic potentials

(EPSPs) – depolarization of membrane nears threshold

2. Inhibitory postsynaptic potential (IPSPs)- selective channels open creating a

hyperpolarizing effect that prevents membrane from nearing threshold

Summation• Unlike action potentials, PSPs are graded responses• Signal strength diminishes and does not regenerate itself at nodes• PSPs also depend on amount of neurotransmitter released • Summation refers to a process in which PSPs can add together to

near threshold and generate an action potential or oppose the chances of threshold being near

Temporal summation – two or more EPSPs occur in rapid succession from the same presynaptic neuron before return to resting potential (near threshold)

Spatial summation – two or more EPSPs occur in succession but from different presynaptic neurons

• Summation can happen in the same way with IPSP’s and prevent a membrane from reaching threshold

Summation

Indirect Synaptic Transmission

• Neurotranmitters bind to a receptor not on the ion channel and cause the activation of a signal transduction pathway

- second messenger system has cascade of reactions that create a slower onset but

longer lasting effect - can be an amplified effect that opens

or closes many kinds of channels

Neurotranmitters

Specialization of Nevous System

• Vertebrate CNS is derived from a nerve cord which is hollow and develops into a central canal in the spinal cord and ventricles within brain

- these hollow spaces are filled with cerebrospinal fluid that aids in transport of nutrients, hormones and removal of wastes- also adds cushion to the brain*White matter – of the brain refers to myelinated axons*Gray matter – refers to cell bodies and dendrites

Peripheral Nervous System

• Consists of right-left pairs of cranial and spinal nerves

12 pairs of cranial nerves that originate in the brain and terminate in organs of head and upper body31 pairs of spinal nerves that originate in spinal cord and extend to many parts of the body

• Somatic nervous system is a function component of the PNS that carries signals to and from skeletal muscles in response to external stimuli

- Under conscious control or is voluntary

Peripheral Nervous System

Autonomic nervous system – another functional component of the PNS; regulates internal environment by controlling smooth, cardiac muscles, digestive organs, excretory and endocrine systems

3 division of Autonomic nervous system:1. Sympathetic – corresponds to arousal and energy

generation (adrenaline response)2. Parasympathetic – counteracts acts of the

sympathetic division – calming and return to homeostasis3. Enteric – consists of neuron of digestive tract, pancreas,

and gallbladder *controls organ secretions

Autonomic Nervous System

Brain Development

BrainstemBrain stem consists of 3 parts:

1. Midbrain – integrated relay center for sensory input (visual and auditory)

2. Pons – involuntary functions much like medulla, but also regulation of functions in the medulla (breathing)

3. Medulla oblongata – involuntary visceral functions like breathing, heart rate, swallowing, vomiting, and digestion• Reticular formation – diffuse cluster of

neurons in brainstem the regulate sleep and arousal

Cerebellum

• Developed from part of metencephalon• Functions include:

1. coordination and error checking with motor functions2. balance and movement 3. learning and remembering motor skills4. Hand-eye coordination

Diencephalon

• 3 main parts:1. epithalamus – includes pineal gland (melatonin);

secretion in response to light2. Thalamus – main input relay center for sensory

input to the cerebrum and motor info leaving 3. Hypothalamus – control of hormone release,

homeostatic mechanisms including hunger/thirst, basic survival mechanisms, sleep/wake (biological clock), sexual and mating behaviors

Cerebrum• Developed as outgrowth of forebrain and is divided into 2

hemispheres- outer portion of gray matter; inner portion white matter- largest and most complex portion of the brain- sensory analyzing, motor commands, language generation and

interpretation*Neocortex outermost portion that separates mammals from

other organisms – convoluted folds that increase surface area but still fits inside the skull• Each hemisphere corresponds to opposite side of the body

Ex: right hemisphere controls and receives input on left side of body

• Corpus callosum – band of axons that serve as a communication link between the 2 hemispheres

Cerebrum

Lobes of Brain

• Cerebrum is divided into 4 lobes dealing with sensory and motor inputs

1. Frontal lobe – olfactory sense; emotions; memory

2. Parietal lobe – somatosensory cortex dealing with touch, pain, pressure, and temperature

3. Occipital lobe – primary visual cortex4. Temporal lobe – auditory cortex; language comprehension

Information Processing

• Neurons of the somatosensory cortex and motor cortex are distributed in an orderly fashion according to part of body it corresponds to

*Cortical surface area devoted to each body part is not related to size, but to the number of sensory neurons that innervate it or the skill needed to control the muscles

Language and Speech

• Broca’s Area – located on left side of frontal lobe near primary motor cortex controls ability to formulate speech

• Wernicke’s Area – located in posterior portion of temporal lobe that controls ability to comprehend speech and language

PET of Language Mapping

Emotions

• Limbic system – ring of structures found around brainstem

3 parts :1. Amygdala2. Hippocampus3. Olfactory bulb

*system mediates feeling like laughing, crying, aggression, feeding, and sexuality

• Also plays role in emotional memory

Memory and Learning

• Frontal lobe control mainly – short term memory – if irrelevant is released

• Long term memory – frontal lobe control but also incorporation of hippocampus

• Consolidation – transfer of short term to long term memory

Long term potentiation – increase in the strength of synaptic transmission by brief high frequency action potentials

- strengthens neural pathway of memory - fundamental to learning