Nervous system, integration: Overview, and peripheral nervous system:
Some review & misc. parts [Fig. 28.11B, p. 573]:
- white matter --> looks white due to the myelinated sheaths, which are quite fatty.
- gray matter --> consists mostly of nerve cell bodies, dendrites, and axons without fatty
sheath.
- ventricles --> spaces in the CNS that contain cerebrospinal fluid. This moves nutrients,
hormones and other substances around, and, particularly in the brain acts as a shock
absorber.
- meninges [Fig., not in text]:
- nervous tissue generally has the consistency of watery jello. It’s very fragile.
- meninges surround the nervous tissue and help maintain the structural integrity;
they also allow for cerebrospinal fluid to circulate. Consists of:
an outer covering, the dura mater
a space in between made up of cerebrospinal fluid and the
arachnoid membrane
an inner membrane lying right over the brain, the pia mater
The PNS, or Peripheral Nervous System:
consists of paired spinal nerves and paired cranial nerves [Fig. 28.11A].
- each set comes/goes to different structures. Except for a few cranial
nerves (e.g. optic nerve), each set contains both sensory and motor
components.
- mammals have 12 pairs of cranial nerves.
- reptiles have 10.
- in anatomy classes, the function of each pair of nerves is explained.
The PNS is divided into two broad groups [Fig. 28.12, p. 574. NOTE: the fourth
edition figure (28.12A) is NOT accurate]:
1) Somatic - sensory and motor nerves, generally parts that you have
voluntary control over and that sense the external environment.
2) autonomic - generally non-voluntary. Nerves that serve the internal
organs. Divided into three parts, but let's skip these details.
Note: the description of some of this is not accurate in the 4th edition.
Finally, how does all this work and fit together?
Example: reflex arc [Fig. 28.1B, p. 564,]:
sensory neuron --> CNS --> motor neuron
A reflex is a response that does not involve the brain.
But - you can feel the response. So the signal does eventually get to the brain.
Reflex arc. What happens to signal as it does go to the brain?
- there are two pathways that lead to the brain. Both cross over to the other side
as follows [Fig., not in book]:
- sensory neuron --> thalamus --> sensory cortex [Fig. 28.15A, p. 577]
- thalamus — coordinates information coming from different parts
of the body and sends it to the correct spot in the cortex.
- cortex - the outer layer of the cerebrum.
- cerebrum - higher functions take place here (more shortly)
- the nerve cell bodies are at the surface (i.e. on the cortex).
- sensory neuron --> cerebellum
- cerebellum is major area of motor coordination and motor
“memory” (e.g., learning how to play the piano or ride a bike)
- sensory cortex
- receives information from different parts of the body. Each part of the
body can be represented on the surface of the sensory cortex [Fig., not in
book].
- amount of area on the sensory cortex is related to the # of receptors at
each part of the body (e.g., fingers have many more receptors, and also a
much higher representation on the sensory cortex).
2) Other pathways going into the brain [illustrate on board]:
- optic nerve leads (eventually) to the thalamus --> primary visual cortex
- taste receptors lead to brainstem (pons, medulla) --> thalamus --> parietal lobe.
- auditory nerve leads to the “superior olive” --> thalamus --> primary auditory
cortex.
- “superior olive” helps determine direction of sound.
smell receptors --> cortex --> thalamus --> other parts of cortex [note the
difference].
3) Summary: all these pathways eventually wind up in the cortex, which as mentioned, is
the outer part of the cerebrum.
- Cerebrum [Fig. 28.16, p. 578]:
- highly folded structure: it is surface area rather than volume that appears
to contribute to intelligence (remember - the surface is where nerve cell
bodies sit).
- No other animal has as many ridges and grooves as man
(corrected for size) (next in line: dolphins).
- processes and integrates information from all parts of the body.
- note that areas of the cortex can be mapped to different functions such as
speech, pattern recognition, etc.
- the cortex, particularly the frontal part, is where “intelligence” resides.
- so, information is received, processed, and then if needed, the appropriate action
is taken (sometimes no action is needed at all!). More below.
- nerve tracts cross both on the way up and on the way down. Thus the right
hemisphere controls the left side and vice-versa.
- corpus callosum - communicates information between the two halves of the
cerebrum [Fig. 18.15B, p. 577].
4) If some action (response) is taken or needed, then the motor cortex receives
information from [point out structures on overhead]:
1) occipital lobe
2) temporal (hearing, memory, emotion)
3) parietal lobes (sensory, spatial)
--> all via the frontal association cortex, or lobe.
5) Motor cortex:
- maps parts of the body in a manner very similar to the sensory cortex [Fig., not
in book].
- If an electrical current is applied here, the appropriate body part can be made to
move.
- BUT, it’s not just a simple connection from the motor cortex to the muscle:
motor cortex --> medulla (here connection crosses over to the other side)
| |
| |
| --> motor neuron
|
---> pons --> cerebellum
[Aside: please don’t confuse cerebrum with cerebellum. Cerebellum
means “little brain” and is much smaller in man than the cerebrum]
6) Why go to the cerebellum?
- the cerebellum coordinates movement.
- information is processed, then taken BACK to the thalamus, then BACK to the
motor cortex, and back down.
cerebellum (processing) --> thalamus --> motor cortex
- in this way the path has a loop in it, so movements can be refined as they take
place
-also remember that sensory nerves often stop in cerebellum, so the
cerebellum can coordinate the signal headed down with sensory
information coming from the body.
7) A few other brain parts to be aware of [Fig. 28.15A, p. 577]:
- medulla - controls breathing, heart and blood vessel activity, other autonomous
nervous system functions.
- pons - similar to the medulla, but, for example, can control breathing centers of
medulla.
- hypothalamus
- gets signals (eventually from sensory pathways of the vagus nerve), and
uses this to help regulate body functions by communicating with
endocrine system.
- also is responsible for sensations associated with “pleasure”.
8) Other systems associated with the brain [Fig. 28.20, p. 581 & not in book]:
-limbic system - associated with cerebrum, thalamus, hypothalamus. Part of
frontal cortex (prefrontal) forms associations with this system to determine
emotional content of “messages/information” circulating around the brain.
- reticular formation - an area in the midbrain (between the brain stem and the
thalamus) that helps determine states of arousal (= alertness).
- by filtering information, the reticular formation can decrease alertness.
This might be useful when trying to get to sleep.
9) Higher brain functions - some examples/comments [Fig. 28.16, p. 578]:
- as mentioned, various functions have been mapped to different parts of the
cerebral cortex:
- speech - laterally, left side
- sensing object with the left hand - this winds up on the RIGHT side
(remember that nerve tracts cross over).
- this information then travels through the corpus callosum.
- If this becomes severed, a person can’t name the object, because
information can’t get from the right to the left (where the speech
centers are).
- right brain/left brain - this is not as straight forward as it appears:
- Cultural differences have been found. In different cultures, some
association areas, may be on the other side.
- it appears that the methods of learning/teaching are very
important for these areas to develop.
- But in general, we have:
- left - analytical/mathematical
- right - spatial/music recognition
- Another example: reading
- symbols wind up in the visual cortex, then moved to a part of the
brain where they are translated into the equivalent of “sounds”.
- from here they go to Wernicke’s area (also on the cerebrum, left
side), where they are finally comprehended.
- This complicated path is probably due to the fact that
sound communication is much older than writing, so
writing needs an extra step for interpretation.
- Finally, just a little bit about long and short term memory.
- again, this is a little more complicated, since there are also
“immediate memories”.
- the texture of the wall behind you, or the car in front of
you at a stop light.
- short term - memories that you can recall for a few minutes.
- The classic example is a phone number, that you
remember long enough to dial.
- If you dial this number repeatedly, then eventually:
- long term memories - memories that you can recall months or
even years later.
- As usual, this is not straight forward. For example, if you study,
you’ll do fairly well on the final. BUT, a few weeks from now,
you probably won’t remember a thing about anything on the final!
- the process of going from short to long term memory is not well
understood, but repetition helps (like that phone number example).
- learning involves the hippocampus. If this is damaged or
removed, person’s can no longer learn (but previous events are still
recalled clearly!).