The Human Brain

The Human Brain

The brain of all vertebrates develops from three swellings at the anterior end of the neural canal of the embryo. From front to back these develop into the

• forebrain (also known as the prosencephalon)
• midbrain (mesencephalon)
• hindbrain (rhombencephalon)
  
  
  
  
  

The Hindbrain

The main structures of the hindbrain are the

• medulla oblongata
• pons and
• cerebellum

Medulla oblongata
The medulla looks like a swollen tip to the spinal cord. Nerve impulses arising here:

• rhythmically stimulate the intercostal muscles and diaphragm - making breathing possible
• regulate heartbeat
• regulate the diameter of arterioles thus adjusting blood flow.

The neurons controlling breathing have mu receptors, the receptors to which opiates, like heroin bind. This accounts for the suppressive effect of opiates on breathing.  Destruction of the medulla causes instant death.
Pons

• The pons seems to serve as a relay station carrying signals from various parts of the cerebral cortex to the cerebellum. 
• Nerve impulses coming from the eyes, ears, and touch receptors are sent on the cerebellum. 
• The pons also participates in the reflexes that regulate breathing.

Cerebellum
The cerebellum consists of two deeply-convoluted hemispheres. Although it represents only 10% of the weight of the brain, it contains as many neurons as all the rest of the brain combined.
Its most clearly-understood function is to coordinate body movements. People with damage to their cerebellum are able to perceive the world as before and to contract their muscles, but their motions are jerky and uncoordinated.
The reticular formation is a region running through the middle of the hindbrain (and on into the midbrain). It receives sensory input (e.g., sound) from higher in the brain and passes these back up to the thalamus. The reticular formation is involved in sleep and arousal.

The Midbrain

The midbrain occupies only a small region in humans (it is relatively much larger in "lower" vertebrates). We shall look at only three features:

• the reticular formation: collects input from higher brain centers and passes it on to motor neurons.
• the substantia nigra: helps "smooth" out body movements; damage to the substantia nigra causes Parkinson's disease.
• the ventral tegmental area (VTA): packed with dopamine-releasing neurons that synapse deep within the forebrain. The VTA seems to be involved in pleasure: amphetamines and cocaine bind to the same receptors that it activates and this may account - at least in part (see below)- for their addictive qualities.

The Forebrain

The human forebrain is made up of

• a pair of large cerebral hemispheres, called the telencephalon. Because of crossing over of the spinal tracts, the left hemisphere of the forebrain deals with the right side of the body and vice versa.
• a group of unpaired structures located deep within the cerebrum, called the diencephalon.

Diencephalon

• Thalamus.
  • All sensory input (except for olfaction) passes through it on the way up to the somatic-sensory regions of the cerebral cortex and then returns to it from there.
  • signals from the cerebellum pass through it on the way to the motor areas of the cerebral cortex.
• Lateral geniculate nucleus (LGN). All signals entering the brain from the optic nerves enter the LGN and undergo some processing before moving on the various visual areas of the cerebral cortex.
• Hypothalamus.
  • The seat of the autonomic nervous system. Damage to the hypothalamus is quickly fatal as the normal homeostasis of body temperature, blood chemistry, etc. goes out of control.
  • The source of 8 hormones, two of which pass into the posterior lobe of the pituitary gland.

Telencephalon

The cerebrum, the largest part of the human brain, is divided into left and right hemispheres connected to each other by the corpus callosum. The hemispheres are covered by a thin layer of gray matter known as the cerebral cortex, the most recently evolved region of the vertebrate brain. Fish have no cerebral cortex, amphibians and reptiles have only rudiments of this area.
The cortex in each hemisphere of the cerebrum is between 1 and 4 mm thick. Folds divide the cortex into four lobes: occipital, temporal, parietal, and frontal. No region of the brain functions alone, although major functions of various parts of the lobes have been determined.
The occipital lobe (back of the head) receives and processes visual information. The temporal lobe receives auditory signals, processing language and the meaning of words. The parietal lobe is associated with the sensory cortex and processes information about touch, taste, pressure, pain, and heat and cold. The frontal lobe conducts three functions:

1. motor activity and integration of muscle activity
2. speech
3. thought processes

Functional areas of the brain. 

Most people who have been studied have their language and speech areas on the left hemisphere of their brain. Language comprehension is found in Wernicke's area. Speaking ability is in Broca's area. Damage to Broca's area causes speech impairment but not impairment of language comprehension. Lesions in Wernicke's area impairs ability to comprehend written and spoken words but not speech. The remaining parts of the cortex are associated with higher thought processes, planning, memory, personality and other human activities.