Biology 30 AP - Nervous System & Senses

Central nervous system (CNS)

the body’s coordinating center for mechanical and chemical actions

brain & spinal cord

Outer region of CNS

white matter

protected by oligodendrocytes which have a myelin sheath

Inner region of CNS

does NOT have a myelin sheath

Functions of CNS

relays nerve impulses to and from the brain

uses sensory and motor neutrons

controls spinal reflexes

Peripheral nervous system (PNS)

all parts of the nervous system excluding brain and spinal cord

relay information between the CNS and other parts of the body (muscles & glands) for a voluntary (somatic) or involuntary (autonomic) response

Glial (neuroglial) cell

non-conducting cells

important for structural support and metabolism of nerve cells

Neurons

functional units of nervous system

conducts nerve impulses

Dendrites

receive information from either environment or other neurons

projection of cytoplasm that carries impulses towards cell body

Axon

extension of cytoplasm that carries the nerve impulses away from the cell body

Myelin sheath

white coat of fatty protein

acts as insulation for the neurons

Schwann cell

special type of glial cell that produces myelin sheath

Nodes of Ranvier

regularly occurring gaps between section of myelin sheath along axon

Neurilemma

delicate membrane

surrounds axon of SOME nerve cells

promotes the regeneration of damaged axons

not all nerve cells that have a myelin sheath have a neurilemma (CNS lacks neurilemma)

Types of neurons

sensory (afferent)

interneuron

motor (efferent)

Sensory (afferent)

relay information or stimuli received by sensory receptors about the internal/external environment to the CNS for processing

Interneurons

links neurons to other neurons

found only in brain and spinal cord

integrate and interpret the sensory information and connect to outgoing motor neurons

Motor (efferent)

relay information to the effectors (cell/organ that responds)

effectors: muscles, organs, glands

Reflex arc

simplest nerve pathway

involuntary/unconscious

do not require coordination by the brain

stimulus → receptor → sensory neuron → interneuron → motor neuron → effector → response

Steps of the reflex arc

1. The stimulus is detected by receptors in the skin

2. Receptors initiate nerve impulses in the sensory neurons leading from them to the spinal cord

3. Impulses enter the spinal cord and initiate impulses in one or more association/interneurons

4. Association/interneurons initiate impulses in the appropriate motor neurons

5. When these impulses reach the junction between the motor neurons and the muscles, the muscles are stimulated to contract and the hand is withdrawn

Resting potential

voltage difference across a nerve cell membrane when it is not transmitting a nerve impulse

more negative inside

average: -0.70 millivolts (mV)

Polarization

generating resting potential

unequal distribution of positively charged ions during a resting membrane

Factors to achieve polarization/resting potential

large, negatively charged protein molecules found in intracellular fluid (too big to exit)

membrane is impermeable to some smaller negatively charged ions such as Cl^-

sodium-potassium exchange pump (most important)

3 Na⁺ out → 2 K⁺ = positive charge accumulates outside the cell

Sodium-potassium exchange pump

uses ATP to actively transport sodium ions out of the cell and potassium ions into the cell

Action potential

the voltage difference across a nerve cell membrane when the nerve is excited

reversal of electric charges inside the cell membrane

Nerve impulse

series of action potentials

Depolarization

neuron receives a stimulus, membrane becomes more permeable to Na⁺ than K⁺

Na⁺ channels opened, while K⁺ stays closed

Na⁺ rush into the cell by diffusion and charge attraction

charge reversal occurs (depolarization)

Repolarization

positive inside causes Na⁺ channels to close, stopping Na⁺ inflow

K⁺ channels open → K⁺ diffuse out, charge outside cells becomes positive again

restoring original polarity (repolarization)

Refractory period

K⁺ gates close too slowly, outside cell becomes more positively charged than resting potential (hyperpolarization)

sodium-potassium pump restores resting membrane potential

membrane cannot generate another action potential during this time (refractory period)

Does action potential move?

No

Wave of depolarization

many action potentials in a row

stimulus triggers first action potential → generates depolarization → triggers next depolarization → and so on

Why is the refractory period crucial/important?

stops the action potential from happening in reverse (via hyperpolarization)

What does a stimulus have to reach?

threshold to initiate an action potential

below this value do not initiate a response (all-or-none response)

increasing intensity of stimuli above threshold will NOT produce an increased response → intensity and speed of transmission are the same

neurons either fire maximally or not at all (no in-between)

Threshold

different for each neuron

What does the brain recognize for each nerve impulse?

the more intense the stimulus, the higher the frequency of impulses

the greater the frequency of impulses is a difference the brain can recognize

Synapses

small spaces between neurons or between neurons and effectors (e.g. muscles)

terminal branches of one neuron may join with many different neurons (rarely just two)

Neurotransmitters

small vesicles containing chemicals

released from end plates of axon terminals

Pathway of neurotransmitters from presynaptic neuron

released from presynaptic neuron

diffuses across synaptic cleft

bind to receptors

depolarize dendrites on post synaptic

Is diffusion across synapses fast or slow?

relatively slow

the more synapses over a specified distance = slower speed of transmission

Acetylcholine

found in most nerve cells

acts as an excitatory neurotransmitter by opening Na⁺ channels

Na⁺ channels open → Na⁺ rush into postsynaptic neuron → depolarization → action potential → nerve impulse

Cholinesterase

enzyme released by presynaptic membrane to destroy acetylcholine

Na⁺ channels close → neuron recovery stage

Inhibitory neurotransmitters

make postsynaptic membrane more permeable to K⁺

more K⁺ gates open → more K⁺ diffusion out of cell → hyperpolarization → action potentials inhibited

Steps for impulse to be transmitted through synaptic cleft

1. Action potential reaches presynaptic terminal

2. Voltage-gated Ca²⁺ channels open

3. Influx of Ca²⁺

4. Synaptic vesicles fuse with membrane (exocytosis)

5. Neurotransmitters released into synaptic cleft and diffuse to postsynaptic terminal

6. Neurotransmitter binds to receptor on postsynaptic membrane

7. Excitatory or inhibitory

8. If threshold reached → action potential initiated

9. Neurotransmitters broken down by specific enzymes in synaptic cleft

Summation

effect in the postsynaptic neuron produced by accumulation of neurotransmitters from two or more neurons

Neurotoxins

bacterial proteins that are exocytosed into nerve endings

inhibit the release of essential neurotransmitters

e.g. botulinum, tetanus

Why do nerve impulses move faster along myelinated nerve fibres?

myelin sheath acts as an electrical insulator, forcing the impulse to jump b/w the nodes of Ranvier instead of moving along/down the axon

Meninges

protective layers surrounding the brain and spinal cord

3 layers

3 layers of meninges

outer: dura mater

middle: arachnoid mater

inner: pia mater

Cerebrospinal fluid

circulates between innermost and middle meninges

circulates through central canal of spinal cord

acts as a shock absorber

transports medium (carrying nutrients to/removing wastes from)

Meningitis

inflammation of meninges

bacterial/viral infection

can spread to underlying brain tissue

Blood-brain barrier (right)

tight seal

only allows certain substances to pass through

Foramen magnum

opening from the skull

where the spinal cord extends downward through a canal within the backbone

Central grey matter nerve tissue

non-myelinated interneurons

White matter nerve tissue

myelinated nerve fibers from sensory and motor neurons

Dorsal root

brings sensory info into the spinal cord

Ventral root

carries motor info from the spinal cord to the peripheral nervous system

Forebrain

consists of: cerebrum, thalamus, hypothalamus, olfactory bulbs

Cerebrum

largest part of brain

left and right hemispheres - act as coordinating centers from which sensory info and motor actions originate

Right side of brain

visual patterns

spatial awareness

Left side of brain

linked to verbal skills

Corpus callosum

allows for communication b/w left and right hemispheres

Cerebral cortex

surface of cerebrum

composed of grey matter

many fissures (folds or “sulci”) - increase surface area

bumps or hills = “gyri”

Thalamus

relay station for sensory info going to cerebrum

Hypothalamus

small size

big role in maintaining body’s internal equilibrium

direct connection w/ pituitary glands unites nervous and endocrine systems

Lateral ventricle

produce and store cerebrospinal fluid

Olfactory bulbs

receive and interpret information about smell

Midbrain

lies just below thalamus

relay center for some eye/ear reflexes

Hindbrain

posterior to midbrain

joins with spinal cord

consists of: cerebellum, pons, and medulla oblongata

Cerebellum

controls limb movements, balance, and muscle tone

Pons

relay station for regions of cerebellum and b/w cerebellum and medulla

Medulla oblongata

connection b/w CNS and PNS

controls involuntary muscle action (e.g. breathing, heart rate)

coordinating center for the autonomic nervous system

What are MRIs, CTs, & PETs used for?

non-invasive tools

map human brain function

screen for diseases

Frontal lobe

motor area control movement to voluntary muscles

linked to intellect, memory, emotions, and personality

inhibits unsuitable behaviors

Temporal lobe

process hearing (and some visual) information

linked to visual and verbal memory, emotions, learning, and interpretations of language

Parietal lobe

associated with touch and temperature awareness from the skin

linked to emotions and interpretations of language

body senses & orientation

Occipital lobe

associated with vision

interpret visual information

Somatic vs. autonomic

both consist of sensory and motor neurons

somatic: responds to external stimuli

autonomic: responds to internal stimuli

Sensory somatic system (SSNS)

info about external enviro to CNS

sends messages to skeletal muscles (voluntary)

somatic reflex arcs do NOT fit

12 pairs of cranial nerves

31 pairs of spinal nerves

What do cranial nerves control?

vision

hearing

balance

taste & smell

facial & tongue movements

muscles of the head & neck

Spinal nerves

operate skeletal actions that we consciously make to cope with our surroundings

e.g. putting on sunglasses

Autonomic nervous system (ANS)

info about body’s internal enviro to CNS

body responds to maintain homeostasis (involuntary)

controls smooth/cardiac muscle and internal organs/glands

2 systems: sympathetic and parasympathetic

2 groups of motor neurons of PNS

stimulate target effectors

1. Preganglionic neurons

2. Postganglionic neurons

Preganglionic neurons

run from the CNS to a ganglion (mass of nerves) where they connect with a second group

Postganglionic neurons

run to target organ, muscle, or gland

Sympathetic system

fight or flight

prepares the body for stress

Parasympathetic system

rest and digest

restore the body to normal

Vagus nerve

important cranial nerve of the parasympathetic system

wandering “vagabond”

innervates the heart, bronchi of the lungs, liver, pancreas, and the digestive tract

Why might meninges not be visible?

extremely thin and tightly attached to the nervous system

pulled/cut off during shipping

Function of the meninges

protection: cushion and protect the brain and spinal cord by enclosing CSF (absorbs chock and reduces injury)

delivery and regulation of the CSF

dura mater: outer layer that supports large veins

arachnoid mater: middle layer that provides a cushioning effect for CNS
pia mater: inner layer where blood vessels travel between brain and meninges

What makes the corpus callosum lighter in colour than the cerebrum?

made mostly of white matter - myelinated axons - that appears pale

cerebrum outer layer made mostly of grey matter

Why are arteries and veins not visible inside the brain? What replaces the blood?

no arteries or veins inside the brain

replaced by cerebrospinal fluid

What type of nerve conducts impulses to the olfactory bulbs?

sensory neuron in the olfactory nerve